# Look-and-say sequence

Look-and-say sequence
You are encouraged to solve this task according to the task description, using any language you may know.

The   Look and say sequence   is a recursively defined sequence of numbers studied most notably by   John Conway.

Sequence Definition

• Take a decimal number
• Look at the number, visually grouping consecutive runs of the same digit.
• Say the number, from left to right, group by group; as how many of that digit there are - followed by the digit grouped.
This becomes the next number of the sequence.

An example:

• Starting with the number 1,   you have one 1 which produces 11
• Starting with 11,   you have two 1's.   I.E.:   21
• Starting with 21,   you have one 2, then one 1.   I.E.:   (12)(11) which becomes 1211
• Starting with 1211,   you have one 1, one 2, then two 1's.   I.E.:   (11)(12)(21) which becomes 111221

Write a program to generate successive members of the look-and-say sequence.

function "+" (S : String) return String is
Item : constant Character := S (S'First);
begin
for Index in S'First + 1..S'Last loop
if Item /= S (Index) then
return Trim (Integer'Image (Index - S'First), Both) & Item & (+(S (Index..S'Last)));
end if;
end loop;
return Trim (Integer'Image (S'Length), Both) & Item;
end "+";

This function can be used as follows:

Put_Line (+"1");
Put_Line (+(+"1"));
Put_Line (+(+(+"1")));
Put_Line (+(+(+(+"1"))));
Put_Line (+(+(+(+(+"1")))));
Put_Line (+(+(+(+(+(+"1"))))));
Put_Line (+(+(+(+(+(+(+"1")))))));
Put_Line (+(+(+(+(+(+(+(+"1"))))))));
Put_Line (+(+(+(+(+(+(+(+(+"1")))))))));
Put_Line (+(+(+(+(+(+(+(+(+(+"1"))))))))));
Output:
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

## ALGOL 68

Works with: ALGOL 68 version Standard - no extensions to language used
Works with: ALGOL 68G version Any - tested with release mk15-0.8b.fc9.i386
Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8.8d.fc9.i386
OP + = (STRING s)STRING:
BEGIN
CHAR item = s[LWB s];
STRING out;
FOR index FROM LWB s + 1 TO UPB s DO
IF item /= s [index] THEN
out := whole(index - LWB s, 0) + item + (+(s [index:UPB s]));
GO TO return out
FI
OD;
out := whole (UPB s, 0) + item;
return out: out
END # + #;

OP + = (CHAR s)STRING:
+ STRING(s);

print ((+"1", new line));
print ((+(+"1"), new line));
print ((+(+(+"1")), new line));
print ((+(+(+(+"1"))), new line));
print ((+(+(+(+(+"1")))), new line));
print ((+(+(+(+(+(+"1"))))), new line));
print ((+(+(+(+(+(+(+"1")))))), new line));
print ((+(+(+(+(+(+(+(+"1"))))))), new line));
print ((+(+(+(+(+(+(+(+(+"1")))))))), new line));
print ((+(+(+(+(+(+(+(+(+(+"1"))))))))), new line))
Output:
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

## AutoHotkey

AutoExecute:
Gui, -MinimizeBox
Gui, Add, Edit, w500 r20 vInput, 1
Gui, Add, Button, x155 w100 Default, &Calculate
Gui, Add, Button, xp+110 yp wp, E&xit
Gui, Show,, Look-and-Say sequence
Return

ButtonCalculate:
Gui, Submit, NoHide
GuiControl,, Input, % LookAndSay(Input)
Return

GuiClose:
ButtonExit:
ExitApp
Return

;---------------------------------------------------------------------------
LookAndSay(Input) {
;---------------------------------------------------------------------------
; credit for this function goes to AutoHotkey forum member Laslo
; http://www.autohotkey.com/forum/topic44657-161.html
;-----------------------------------------------------------------------
Loop, Parse, Input ; look at every digit
If (A_LoopField = d) ; I've got another one! (of the same value)
c += 1 ; Let's count them ...
Else { ; No, this one is different!
r .= c d ; remember what we've got so far
c := 1 ; It is the first one in a row
d := A_LoopField ; Which one is it?
}
Return, r c d
}

## APL

⎕IO←0
d←{(1↓⍵)-¯1↓⍵}
f←{m←(0≠d ⍵),1 ⋄ ,(d ¯1,m/⍳⍴⍵),[.5](m/⍵)}
{(f⍣⍵) ,1}¨⍳10

## AWK

function lookandsay(a)
{
s = ""
c = 1
p = substr(a, 1, 1)
for(i=2; i <= length(a); i++) {
if ( p == substr(a, i, 1) ) {
c++
} else {
s = s sprintf("%d%s", c, p)
p = substr(a, i, 1)
c = 1
}
}
s = s sprintf("%d%s", c, p)
return s
}

BEGIN {
b = "1"
print b
for(k=1; k <= 10; k++) {
b = lookandsay(b)
print b
}
}

## BASIC256

# look and say

dim a\$(2)

i = 0 # input string index

a\$[i] = "1"

print a\$[i]

for n=1 to 10
j = 1 - i # output string index
a\$[j] = ""
k = 1
while (k <= length(a\$[i]))
k0 = k + 1
while ((k0 <= length(a\$[i])) and (mid(a\$[i], k, 1) = mid(a\$[i], k0, 1)))
k0 = k0 + 1
end while
a\$[j] += string(k0 - k) + mid(a\$[i], k, 1)
k = k0
end while
i = j
print a\$[j]
next n

## BBC BASIC

number\$ = "1"
FOR i% = 1 TO 10
number\$ = FNlooksay(number\$)
PRINT number\$
NEXT
END

DEF FNlooksay(n\$)
LOCAL i%, j%, c\$, o\$
i% = 1
REPEAT
c\$ = MID\$(n\$,i%,1)
j% = i% + 1
WHILE MID\$(n\$,j%,1) = c\$
j% += 1
ENDWHILE
o\$ += STR\$(j%-i%) + c\$
i% = j%
UNTIL i% > LEN(n\$)
= o\$
Output:
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

## Bracmat

In this example we use a non-linear pattern and a negation of a pattern: the end of e sequence of equal digits is (1) the end of the string or (2) the start of a sequence starting with a different digit.

( 1:?number
& 0:?lines
& whl
' ( 1+!lines:~>10:?lines
& :?say { This will accumulate all that has to be said after one iteration. }
& 0:?begin
& ( @( !number { Pattern matching. The '@' indicates we're looking in a string rather than a tree structure. }
:  ?
( [!begin
%@?digit
?
[?end
( (|(%@:~!digit) ?) { The %@ guarantees we're testing one character - not less (%) and not more (@). The ? takes the rest. }
& !say !end+-1*!begin !digit:?say
& !end:?begin { When backtracking, 'begin' advances to the begin of the next sequence, or to the end of the string. }
)
& ~ { fail! This forces backtracking. Backtracking stops when all begin positions have been tried. }
)
)
| out\$(str\$!say:?number) { After backtracking, output string and set number to string for next iteration. }
)
)
);
Output:
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

## C

This program will not stop until killed or running out of memory.

#include <stdio.h>
#include <stdlib.h>

int main()
{
char *a = malloc(2), *b = 0, *x, c;
int cnt, len = 1;

for (sprintf(a, "1"); (b = realloc(b, len * 2 + 1)); a = b, b = x) {
puts(x = a);
for (len = 0, cnt = 1; (c = *a); ) {
if (c == *++a)
cnt++;
else if (c) {
len += sprintf(b + len, "%d%c", cnt, c);
cnt = 1;
}
}
}

return 0;
}

## C++

#include <iostream>
#include <sstream>
#include <string>

std::string lookandsay(const std::string& s)
{
std::ostringstream r;

for (std::size_t i = 0; i != s.length();) {
auto new_i = s.find_first_not_of(s[i], i + 1);

if (new_i == std::string::npos)
new_i = s.length();

r << new_i - i << s[i];
i = new_i;
}
return r.str();
}

int main()
{
std::string laf = "1";

std::cout << laf << '\n';
for (int i = 0; i < 10; ++i) {
laf = lookandsay(laf);
std::cout << laf << '\n';
}
}

## C#

using System;
using System.Text;
using System.Linq;

class Program
{
static string lookandsay(string number)
{
StringBuilder result = new StringBuilder();

char repeat = number[0];
number = number.Substring(1, number.Length-1)+" ";
int times = 1;

foreach (char actual in number)
{
if (actual != repeat)
{
result.Append(Convert.ToString(times)+repeat);
times = 1;
repeat = actual;
}
else
{
times += 1;
}
}
return result.ToString();
}

static void Main(string[] args)
{
string num = "1";

foreach (int i in Enumerable.Range(1, 10)) {
Console.WriteLine(num);
num = lookandsay(num);
}
}
}
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

Alternate version using Regex (C#2 syntax only):

using System;
using System.Text.RegularExpressions;

namespace RosettaCode_Cs_LookAndSay
{
public class Program
{
public static int Main(string[] args)
{
Array.Resize<string>(ref args, 2);
string ls = args[0] ?? "1";
int n;
if (!int.TryParse(args[1], out n)) n = 10;
do {
Console.WriteLine(ls);
if (--n <= 0) break;
ls = say(look(ls));
} while(true);

return 0;
}

public static string[] look(string input)
{
int i = -1;
return Array.FindAll(Regex.Split(input, @"((\d)\2*)"),
delegate(string p) { ++i; i %= 3; return i == 1; }
);
}

public static string say(string[] groups)
{
return string.Concat(
Array.ConvertAll<string, string>(groups,
delegate(string p) { return string.Concat(p.Length, p[0]); }
)
);
}
}
}
Output:
(with args: 1 15):
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

## Ceylon

shared void run() {

function lookAndSay(Integer|String input) {

variable value digits = if (is Integer input) then input.string else input;
value builder = StringBuilder();

while (exists currentChar = digits.first) {
if (exists index = digits.firstIndexWhere((char) => char != currentChar)) {
digits = digits[index...];
builder.append("``index````currentChar``");
}
else {
builder.append("``digits.size````currentChar``");
break;
}
}

return builder.string;
}

variable String|Integer result = 1;
print(result);
for (i in 1..14) {
result = lookAndSay(result);
print(result);
}
}

## Clojure

No ugly int-to-string-and-back conversions.

(defn digits-seq
"Returns a seq of the digits of a number (L->R)."
[n]
(loop [digits (), number n]
(if (zero? number) (seq digits)
(recur (cons (mod number 10) digits)
(quot number 10)))))

(defn join-digits
"Converts a digits-seq back in to a number."
[ds]
(reduce (fn [n d] (+ (* 10 n) d)) ds))

(defn look-and-say [n]
(->> n digits-seq (partition-by identity)
(mapcat (juxt count first)) join-digits))
Output:
user> (take 8 (iterate look-and-say 1))
(1 11 21 1211 111221 312211 13112221 1113213211)

## Common Lisp

(defun compress (array &key (test 'eql) &aux (l (length array)))
"Compresses array by returning a list of conses each of whose car is
a number of occurrences and whose cdr is the element occurring. For
instance, (compress \"abb\") produces ((1 . #\a) (2 . #\b))."

(if (zerop l) nil
(do* ((i 1 (1+ i))
(segments (acons 1 (aref array 0) '())))
((eql i l) (nreverse segments))
(if (funcall test (aref array i) (cdar segments))
(incf (caar segments))
(setf segments (acons 1 (aref array i) segments))))))

(defun next-look-and-say (number)
(reduce #'(lambda (n pair)
(+ (* 100 n)
(* 10 (car pair))
(parse-integer (string (cdr pair)))))
(compress (princ-to-string number))
:initial-value 0))

Example use:

(next-look-and-say 9887776666) ;=> 19283746

Straight character counting:

(defun look-and-say (s)
(let ((out (list (char s 0) 0)))
(loop for x across s do
(if (char= x (first out))
(incf (second out))
(setf out (list* x 1 out))))
(format nil "~{~a~^~}" (nreverse out))))

(loop for s = "1" then (look-and-say s)
repeat 10
do (write-line s))

## D

### Short Functional Version

import std.stdio, std.algorithm, std.range;

enum say = (in string s) pure => s.group.map!q{ text(a[1],a[0]) }.join;

void main() {
"1".recurrence!((t, n) => t[n - 1].say).take(8).writeln;
}
Output:
["1", "11", "21", "1211", "111221", "312211", "13112221", "1113213211"]

### Fast Imperative Version

Same output.

import core.stdc.stdio, std.math, std.conv, std.algorithm, std.array;

void showLookAndSay(bool showArrays)(in uint n) nothrow {
if (n == 0) // No sequences to generate and show.
return;

enum Digit : char { nil = '\0', one = '1', two = '2', thr = '3' }

// Allocate an approximate upper bound size for the array.
static Digit* allocBuffer(in uint m) nothrow {
immutable len = cast(size_t)(100 + 1.05 *
exp(0.269 * m + 0.2686)) + 1;
auto a = len.uninitializedArray!(Digit[]);
printf("Allocated %d bytes.\n", a.length * Digit.sizeof);
return a.ptr;
}

// Can't be expressed in the D type system:
// a1 and a2 are immutable pointers to mutable data.
auto a1 = allocBuffer(n % 2 ? n : n - 1);
auto a2 = allocBuffer(n % 2 ? n - 1 : n);
printf("\n");

a1[0] = Digit.one;
size_t len1 = 1;
a1[len1] = Digit.nil;

foreach (immutable i; 0 .. n - 1) {
static if (showArrays)
printf("%2u: %s\n", i + 1, a1);
else
printf("%2u: n. digits: %u\n", i + 1, len1);
auto p1 = a1,
p2 = a2;

S0: final switch (*p1++) with (Digit) { // Initial state.
case nil: goto END;
case one: goto S1;
case two: goto S2;
case thr: goto S3;
}
S1: final switch (*p1++) with (Digit) {
case nil: *p2++ = one; *p2++ = one; goto END;
case one: goto S11;
case two: *p2++ = one; *p2++ = one; goto S2;
case thr: *p2++ = one; *p2++ = one; goto S3;
}
S2: final switch (*p1++) with (Digit) {
case nil: *p2++ = one; *p2++ = two; goto END;
case one: *p2++ = one; *p2++ = two; goto S1;
case two: goto S22;
case thr: *p2++ = one; *p2++ = two; goto S3;
}
S3: final switch (*p1++) with (Digit) {
case nil: *p2++ = one; *p2++ = thr; goto END;
case one: *p2++ = one; *p2++ = thr; goto S1;
case two: *p2++ = one; *p2++ = thr; goto S2;
case thr: goto S33;
}
S11: final switch (*p1++) with (Digit) {
case nil: *p2++ = two; *p2++ = one; goto END;
case one: *p2++ = thr; *p2++ = one; goto S0;
case two: *p2++ = two; *p2++ = one; goto S2;
case thr: *p2++ = two; *p2++ = one; goto S3;
}
S22: final switch (*p1++) with (Digit) {
case nil: *p2++ = two; *p2++ = two; goto END;
case one: *p2++ = two; *p2++ = two; goto S1;
case two: *p2++ = thr; *p2++ = two; goto S0;
case thr: *p2++ = two; *p2++ = two; goto S3;
}
S33: final switch (*p1++) with (Digit) {
case nil: *p2++ = two; *p2++ = thr; goto END;
case one: *p2++ = two; *p2++ = thr; goto S1;
case two: *p2++ = two; *p2++ = thr; goto S2;
case thr: *p2++ = thr; *p2++ = thr; goto S0;
}
END:
immutable len2 = p2 - a2;
a2[len2] = Digit.nil;
a1.swap(a2);
len1 = len2;
}

static if (showArrays)
printf("%2u: %s\n", n, a1);
else
printf("%2u: n. digits: %u\n", n, len1);
}

void main(in string[] args) {
immutable n = (args.length == 2) ? args[1].to!uint : 10;
n.showLookAndSay!true;
}
Output:
Allocated 116 bytes.
Allocated 121 bytes.

1: 1
2: 11
3: 21
4: 1211
5: 111221
6: 312211
7: 13112221
8: 1113213211
9: 31131211131221
10: 13211311123113112211

With:

70.showLookAndSay!false;
Output:
Allocated 158045069 bytes.
Allocated 206826462 bytes.

1: n. digits: 1
2: n. digits: 2
3: n. digits: 2
4: n. digits: 4
5: n. digits: 6
...
60: n. digits: 12680852
61: n. digits: 16530884
62: n. digits: 21549544
63: n. digits: 28091184
64: n. digits: 36619162
65: n. digits: 47736936
66: n. digits: 62226614
67: n. digits: 81117366
68: n. digits: 105745224
69: n. digits: 137842560
70: n. digits: 179691598

Using the LDC2 compiler with n=70 the run-time is about 3.74 seconds.

### Intermediate Version

This mostly imperative version is intermediate in both speed and code size:

void main(in string[] args) {
import std.stdio, std.conv, std.algorithm, std.array, std.string;

immutable n = (args.length == 2) ? args[1].to!uint : 10;
if (n == 0)
return;

auto seq = ['1'];
writefln("%2d: n. digits: %d", 1, seq.length);
foreach (immutable i; 2 .. n + 1) {
Appender!(typeof(seq)) result;
foreach (const digit, const count; seq.representation.group) {
result ~= "123"[count - 1];
result ~= digit;
}
seq = result.data;
writefln("%2d: n. digits: %d", i, seq.length);
}
}

The output is the same as the second version.

If you modify the first program to print only the lengths of the strings (with a .map!(s => s.length)), compiling with LDC2 the run-times of the three versions with n=55 are about 31.1, 0.10 and 0.23 seconds.

### More Direct Version

Translated and modified from C code by Reddit user "skeeto": http://www.reddit.com/r/dailyprogrammer/comments/2ggy30/9152014_challenge180_easy_looknsay/

This recursive version is able to generate very large sequences in a short time without memory for the intermediate sequence (and with stack space proportional to the sequence order).

import core.stdc.stdio, std.conv;

// On Windows this uses the printf from the Microsoft C runtime,
// that doesn't handle real type and some of the C99 format
// specifiers, but it's faster for bulk printing.
version (LDC) version (Windows)
extern(C) nothrow @nogc int printf(const char*, ...);

// http://www.njohnston.ca/2010/10/a-derivation-of-conways-degree-71-look-and-say-polynomial/
struct Sequence {
string seq;
uint[6] next;
}

immutable Sequence[93] sequences = [
{"", []},
{"1112", [63]},
{"1112133", [64, 62]},
{"111213322112", [65]},
{"111213322113", [66]},
{"1113", [68]},
{"11131", [69]},
{"111311222112", [84, 55]},
{"111312", [70]},
{"11131221", [71]},
{"1113122112", [76]},
{"1113122113", [77]},
{"11131221131112", [82]},
{"111312211312", [78]},
{"11131221131211", [79]},
{"111312211312113211", [80]},
{"111312211312113221133211322112211213322112", [81, 29, 91]},
{"111312211312113221133211322112211213322113", [81, 29, 90]},
{"11131221131211322113322112", [81, 30]},
{"11131221133112", [75, 29, 92]},
{"1113122113322113111221131221", [75, 32]},
{"11131221222112", [72]},
{"111312212221121123222112", [73]},
{"111312212221121123222113", [74]},
{"11132", [83]},
{"1113222", [86]},
{"1113222112", [87]},
{"1113222113", [88]},
{"11133112", [89, 92]},
{"12", [1]},
{"123222112", [3]},
{"123222113", [4]},
{"12322211331222113112211", [2, 61, 29, 85]},
{"13", [5]},
{"131112", [28]},
{"13112221133211322112211213322112", [24, 33, 61, 29, 91]},
{"13112221133211322112211213322113", [24, 33, 61, 29, 90]},
{"13122112", [7]},
{"132", [8]},
{"13211", [9]},
{"132112", [10]},
{"1321122112", [21]},
{"132112211213322112", [22]},
{"132112211213322113", [23]},
{"132113", [11]},
{"1321131112", [19]},
{"13211312", [12]},
{"1321132", [13]},
{"13211321", [14]},
{"132113212221", [15]},
{"13211321222113222112", [18]},
{"1321132122211322212221121123222112", [16]},
{"1321132122211322212221121123222113", [17]},
{"13211322211312113211", [20]},
{"1321133112", [6, 61, 29, 92]},
{"1322112", [26]},
{"1322113", [27]},
{"13221133112", [25, 29, 92]},
{"1322113312211", [25, 29, 67]},
{"132211331222113112211", [25, 29, 85]},
{"13221133122211332", [25, 29, 68, 61, 29, 89]},
{"22", [61]},
{"3", [33]},
{"3112", [40]},
{"3112112", [41]},
{"31121123222112", [42]},
{"31121123222113", [43]},
{"3112221", [38, 39]},
{"3113", [44]},
{"311311", [48]},
{"31131112", [54]},
{"3113112211", [49]},
{"3113112211322112", [50]},
{"3113112211322112211213322112", [51]},
{"3113112211322112211213322113", [52]},
{"311311222", [47, 38]},
{"311311222112", [47, 55]},
{"311311222113", [47, 56]},
{"3113112221131112", [47, 57]},
{"311311222113111221", [47, 58]},
{"311311222113111221131221", [47, 59]},
{"31131122211311122113222", [47, 60]},
{"3113112221133112", [47, 33, 61, 29, 92]},
{"311312", [45]},
{"31132", [46]},
{"311322113212221", [53]},
{"311332", [38, 29, 89]},
{"3113322112", [38, 30]},
{"3113322113", [38, 31]},
{"312", [34]},
{"312211322212221121123222113", [36]},
{"312211322212221121123222112", [35]},
{"32112", [37]}
];

void evolve(in uint seq, in uint n) nothrow @nogc {
if (n <= 0) {
printf(sequences[seq].seq.ptr);
} else {
foreach (immutable next; sequences[seq].next) {
if (next == 0)
break;
evolve(next, n - 1);
}
}
}

void main(in string[] args) {
immutable uint n = (args.length != 2) ? 10 : args[1].to!uint;

immutable base = 8;
immutable string[base] results = ["", "1", "11", "21", "1211",
"111221", "312211", "13112221"];
if (n < base) {
printf("%s\n", results[n].ptr);
return;
}

evolve(24, n - base);
evolve(39, n - base);
'\n'.putchar;
}

## E

def lookAndSayNext(number :int) {
var seen := null
var count := 0
var result := ""
def put() {
if (seen != null) {
result += count.toString(10) + E.toString(seen)
}
}
for ch in number.toString(10) {
if (ch != seen) {
put()
seen := ch
count := 0
}
count += 1
}
put()
return __makeInt(result, 10)
}

var number := 1
for _ in 1..20 {
println(number)
number := lookAndSayNext(number)
}

## EchoLisp

(lib 'math) ;; for (number->list) = explode function
(lib 'list) ;; (group)

(define (next L)
(for/fold (acc null) ((g (group L)))
(append acc (list (length g) (first g)))))

(for/fold (L (number->list starter)) ((i n))
(writeln (list->string L))
(next L)))

Output:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

## Elixir

defmodule LookAndSay do
def next(n) do
Enum.chunk_by(to_char_list(n), &(&1))
|> Enum.map(fn cl=[h|_] -> Enum.concat(to_char_list(length cl), [h]) end)
|> Enum.concat
|> List.to_integer
end

def sequence_from(n) do
Stream.iterate n, &(next/1)
end

def main([start_str|_]) do
{start_val,_} = Integer.parse(start_str)
IO.inspect sequence_from(start_val) |> Enum.take 9
end

def main([]) do
main(["1"])
end
end

LookAndSay.main(System.argv)
Output:
[1, 11, 21, 1211, 111221, 312211, 13112221, 1113213211, 31131211131221]

Regex version:

defmodule RC do
def look_and_say(n) do
Regex.replace(~r/(.)\1*/, to_string(n), fn x,y -> [to_string(String.length(x)),y] end)
|> String.to_integer
end
end

IO.inspect Enum.reduce(1..9, [1], fn _,acc -> [RC.look_and_say(hd(acc)) | acc] end) |> Enum.reverse
Output:
[1, 11, 21, 1211, 111221, 312211, 13112221, 1113213211, 31131211131221,
13211311123113112211]

## Erlang

-module(str).
-export([look_and_say/1, look_and_say/2]).

%% converts a single number
look_and_say([H|T]) -> lists:reverse(look_and_say(T,H,1,"")).

%% converts and accumulates as a loop
look_and_say(_, 0) -> [];
look_and_say(Start, Times) when Times > 0 ->
[Start | look_and_say(look_and_say(Start), Times-1)].

%% does the actual conversion for a number
look_and_say([], Current, N, Acc) ->
[Current, \$0+N | Acc];
look_and_say([H|T], H, N, Acc) ->
look_and_say(T, H, N+1, Acc);
look_and_say([H|T], Current, N, Acc) ->
look_and_say(T, H, 1, [Current, \$0+N | Acc]).
Output:
1> c(str).
{ok,str}
2> str:look_and_say("1").
"11"
3> str:look_and_say("111221").
"312211"
4> str:look_and_say("1",10).
["1","11","21","1211","111221","312211","13112221",
"1113213211","31131211131221","13211311123113112211"]

## ERRE

PROGRAM LOOK

PROCEDURE LOOK_AND_SAY(N\$->N\$)
LOCAL I%,J%,C\$,O\$
I%=1
REPEAT
C\$=MID\$(N\$,I%,1)
J%=I%+1
WHILE MID\$(N\$,J%,1)=C\$ DO
J%+=1
END WHILE
O\$+=MID\$(STR\$(J%-I%),2)+C\$
I%=J%
UNTIL I%>LEN(N\$)
N\$=O\$
END PROCEDURE

BEGIN
NUMBER\$="1"
FOR I%=1 TO 10 DO
LOOK_AND_SAY(NUMBER\$->NUMBER\$)
PRINT(NUMBER\$)
END FOR
END PROGRAM

11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

## F#

Library functions somehow missing in F# out of the box (but present in haskell)

let rec brk p lst =
match lst with
| [] -> (lst, lst)
| x::xs ->
if p x
then ([], lst)
else
let (ys, zs) = brk p xs
(x::ys, zs)

let span p lst = brk (not << p) lst

let rec groupBy eq lst =
match lst with
| [] -> []
| x::xs ->
let (ys,zs) = span (eq x) xs
(x::ys)::groupBy eq zs

let group lst : list<list<'a>> when 'a : equality = groupBy (=) lst

Implementation

let lookAndSay =
let describe (xs: char list) =
List.append (List.ofSeq <| (List.length xs).ToString()) [List.head xs]
let next xs = List.collect describe (group xs)
let toStr xs = String (Array.ofList xs)
Seq.map toStr <| Seq.unfold (fun xs -> Some (xs, next xs)) ['1']

let getNthLookAndSay n = Seq.nth n lookAndSay

Seq.take 10 lookAndSay

## Factor

: (look-and-say) ( str -- )
unclip-slice swap [ 1 ] 2dip [
2dup = [ drop [ 1 + ] dip ] [
[ [ number>string % ] dip , 1 ] dip
] if
] each [ number>string % ] [ , ] bi* ;

: look-and-say ( str -- str' ) [ (look-and-say) ] "" make ;

"1" 10 [ dup print look-and-say ] times print

## Forth

create buf1 256 allot
create buf2 256 allot
buf1 value src
buf2 value dest

s" 1" src place

: append-run ( digit run -- )
dest count +
tuck c! 1+ c!
dest [email protected] 2 + dest c! ;

: next-look-and-say
0 dest c!
src 1+ [email protected] [char] 0 ( digit run )
src count bounds do
over i [email protected] =
if 1+
else append-run i [email protected] [char] 1
then
loop
append-run
src dest to src to dest ;

: look-and-say ( n -- )
0 do next-look-and-say cr src count type loop ;

10 look-and-say

## Fortran

module LookAndSay
implicit none

contains

subroutine look_and_say(in, out)
character(len=*), intent(in) :: in
character(len=*), intent(out) :: out

integer :: i, c
character(len=1) :: x
character(len=2) :: d

out = ""
c = 1
x = in(1:1)
do i = 2, len(trim(in))
if ( x == in(i:i) ) then
c = c + 1
else
write(d, "(I2)") c
out = trim(out) // trim(adjustl(d)) // trim(x)
c = 1
x = in(i:i)
end if
end do
write(d, "(I2)") c
out = trim(out) // trim(adjustl(d)) // trim(x)
end subroutine look_and_say

end module LookAndSay
program LookAndSayTest
use LookAndSay
implicit none

integer :: i
character(len=200) :: t, r
t = "1"
print *,trim(t)
call look_and_say(t, r)
print *, trim(r)
do i = 1, 10
call look_and_say(r, t)
r = t
print *, trim(r)
end do

end program LookAndSayTest

## Gambas

Code is modified from the [PureBasic] example

Public Sub Main()
Dim i, j, cnt As Integer
Dim txt\$, curr\$, result\$ As String

txt\$ = "1211"
i = 1

Print "Sequence: " & txt\$ & " = ";
Repeat
j = 1
result\$ = ""
Repeat
curr\$ = Mid(txt\$, j, 1)
cnt = 0
Repeat
Inc cnt
Inc j
Until Mid(txt\$, j, 1) <> curr\$
result\$ &= Str(cnt) & curr\$
Until j > Len(txt\$)
Print result\$
txt\$ = result\$
Dec i
Until i <= 0
End

Output:

Sequence: 1211 = 111221

## GAP

LookAndSay := function(s)
local c, r, cur, ncur, v;
v := "123";
r := "";
cur := 0;
ncur := 0;
for c in s do
if c = cur then
ncur := ncur + 1;
else
if ncur > 0 then
fi;
cur := c;
ncur := 1;
fi;
od;
return r;
end;

LookAndSay("1"); # "11"
LookAndSay(last); # "21"
LookAndSay(last); # "1211"
LookAndSay(last); # "111221"
LookAndSay(last); # "312211"
LookAndSay(last); # "13112221"
LookAndSay(last); # "1113213211"
LookAndSay(last); # "31131211131221"
LookAndSay(last); # "13211311123113112211"
LookAndSay(last); # "11131221133112132113212221"
LookAndSay(last); # "3113112221232112111312211312113211"
LookAndSay(last); # "1321132132111213122112311311222113111221131221"
LookAndSay(last); # "11131221131211131231121113112221121321132132211331222113112211"
LookAndSay(last); # "311311222113111231131112132112311321322112111312211312111322212311322113212221"

## Go

package main

import (
"fmt"
"strconv"
)

func lss(s string) (r string) {
c := s[0]
nc := 1
for i := 1; i < len(s); i++ {
d := s[i]
if d == c {
nc++
continue
}
r += strconv.Itoa(nc) + string(c)
c = d
nc = 1
}
return r + strconv.Itoa(nc) + string(c)
}

func main() {
s := "1"
fmt.Println(s)
for i := 0; i < 8; i++ {
s = lss(s)
fmt.Println(s)
}
}
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221

## Groovy

def lookAndSay(sequence) {
def encoded = new StringBuilder()
(sequence.toString() =~ /(([0-9])\2*)/).each { matcher ->
encoded.append(matcher[1].size()).append(matcher[2])
}
encoded.toString()
}

Test Code

def sequence = "1"
(1..12).each {
println sequence
sequence = lookAndSay(sequence)
}
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211

import Data.List (group)

-- this function is composed out of many functions; data flows from the bottom up
lookAndSay :: Integer -> Integer
lookAndSay = read -- convert digits to integer
. concatMap -- concatenate for each run,
(liftM2 (++) (show . length) -- the length of it
(take 1)) -- and an example member
. group -- collect runs of the same digit
. show -- convert integer to digits

lookAndSay2 :: Integer -> Integer
lookAndSay2 = read . concatMap (liftM2 (++) (show . length)
(take 1))
. group . show

-- same thing with more variable names
lookAndSay3 :: Integer -> Integer
lookAndSay3 n = read (concatMap describe (group (show n)))
where describe run = show (length run) ++ take 1 run

main = mapM_ print (iterate lookAndSay 1) -- display sequence until interrupted

## Haxe

using Std;

class Main
{

static function main()
{
var test = "1";
for (i in 0...11) {
Sys.println(test);
test = lookAndSay(test);
}
}

static function lookAndSay(s:String)
{
if (s == null || s == "") return "";

var results = "";
var repeat = s.charAt(0);
var amount = 1;
for (i in 1...s.length)
{
var actual = s.charAt(i);
if (actual != repeat)
{
results += amount.string();
results += repeat;
repeat = actual;
amount = 0;
}
amount++;
}
results += amount.string();
results += repeat;

return results;
}
}

## Icon and Unicon

procedure main()
every 1 to 10 do
write(n := nextlooknsayseq(\n | 1))
end

procedure nextlooknsayseq(n) #: return next element in look and say sequence
n2 := ""
n ? until pos(0) do {
i := tab(any(&digits)) | fail # or fail if not digits
move(-1)
n2 ||:= *tab(many(i)) || i # accumulate count+digit
}
return n2
end
Output:
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

## J

Solution:

las=: ,@((# , {.);.1~ 1 , 2 ~:/\ ])&.(10x&#.inv)@]^:(1[email protected][)

Example:

10 las 1
1 11 21 1211 111221 312211 13112221 1113213211 31131211131221 13211311123113112211 11131221133112132113212221

Note the result is an actual numeric sequence (cf. the textual solutions given in other languages).

## Java

Translation of: C#
Works with: Java version 1.5+
public static String lookandsay(String number){
StringBuilder result= new StringBuilder();

char repeat= number.charAt(0);
number= number.substring(1) + " ";
int times= 1;

for(char actual: number.toCharArray()){
if(actual != repeat){
result.append(times + "" + repeat);
times= 1;
repeat= actual;
}else{
times+= 1;
}
}
return result.toString();
}

Testing:

public static void main(String[] args){
String num = "1";

for (int i=1;i<=10;i++) {
System.out.println(num);
num = lookandsay(num);
}
}
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

## JavaScript

Translation of: Perl
function lookandsay(str) {
return str.replace(/(.)\1*/g, function(seq, p1){return seq.length.toString() + p1})
}

var num = "1";
for (var i = 10; i > 0; i--) {
num = lookandsay(num);
}

Without RegExp

function lookSay(digits) {
var result = '',
chars = (digits + ' ').split(''),
lastChar = chars[0],
times = 0;

chars.forEach(function(nextChar) {
if (nextChar === lastChar) {
times++;
}
else {
result += (times + '') + lastChar;
lastChar = nextChar;
times = 1;
}
});

return result;
}

(function output(seed, iterations) {
for (var i = 0; i < iterations; i++) {
console.log(seed);
seed = lookSay(seed);
}
})("1", 10);

## jq

Works with: jq version 1.4
def look_and_say:
def head(c; n): if .[n:n+1] == c then head(c; n+1) else n end;
tostring
| if length == 0 then ""
| (\$len | tostring) + \$head[0:1] + (.[\$len:] | look_and_say)
end ;

# look and say n times
def look_and_say(n):
if n == 0 then empty
else look_and_say as \$lns
| \$lns, (\$lns|look_and_say(n-1))
end ;

Example

1 | look_and_say(10)
Output:
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

## Julia

# v0.6

function lookandsay(n::String)
rst = IOBuffer()
i, c = 1, 1
while i ≤ length(n)
if i != length(n) && n[i] == n[i+1]
c += 1
else
print(rst, c, n[i])
c = 1
end
i += 1
end
return String(take!(rst))
end

function lookandsayseq(n::Integer)
rst = Array{String}(n)
rst[1] = "1"
for i in 2:n
rst[i] = lookandsay(rst[i-1])
end
return rst
end

println(lookandsayseq(10))
Output:
String["1", "11", "21", "1211", "111221", "312211", "13112221", "1113213211", "31131211131221", "13211311123113112211"]

## K

las: {x{0\$,//\$(#:'n),'*:'n:(&1,~=':x)_ x:0\$'\$x}\1}
las 8
1 11 21 1211 111221 312211 13112221 1113213211 31131211131221

## Kotlin

// version 1.0.6

fun lookAndSay(s: String): String {
val sb = StringBuilder()
var digit = s[0]
var count = 1
for (i in 1 until s.length) {
if (s[i] == digit)
count++
else {
sb.append("\$count\$digit")
digit = s[i]
count = 1
}
}
return sb.append("\$count\$digit").toString()
}

fun main(args: Array<String>) {
var las = "1"
for (i in 1..15) {
println(las)
las = lookAndSay(las)
}
}
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

## Lasso

The Look-and-say sequence is a recursive RLE, so the solution can leverage the same method as used for RLE.

define rle(str::string)::string => {
local(orig = #str->values->asCopy,newi=array, newc=array, compiled=string)
while(#orig->size) => {
if(not #newi->size) => {
#newi->insert(1)
#newc->insert(#orig->first)
#orig->remove(1)
else
if(#orig->first == #newc->last) => {
#newi->get(#newi->size) += 1
else
#newi->insert(1)
#newc->insert(#orig->first)
}
#orig->remove(1)
}
}
loop(#newi->size) => {
#compiled->append(#newi->get(loop_count)+#newc->get(loop_count))
}
return #compiled
}
define las(n::integer,run::integer) => {
local(str = #n->asString)
loop(#run) => { #str = rle(#str) }
return #str
}
loop(15) => {^ las(1,loop_count) + '\r' ^}
Output:
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221
132113213221133112132113311211131221121321131211132221123113112221131112311332111213211322211312113211

## LiveCode

This function takes a string and returns the next Look-And-Say iteration of it:

function lookAndSay S
put 0 into C
put char 1 of S into lastChar
repeat with i = 2 to length(S)
if char i of S is lastChar then next repeat
put C & lastChar after R
put 0 into C
put char i of S into lastChar
end repeat
return R & C + 1 & lastChar
end lookAndSay

on demoLookAndSay
put 1 into x
repeat 10
put x & cr after message
put lookAndSay(x) into x
end repeat
put x after message
end demoLookAndSay
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

## Logo

to look.and.say.loop :in :run :c :out
if empty? :in [output (word :out :run :c)]
if equal? first :in :c [output look.and.say.loop bf :in :run+1 :c :out]
output look.and.say.loop bf :in 1 first :in (word :out :run :c)
end
to look.and.say :in
if empty? :in [output :in]
output look.and.say.loop bf :in 1 first :in "||
end

show cascade 10 [print ? look.and.say ?] 1

## Lua

--returns an iterator over the first n copies of the look-and-say sequence
function lookandsayseq(n)
local t = {1}
return function()
local ret = {}
for i, v in ipairs(t) do
if t[i-1] and v == t[i-1] then
ret[#ret - 1] = ret[#ret - 1] + 1
else
ret[#ret + 1] = 1
ret[#ret + 1] = v
end
end
t = ret
n = n - 1
if n > 0 then return table.concat(ret) end
end
end
for i in lookandsayseq(10) do print(i) end

Alternative solution, using LPeg:

require "lpeg"
local P, C, Cf, Cc = lpeg.P, lpeg.C, lpeg.Cf, lpeg.Cc
lookandsay = Cf(Cc"" * C(P"1"^1 + P"2"^1 + P"3"^1)^1, function (a, b) return a .. #b .. string.sub(b,1,1) end)
t = "1"
for i = 1, 10 do
print(t)
t = lookandsay:match(t)
end

## M4

Using regular expressions:

Translation of: Perl
divert(-1)
define(`for',
`ifelse(\$#,0,``\$0'',
`ifelse(eval(\$2<=\$3),1,
`pushdef(`\$1',\$2)\$4`'popdef(`\$1')\$0(`\$1',incr(\$2),\$3,`\$4')')')')

define(`las',
`patsubst(`\$1',`\(\(.\)\2*\)',`len(\1)`'\2')')

define(`v',1)
divert
for(`x',1,10,
`v
define(`v',las(v))')dnl
v

## Mathematica

Custom Functions:

RunLengthEncode[x_List]:=(Through[{First,Length}[#]]&)/@Split[x]
LookAndSay[n_,d_:1]:=NestList[Flatten[Reverse/@RunLengthEncode[#]]&,{d},n-1]

If second argument is omitted the sequence is started with 1. Second argument is supposed to be a digits from 0 to 9. If however a larger number is supplied it will be seen as 1 number, not multiple digits. However if one wants to start with a 2 or more digit number, one could reverse the sequence to go back to a single digit start. First example will create the first 13 numbers of the sequence starting with 1, the next example starts with 7:

FromDigits /@ LookAndSay[13] // Column
FromDigits /@ LookAndSay[13, 7] // Column

gives back:

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221

7
17
1117
3117
132117
1113122117
311311222117
13211321322117
1113122113121113222117
31131122211311123113322117
132113213221133112132123222117
11131221131211132221232112111312111213322117
31131122211311123113321112131221123113111231121123222117

## Maxima

collect(a) := block(
[n: length(a), b: [ ], x: a[1], m: 1],
for i from 2 thru n do
(if a[i] = x then m: m + 1 else (b: endcons([x, m], b), x: a[i], m: 1)),
b: endcons([x, m], b)
)\$

look_and_say(s) := apply(sconcat, map(lambda([p], sconcat(string(p[2]), p[1])), collect(charlist(s))))\$

block([s: "1"], for i from 1 thru 10 do (disp(s), s: look_and_say(s)));
/* "1"
"11"
"21"
"1211"
"111221"
"312211"
"13112221"
"1113213211"
"31131211131221"
"13211311123113112211" */

## MAXScript

fn lookAndSay num =
(
local result = ""
num += " "
local current = num[1]
local numReps = 1

for digit in 2 to num.count do
(
if num[digit] != current then
(
result += (numReps as string) + current
numReps = 1
current = num[digit]
)
else
(
numReps += 1
)
)
result
)

local num = "1"

for i in 1 to 10 do
(
print num
num = lookAndSay num
)

## Metafont

vardef lookandsay(expr s) =
string r; r := "";
if string s:
i := 0;
forever: exitif not (i < length(s));
c := i+1;
forever: exitif ( (substring(c,c+1) of s) <> (substring(i,i+1) of s) );
c := c + 1;
endfor
r := r & decimal (c-i) & substring(i,i+1) of s;
i := c;
endfor
fi
r
enddef;

string p; p := "1";
for el := 1 upto 10:
message p;
p := lookandsay(p);
endfor

end

## Nim

proc NextInLookAndSaySequence (current: string): string =
assert(len(current) > 0)
Result = ""
var ch = current[0]
var count = 1
for i in countup(1, len(current)-1):
if current[i] != ch:
Result &= \$count & ch
ch = current[i]
count = 1
else:
count += 1
Result &= \$count & ch

proc LookAndSay (n = 10) =
var next = "1"
for i in countup(1, n):
next = NextInLookAndSaySequence(next)
echo next

LookAndSay()

## Objective-C

#import <Foundation/Foundation.h>

-(NSString*)lookAndSay:(NSString *)word{
if (!word) {
return nil;
}
NSMutableString *result = [NSMutableString new];

char repeat = [word characterAtIndex:0];
int times = 1;
word = [NSString stringWithFormat:@"%@ ",[word substringFromIndex:1] ];

for (NSInteger index = 0; index < word.length; index++) {
char actual = [word characterAtIndex:index];
if (actual != repeat) {
[result appendFormat:@"%d%c", times, repeat];
times = 1;
repeat = actual;
} else {
times ++;
}
}

return [result copy];
}

{
NSString *num = @"1";
for (int i=1;i<=10;i++) {
NSLog(@"%@", num);

num = [self lookAndSay:num];
}
}

## OCaml

### Functional

This function computes a see-and-say sequence from the previous one:

let rec seeAndSay = function
| [], nys -> List.rev nys
| x::xs, [] -> seeAndSay(xs, [x; 1])
| x::xs, y::n::nys when x=y -> seeAndSay(xs, y::1+n::nys)
| x::xs, nys -> seeAndSay(xs, x::1::nys)

It can be used like this:

> let gen n =
let xs = Array.create n [1] in
for i=1 to n-1 do
xs.(i) <- seeAndSay(xs.(i-1), [])
done;
xs;;
val gen : int -> int list array = <fun>

> gen 10;;
- : int list array =
[|[1]; [1; 1]; [2; 1]; [1; 2; 1; 1]; [1; 1; 1; 2; 2; 1]; [3; 1; 2; 2; 1; 1];
[1; 3; 1; 1; 2; 2; 2; 1]; [1; 1; 1; 3; 2; 1; 3; 2; 1; 1];
[3; 1; 1; 3; 1; 2; 1; 1; 1; 3; 1; 2; 2; 1];
[1; 3; 2; 1; 1; 3; 1; 1; 1; 2; 3; 1; 1; 3; 1; 1; 2; 2; 1; 1]|]

### With regular expressions in the Str library

let lookandsay =
Str.global_substitute (Str.regexp "\\(.\\)\\1*")
(fun s -> string_of_int (String.length (Str.matched_string s)) ^
Str.matched_group 1 s)

let () =
let num = ref "1" in
print_endline !num;
for i = 1 to 10 do
num := lookandsay !num;
print_endline !num;
done

### With regular expressions in the Pcre library

open Pcre

let lookandsay str =
let rex = regexp "(.)\\1*" in
let subs = exec_all ~rex str in
let ar = Array.map (fun sub -> get_substring sub 0) subs in
let ar = Array.map (fun s -> String.length s, s.[0]) ar in
let ar = Array.map (fun (n,c) -> (string_of_int n) ^ (String.make 1 c)) ar in
let res = String.concat "" (Array.to_list ar) in
(res)

let () =
let num = ref(string_of_int 1) in
for i = 1 to 10 do
num := lookandsay !num;
print_endline !num;
done

run this example with 'ocaml -I +pcre pcre.cma script.ml'

### Imperative

(* see http://oeis.org/A005150 *)

let look_and_say s =
let n = String.length s
and buf = Buffer.create 0
and prev = ref s.[0]
and count = ref 0 in
let append () = Buffer.add_char buf (char_of_int (48 + !count));
String.iter (fun c ->
if c = !prev then incr count else
begin
append ();
prev := c;
count := 1
end
) s;
append ();
Buffer.contents buf;;

(* what about length of successive strings ? *)
let iter f a n =
let rec aux r n v = if n = 0
then List.rev(r::v)
else aux (f r) (n - 1) (r::v) in
aux a n [];;

let las = iter look_and_say "1";;

(* the first sixty terms *)

List.map (String.length) (las 59);;
(*
[1; 2; 2; 4; 6; 6; 8; 10; 14; 20; 26; 34; 46; 62; 78; 102; 134; 176; 226;
302; 408; 528; 678; 904; 1182; 1540; 2012; 2606; 3410; 4462; 5808; 7586;
9898; 12884; 16774; 21890; 28528; 37158; 48410; 63138; 82350; 107312;
139984; 182376; 237746; 310036; 403966; 526646; 686646; 894810; 1166642;
1520986; 1982710; 2584304; 3369156; 4391702; 5724486; 7462860; 9727930;
12680852]
*)

(* see http://oeis.org/A005341 *)

## Oforth

: lookAndSay(n)  [ 1 ] n #[ dup .cr group map([#size, #first]) ] times ;
Output:
for n = 10 :
[1]
[1, 1]
[2, 1]
[1, 2, 1, 1]
[1, 1, 1, 2, 2, 1]
[3, 1, 2, 2, 1, 1]
[1, 3, 1, 1, 2, 2, 2, 1]
[1, 1, 1, 3, 2, 1, 3, 2, 1, 1]
[3, 1, 1, 3, 1, 2, 1, 1, 1, 3, 1, 2, 2, 1]
[1, 3, 2, 1, 1, 3, 1, 1, 1, 2, 3, 1, 1, 3, 1, 1, 2, 2, 1, 1]

## Oz

declare
%% e.g. "21" -> "1211"
fun {LookAndSayString S}
for DigitGroup in {Group S} append:Add do
end
end

%% lazy sequence of integers starting with N
fun {LookAndSay N}
fun lazy {Loop S}
{String.toInt S}|{Loop {LookAndSayString S}}
end
in
{Loop {Int.toString N}}
end

fun {Group Xs}
case Xs of nil then nil
[] X|Xr then
Ys Zs
{List.takeDropWhile Xr fun {\$ W} W==X end ?Ys ?Zs}
in
(X|Ys) | {Group Zs}
end
end
in
{ForAll {List.take {LookAndSay 1} 10} Show}

## PARI/GP

step(n)={
my(v=eval(Vec(Str(n))),cur=v[1],ct=1,out="");
v=concat(v,99);
for(i=2,#v,
if(v[i]==cur,
ct++
,
out=Str(out,ct,cur);
cur=v[i];
ct=1
)
);
eval(out)
};
n=1;for(i=1,20,print(n);n=step(n))

## Pascal

Works with: Free_Pascal
Library: SysUtils
program LookAndSayDemo(input, output);

uses
SysUtils;

function LookAndSay (s: string): string;
var
item: char;
index: integer;
count: integer;
begin
LookAndSay := '';
item := s[1];
count := 1;
for index:= 2 to length(s) do
if item = s[index] then
inc(count)
else
begin
LookAndSay := LookAndSay + intTostr(count) + item;
item := s[index];
count := 1;
end;
LookAndSay := LookAndSay + intTostr(count) + item;
end;

var
number: string;

begin
number := '1';
while not eof(input) do
begin
write(number);
number := LookAndSay(number);
end;
end.
Output:
% ./LookAndSay

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211^C

Even faster imperative Version Improvement: setlength of result,no inttoStr and using pChar But the Code Alignment is very important.

Works with: Free_Pascal
Library: SysUtils

program LookAndSayDemo(input, output);
{\$IFDEF FPC}
{\$Mode Delphi} // using result
{\$optimization ON}
// i3-4330 3.5 Ghz
// {\$CodeAlign proc=16,loop=8} //2,6 secs
{\$CodeAlign proc=16,loop=1} //1,6 secs so much faster ???
{\$ENDIF}

uses
SysUtils;
const
cntChar : array[1..9] of char =
('1','2','3','4','5','6','7','8','9');

function LookAndSay2 (const s: string): string;
//using pChar for result
var
source,
destin : pChar;
len,
idxFrom,
idxTo : integer;
cnt: integer;

item: char;
begin
idxFrom := length(s);
source := @s[1];

len := round(length(s)* 1.306+10);
setlength(result,len);
destin := @result[1];
dec(destin);

idxto := 1;
item := source^;
inc(source);
cnt := 1;
for idxFrom := idxFrom downto 2 do
begin
if item <> source^ then
begin
destin[idxTo] := cntChar[cnt];
destin[idxTo+1]:= item;
item := source^;
cnt := 1;
inc(idxto,2);
end
else
inc(cnt);
inc(source);
end;
destin[idxTo] := cntChar[cnt];
destin[idxTo+1]:= item;
setlength(result,idxto+1);
end;

var
number: string;
l1,l2,
i : integer;
begin
number := '1';
writeln(number);
writeln(1:4,length(number):16,1/1:10:6);

For i := 2 to 70 do
begin
l1 := length(number);
number := LookAndSay2(number);
l2 := length(number);
IF i <10 then
writeln(number);
writeln(i:4,length(number):16,l2/l1:10:6);
end;
end.
Output:
1
1               1  1.000000
11
2               2  2.000000
21
3               2  1.000000
1211
4               4  2.000000
111221
5               6  1.500000
312211
6               6  1.000000
13112221
7               8  1.333333
1113213211
8              10  1.250000
31131211131221
9              14  1.400000
10              20  1.428571
11              26  1.300000
12              34  1.307692
13              46  1.352941
14              62  1.347826
15              78  1.258065
16             102  1.307692
........
67        81117366  1.303580
68       105745224  1.303608
69       137842560  1.303535
70       179691598  1.303600

real	0m1.639s
user	0m1.593s
sys	0m0.043s

## Perl

sub lookandsay {
my \$str = shift;
\$str =~ s/((.)\2*)/length(\$1) . \$2/ge;
return \$str;
}

my \$num = "1";
foreach (1..10) {
print "\$num\n";
\$num = lookandsay(\$num);
}

Using string as a cyclic buffer:

for (local \$_ = "1\n"; s/((.)\2*)//s;) {
print \$1;
\$_ .= (\$1 ne "\n" and length(\$1)).\$2
}

## Perl 6

Works with: rakudo version 2018.03

In Perl 6 it is natural to avoid explicit loops; rather we use the sequence operator to define a lazy infinite sequence. We'll print the first 15 values here.

.say for ('1', *.subst(/(.)\$0*/, { .chars ~ .[0] }, :g) ... *)[^15];
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

## Phix

function lookandsay(string s)
string res = ""
integer p = s[1], c = 1
for i=2 to length(s) do
if p=s[i] then
c += 1
else
res &= sprintf("%d%s",{c,p})
p = s[i]
c = 1
end if
end for
res &= sprintf("%d%s",{c,p})
return res
end function

string s = "1"
?s
for i=1 to 10 do
s = lookandsay(s)
?s
end for
Output:
"1"
"11"
"21"
"1211"
"111221"
"312211"
"13112221"
"1113213211"
"31131211131221"
"13211311123113112211"
"11131221133112132113212221"

## PHP

<?php

function lookAndSay(\$str) {

return preg_replace_callback('#(.)\1*#', function(\$matches) {

return strlen(\$matches[0]).\$matches[1];
}, \$str);
}

\$num = "1";

foreach(range(1,10) as \$i) {

echo \$num."<br/>";
\$num = lookAndSay(\$num);
}

?>

## PicoLisp

(de las (Lst)
(make
(while Lst
(let (N 1 C)
(while (= (setq C (pop 'Lst)) (car Lst))
(inc 'N) )
(link N C) ) ) ) )

Usage:

: (las (1))
-> (1 1)
: (las @)
-> (2 1)
: (las @)
-> (1 2 1 1)
: (las @)
-> (1 1 1 2 2 1)
: (las @)
-> (3 1 2 2 1 1)
: (las @)
-> (1 3 1 1 2 2 2 1)
: (las @)
-> (1 1 1 3 2 1 3 2 1 1)
: (las @)
-> (3 1 1 3 1 2 1 1 1 3 1 2 2 1)

## PowerBASIC

This uses the RLEncode function from the PowerBASIC Run-length encoding entry.

FUNCTION RLEncode (i AS STRING) AS STRING
DIM tmp1 AS STRING, tmp2 AS STRING, outP AS STRING
DIM Loop0 AS LONG, count AS LONG

FOR Loop0 = 1 TO LEN(i)
tmp1 = MID\$(i, Loop0, 1)
IF tmp1 <> tmp2 THEN
IF count > 1 THEN
outP = outP & TRIM\$(STR\$(count)) & tmp2
tmp2 = tmp1
count = 1
ELSEIF 0 = count THEN
tmp2 = tmp1
count = 1
ELSE
outP = outP & "1" & tmp2
tmp2 = tmp1
END IF
ELSE
INCR count
END IF
NEXT

outP = outP & TRIM\$(STR\$(count)) & tmp2
FUNCTION = outP
END FUNCTION

FUNCTION lookAndSay(BYVAL count AS LONG) AS STRING
DIM iii AS STRING, tmp AS STRING

IF count > 1 THEN
iii = lookAndSay(count - 1)
ELSEIF count < 2 THEN
iii = "1"
END IF

tmp = RLEncode(iii)
lookAndSay = tmp
END FUNCTION

FUNCTION PBMAIN () AS LONG
DIM v AS LONG
v = VAL(INPUTBOX\$("Enter a number."))
MSGBOX lookAndSay(v)
END FUNCTION

## PowerShell

function Get-LookAndSay (\$n = 1) {
\$re = [regex] '(.)\1*'
\$ret = ""
foreach (\$m in \$re.Matches(\$n)) {
\$ret += [string] \$m.Length + \$m.Value[0]
}
return \$ret
}

function Get-MultipleLookAndSay (\$n) {
if (\$n -eq 0) {
return @()
} else {
\$a = 1
\$a
for (\$i = 1; \$i -lt \$n; \$i++) {
\$a = Get-LookAndSay \$a
\$a
}
}
}
Output:
PS> Get-MultipleLookAndSay 8
1
11
21
1211
111221
312211
13112221
1113213211

## Prolog

Works with SWI-Prolog.

look_and_say(L) :-
maplist(write, L), nl,
encode(L, L1),
look_and_say(L1).

% This code is almost identical to the code of "run-length-encoding"
encode(In, Out) :-
packList(In, R1),
append(R1,Out).

% use of library clpfd allows packList(?In, ?Out) to works
% in both ways In --> Out and In <-- Out.

:- use_module(library(clpfd)).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% ?- packList([a,a,a,b,c,c,c,d,d,e], L).
% L = [[3,a],[1,b],[3,c],[2,d],[1,e]] .
% ?- packList(R, [[3,a],[1,b],[3,c],[2,d],[1,e]]).
% R = [a,a,a,b,c,c,c,d,d,e] .
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
packList([],[]).

packList([X],[[1,X]]) :- !.

packList([X|Rest],[XRun|Packed]):-
run(X,Rest, XRun,RRest),
packList(RRest,Packed).

run(Var,[],[1,Var],[]).

run(Var,[Var|LRest],[N1, Var],RRest):-
N #> 0,
N1 #= N + 1,
run(Var,LRest,[N, Var],RRest).

run(Var,[Other|RRest], [1,Var],[Other|RRest]):-
dif(Var,Other).

Output:
?- look_and_say([1]).
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
..........................

## Pure

using system;

// Remove the trailing "L" from the string representation of bigints.
__show__ x::bigint = init (str x);

say x = val \$ strcat \$ map (sprintf "%d%s") \$ look \$ chars \$ str x with
look [] = [];
look [email protected](x:_) = (#takewhile (==x) xs,x) : look (dropwhile (==x) xs);
end;

iteraten 5 say 1; // [1,11,21,1211,111221]

// This prints the entire sequence, press Ctrl-C to abort.
do (puts.str) (iterate say 1);

## PureBasic

If OpenConsole()
Define i, j, cnt, txt\$, curr\$, result\$
Print("Enter start sequence: "): txt\$=Input()
Print("How many repetitions: "): i=Val(Input())
;
PrintN(#CRLF\$+"Sequence:"+#CRLF\$+txt\$)
Repeat
j=1
result\$=""
Repeat
curr\$=Mid(txt\$,j,1)
cnt=0
Repeat
cnt+1
j+1
Until Mid(txt\$,j,1)<>curr\$
result\$+Str(cnt)+curr\$
Until j>Len(txt\$)
PrintN(result\$)
txt\$=result\$
i-1
Until i<=0
;
PrintN(#CRLF\$+"Press ENTER to exit."): Input()
CloseConsole()
EndIf
Output:
Enter start sequence: 1
How many repetitions: 7

Sequence:
1
11
21
1211
111221
312211
13112221
1113213211

## Python

Translation of: C sharp – C#
def lookandsay(number):
result = ""

repeat = number[0]
number = number[1:]+" "
times = 1

for actual in number:
if actual != repeat:
result += str(times)+repeat
times = 1
repeat = actual
else:
times += 1

return result

num = "1"

for i in range(10):
print num
num = lookandsay(num)

Functional

Works with: Python version 2.4+
>>> from itertools import groupby
>>> def lookandsay(number):
return ''.join( str(len(list(g))) + k
for k,g in groupby(number) )

>>> numberstring='1'
>>> for i in range(10):
print numberstring
numberstring = lookandsay(numberstring)
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

As a generator

>>> from itertools import groupby, islice
>>>
>>> def lookandsay(number='1'):
while True:
yield number
number = ''.join( str(len(list(g))) + k
for k,g in groupby(number) )

>>> print('\n'.join(islice(lookandsay(), 10)))
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

Using regular expressions

Translation of: Perl
import re

def lookandsay(str):
return re.sub(r'(.)\1*', lambda m: str(len(m.group(0))) + m.group(1), str)

num = "1"
for i in range(10):
print num
num = lookandsay(num)

## Q

las:{{raze string[count@'x],'@'[;0]x:where[differ x]_x}\[x;1#"1"]}
las 8
Output:
,"1"
"11"
"21"
"1211"
"111221"
"312211"
"13112221"
"1113213211"
"31131211131221"

## R

Returning the value as an integer limits how long the sequence can get, so the option for integer or character return values are provided.

look.and.say <- function(x, return.an.int=FALSE)
{
#convert number to character vector
xstr <- unlist(strsplit(as.character(x), ""))
#get run length encoding
rlex <- rle(xstr)
#form new string
odds <- as.character(rlex\$lengths)
evens <- rlex\$values
newstr <- as.vector(rbind(odds, evens))
#collapse to scalar
newstr <- paste(newstr, collapse="")
#convert to number, if desired
if(return.an.int) as.integer(newstr) else newstr
}

Example usage:

x <- 1
for(i in 1:10)
{
x <- look.and.say(x)
print(x)
}

## Racket

#lang racket

(define (encode str)
(regexp-replace* #px"(.)\\1*" str (lambda (m c) (~a (string-length m) c))))

(define (look-and-say-sequence n)
(reverse (for/fold ([r '("1")]) ([n n]) (cons (encode (car r)) r))))

(for-each displayln (look-and-say-sequence 10))

Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

## REXX

Programming note:   this version works with any string   (a null is assumed, which causes   1   to be used).

If a negative number is specified (the number of iterations to be used for the calculations), only the length of
the number (or character string) is shown.

### simple version

/*REXX program displays the sequence (and/or lengths) for the    look and say    series.*/
parse arg N ! . /*obtain optional arguments from the CL*/
if N=='' | N=="," then N=20 /*Not specified? Then use the deault. */
if !=='' | !=="," then !=1 /* " " " " " " */

do j=1 for abs(N) /*repeat a number of times to show NUMS*/
if j\==1 then != \$lookAndSay(!) /*invoke function to calculate next #. */
if N<0 then say 'length['j"]:" length(!) /*Also, display the sequence's length.*/
else say '['j"]:"  ! /*display the number to the terminal. */
end /*j*/
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
\$lookAndSay: procedure; parse arg x,,\$ /*obtain the (passed) argument {X}. */
fin = '0'x /*use unique character to end scanning.*/
x=x || fin /*append the FIN character to string.*/
do k=1 by 0 /*now, process the given sequence. */
y= substr(x, k, 1) /*pick off one character to examine. */
if y== fin then return \$ /*if we're at the end, then we're done.*/
_= verify(x, y, , k) - k /*see how many characters we have of Y.*/
\$= \$ || _ || y /*build the "look and say" sequence. */
k= k + _ /*now, point to the next character. */
end /*k*/
output   when using the default input values of:     10   20   30
[1]: 1
[2]: 11
[3]: 21
[4]: 1211
[5]: 111221
[6]: 312211
[7]: 13112221
[8]: 1113213211
[9]: 31131211131221
[10]: 13211311123113112211
[11]: 11131221133112132113212221
[12]: 3113112221232112111312211312113211
[13]: 1321132132111213122112311311222113111221131221
[14]: 11131221131211131231121113112221121321132132211331222113112211
[15]: 311311222113111231131112132112311321322112111312211312111322212311322113212221
[16]: 132113213221133112132113311211131221121321131211132221123113112221131112311332111213211322211312113211
[17]: 11131221131211132221232112111312212321123113112221121113122113111231133221121321132132211331121321231231121113122113322113111221131221
[18]: 31131122211311123113321112131221123113112211121312211213211321322112311311222113311213212322211211131221131211132221232112111312111213111213211231131122212322211331222113112211
[19]: 1321132132211331121321231231121113112221121321132122311211131122211211131221131211132221121321132132212321121113121112133221123113112221131112311332111213122112311311123112111331121113122112132113213211121332212311322113212221
[20]: 11131221131211132221232112111312111213111213211231132132211211131221131211221321123113213221123113112221131112311332211211131221131211132211121312211231131112311211232221121321132132211331121321231231121113112221121321133112132112312321123113112221121113122113121113123112112322111213211322211312113211
output   when using the input values of:     17   ggg
[1]: ggg
[2]: 3g
[3]: 131g
[4]: 1113111g
[5]: 3113311g
[6]: 132123211g
[7]: 11131211121312211g
[8]: 31131112311211131122211g
[9]: 132113311213211231132132211g
[10]: 11131221232112111312211213211312111322211g
[11]: 3113112211121312211231131122211211131221131112311332211g
[12]: 1321132122311211131122211213211321322112311311222113311213212322211g
[13]: 1113122113121122132112311321322112111312211312111322211213211321322123211211131211121332211g
[14]: 31131122211311122122111312211213211312111322211231131122211311123113322112111312211312111322111213122112311311123112112322211g
[15]: 132113213221133122112231131122211211131221131112311332211213211321322113311213212322211231131122211311123113223112111311222112132113311213211221121332211g
[16]: 11131221131211132221231122212213211321322112311311222113311213212322211211131221131211132221232112111312111213322112132113213221133112132113221321123113213221121113122123211211131221222112112322211g
[17]: 31131122211311123113321112132132112211131221131211132221121321132132212321121113121112133221123113112221131112311332111213122112311311123112112322211211131221131211132221232112111312211322111312211213211312111322211231131122111213122112311311221132211221121332211g
output   when using the input value of:     -60
length[1]: 1
length[2]: 2
length[3]: 2
length[4]: 4
length[5]: 6
length[6]: 6
length[7]: 8
length[8]: 10
length[9]: 14
length[10]: 20
length[11]: 26
length[12]: 34
length[13]: 46
length[14]: 62
length[15]: 78
length[16]: 102
length[17]: 134
length[18]: 176
length[19]: 226
length[20]: 302
length[21]: 408
length[22]: 528
length[23]: 678
length[24]: 904
length[25]: 1182
length[26]: 1540
length[27]: 2012
length[28]: 2606
length[29]: 3410
length[30]: 4462
length[31]: 5808
length[32]: 7586
length[33]: 9898
length[34]: 12884
length[35]: 16774
length[36]: 21890
length[37]: 28528
length[38]: 37158
length[39]: 48410
length[40]: 63138
length[41]: 82350
length[42]: 107312
length[43]: 139984
length[44]: 182376
length[45]: 237746
length[46]: 310036
length[47]: 403966
length[48]: 526646
length[49]: 686646
length[50]: 894810
length[51]: 1166642
length[52]: 1520986
length[53]: 1982710
length[54]: 2584304
length[55]: 3369156
length[56]: 4391702
length[57]: 5724486
length[58]: 7462860
length[59]: 9727930
length[60]: 12680852

### faster version

This version appends the generated parts of the sequence, and after it gets to a certain size (chunkSize),
it appends the sequence generated (so far) to the primary sequence, and starts with a null sequence.
This avoids appending a small character string to a growing larger and larger character string.

/*REXX program displays the sequence (and/or lengths) for the    look and say    series.*/
parse arg N ! . /*obtain optional arguments from the CL*/
if N=='' | N=="," then N=20 /*Not specified? Then use the default.*/
if !=='' | !=="," then !=1 /* " " " " " " */
/* [↑]  !: starting char for the seq.*/
do j=1 for abs(N) /*repeat a number of times to show NUMS*/
if j\==1 then != \$lookAndSay(!) /*invoke function to calculate next #. */
if N<0 then say 'length['j"]:" length(!) /*Also, display the sequence's length.*/
else say '['j"]:"  ! /*display the number to the terminal. */
end /*j*/
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
\$lookAndSay: procedure; parse arg x,,\$ ! /*obtain the (passed) argument {X}. */
chSize= 1000 /*define a sensible chunk size. */
fin = '0'x /*use unique character to end scanning.*/
x=x || fin /*append the FIN character to string.*/
do k=1 by 0 /*now, process the given sequence. */
y= substr(x, k, 1) /*pick off one character to examine. */
if y== fin then return \$ /*if we're at the end, then we're done.*/
_= verify(x, y, , k) - k /*see how many characters we have of Y.*/
\$= \$ || _ || y /*build the "look and say" sequence. */
k= k + _ /*now, point to the next character. */
if length(\$)<chSize then iterate /*Less than chunkSize? Then keep going*/
!= ! || \$ /*append \$ to the  ! string. */
\$= /*now, start \$ from scratch. */
chSize= chSize + 100 /*bump the chunkSize (length) counter.*/
end /*k*/
return ! || \$ /*return the ! string plus the \$ string*/
output   is identical to the 1st REXX version   (the simple version).

## Ring

number = "1"
for nr = 1 to 10
number = lookSay(number)
see number + nl
next

func lookSay n
i = 0 j = 0 c="" o=""
i = 1
while i <= len(n)
c = substr(n,i,1)
j = i + 1
while substr(n,j,1) = c
j += 1
end
o += string(j-i) + c
i = j
end
return o

## Ruby

The simplest one:

class String
def look_and_say
gsub(/(.)\1*/){|s| s.size.to_s + s[0]}
end
end

ss = '1'
12.times {puts ss; ss = ss.look_and_say}

Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
Translation of: Perl
def lookandsay(str)
str.gsub(/(.)\1*/) {\$&.length.to_s + \$1}
end

num = "1"
10.times do
puts num
num = lookandsay(num)
end
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

Using Enumerable#chunk

def lookandsay(str)
str.chars.chunk{|c| c}.map{|c,x| [x.size, c]}.join
end

puts num = "1"
9.times do
puts num = lookandsay(num)
end

The output is the same above.

Without regular expression:

module Enumerable
# >> [2,2,2,3,3,4,2,2,1].cluster
# => [[2, 2, 2], [3, 3], [4], [2, 2], [1]]
def cluster
cluster = []
each do |element|
if cluster.last && cluster.last.last == element
cluster.last << element
else
cluster << [element]
end
end
cluster
end
end

Using Array#cluster defined above:

def print_sequence(input_sequence, seq=10)
return unless seq > 0
puts input_sequence.join
result_array = input_sequence.cluster.map do |cluster|
[cluster.count, cluster.first]
end
print_sequence(result_array.flatten, seq-1)
end

print_sequence([1])

The output is the same above.

## Rust

fn next_sequence(in_seq: &[i8]) -> Vec<i8> {
assert!(!in_seq.is_empty());

let mut result = Vec::new();
let mut current_number = in_seq[0];
let mut current_runlength = 1;

for i in &in_seq[1..] {
if current_number == *i {
current_runlength += 1;
} else {
result.push(current_runlength);
result.push(current_number);
current_runlength = 1;
current_number = *i;
}
}
result.push(current_runlength);
result.push(current_number);
result
}

fn main() {
let mut seq = vec![1];

for i in 0..10 {
println!("Sequence {}: {:?}", i, seq);
seq = next_sequence(&seq);
}
}
Output:
Sequence 0: [1]
Sequence 1: [1, 1]
Sequence 2: [2, 1]
Sequence 3: [1, 2, 1, 1]
Sequence 4: [1, 1, 1, 2, 2, 1]
Sequence 5: [3, 1, 2, 2, 1, 1]
Sequence 6: [1, 3, 1, 1, 2, 2, 2, 1]
Sequence 7: [1, 1, 1, 3, 2, 1, 3, 2, 1, 1]
Sequence 8: [3, 1, 1, 3, 1, 2, 1, 1, 1, 3, 1, 2, 2, 1]
Sequence 9: [1, 3, 2, 1, 1, 3, 1, 1, 1, 2, 3, 1, 1, 3, 1, 1, 2, 2, 1, 1]

## Scala

### Recursive

import scala.annotation.tailrec

object LookAndSay extends App {

loop(10, "1")

@tailrec
private def loop(n: Int, num: String): Unit = {
println(num)
if (n <= 0) () else loop(n - 1, lookandsay(num))
}

private def lookandsay(number: String): String = {
val result = new StringBuilder

@tailrec
def loop(numberString: String, repeat: Char, times: Int): String =
if (numberString.isEmpty) result.toString()
else if (numberString.head != repeat) {
result.append(times).append(repeat)
} else loop(numberString.tail, numberString.head, times + 1)

loop(number.tail + " ", number.head, 1)
}

}
Output:
See it running in your browser by (JavaScript, non JVM) or by Scastie (JVM).

### using Iterator

Library: Scala
def lookAndSay(seed: BigInt) = {
val s = seed.toString
( 1 until s.size).foldLeft((1, s(0), new StringBuilder)) {
case ((len, c, sb), index) if c != s(index) => sb.append(len); sb.append(c); (1, s(index), sb)
case ((len, c, sb), _) => (len + 1, c, sb)
} match {
case (len, c, sb) => sb.append(len); sb.append(c); BigInt(sb.toString)
}
}

def lookAndSayIterator(seed: BigInt) = Iterator.iterate(seed)(lookAndSay)

### using Stream

object Main extends App {

def lookAndSay(previous: List[BigInt]): Stream[List[BigInt]] = {

def next(num: List[BigInt]): List[BigInt] = num match {
case Nil => Nil
val size = (num takeWhile (_ == head)).size
}
val x = next(previous)
x #:: lookAndSay(x)
}

(lookAndSay(1 :: Nil) take 10).foreach(s => println(s.mkString("")))
}

## Seed7

\$ include "seed7_05.s7i";

const func string: lookAndSay (in integer: level, in string: stri) is func
result
var string: lookAndSay is "";
local
var integer: index is 2;
begin
if level = 1 then
if stri <> "" then
while index <= length(stri) and stri[index] = stri[1] do
incr(index);
end while;
lookAndSay := str(pred(index)) & stri[1 len 1] & lookAndSay(level, stri[index ..]);
end if;
else
lookAndSay := lookAndSay(1, lookAndSay(pred(level), stri));
end if;
end func;

const proc: main is func
local
var integer: level is 0;
begin
for level range 1 to 14 do
writeln(lookAndSay(level, "1"));
end for;
end func;
Output:
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

## Sidef

Translation of: Perl
func lookandsay(str) {
str.gsub(/((.)\2*)/, {|a,b| a.len.to_s + b });
}

var num = "1";
{
say num;
num = lookandsay(num);
} * 10;
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

## Smalltalk

Works with: GNU Smalltalk
String extend [
lookAndSay [ |anElement nextElement counter coll newColl|
coll := (self asOrderedCollection).
newColl := OrderedCollection new.
counter := 0.
anElement := (coll first).
[ coll size > 0 ]
whileTrue: [
nextElement := coll removeFirst.
( anElement == nextElement ) ifTrue: [
counter := counter + 1.
] ifFalse: [
anElement := nextElement.
counter := 1.
]
].
^(newColl join)
]
].

|r|
r := '1'.
10 timesRepeat: [
r displayNl.
r := r lookAndSay.
]

## SNOBOL4

Works with: Macro Spitbol
Works with: Snobol4+
Works with: CSnobol

The look-and-say sequence is an iterative run-length string encoding. So looksay( ) is just a wrapper around the Run-length Encoding task. This is by far the easiest solution.

*       # Encode RLE
define('rle(str)c,n') :(rle_end)
rle str len(1) . c :f(return)
str span(c) @n =
rle = rle n c :(rle)
rle_end

* # First m members of sequence with seed n
define('looksay(n,m)') :(looksay_end)
looksay output = n; m = gt(m,1) m - 1 :f(return)
n = rle(n) :(looksay)
looksay_end

* Test and display
looksay(1,10)
end
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

## SQL

DROP VIEW delta;
CREATE VIEW delta AS
SELECT sequence1.v AS x,
(sequence1.v<>sequence2.v)*sequence1.c AS v,
sequence1.c AS c
FROM SEQUENCE AS sequence1,
SEQUENCE AS sequence2
WHERE sequence1.c = sequence2.c+1;

DROP VIEW rle0;
CREATE VIEW rle0 AS
SELECT delta2.x AS x,
SUM(delta2.v) AS v,
delta2.c AS c
FROM delta AS delta1,
delta AS delta2
WHERE delta1.c >= delta2.c
GROUP BY delta1.c;

DROP VIEW rle1;
CREATE VIEW rle1 AS
SELECT SUM(x)/x AS a,
x AS b,
c AS c
FROM rle0
GROUP BY v;

DROP VIEW rle2;
CREATE VIEW rle2 AS
SELECT a AS v, 1 AS o, 2*c+0 AS c FROM rle1 UNION
SELECT b AS v, 1 AS o, 2*c+1 AS c FROM rle1;

DROP VIEW normed;
CREATE VIEW normed AS
SELECT r1.v AS v, SUM(r2.o) AS c
FROM rle2 AS r1,
rle2 AS r2
WHERE r1.c >= r2.c
GROUP BY r1.c;

DROP TABLE rle;
CREATE TABLE rle(v INT, c INT);
INSERT INTO rle SELECT * FROM normed ORDER BY c;

DELETE FROM SEQUENCE;
INSERT INTO SEQUENCE VALUES(-1,0);
INSERT INTO SEQUENCE SELECT * FROM rle;

Usage:

% sqlite3
SQLite version 3.4.0
Enter ".help" for instructions
sqlite> CREATE TABLE sequence(v int, c int);
sqlite> INSERT INTO sequence VALUES(-1,0);
sqlite> INSERT INTO sequence VALUES(1,1);
sqlite> SELECT * FROM sequence;
-1|0
1|1
sqlite> SELECT * FROM sequence;
-1|0
1|1
1|2
sqlite> SELECT * FROM sequence;
-1|0
2|1
1|2
sqlite> SELECT * FROM sequence;
-1|0
1|1
2|2
1|3
1|4
sqlite> SELECT * FROM sequence;
-1|0
1|1
1|2
1|3
2|4
2|5
1|6

## SQL PL

Works with: Db2 LUW
version 9.7 or higher.

With SQL PL:

SET SERVEROUTPUT ON @

BEGIN
DECLARE NMBR VARCHAR(100) DEFAULT '1';
DECLARE J SMALLINT DEFAULT 1;

CALL DBMS_OUTPUT.PUT_LINE(NMBR);
WHILE (J < 10) DO
BEGIN
DECLARE I SMALLINT;
DECLARE SIZE SMALLINT;
DECLARE ACTUAL CHAR(1);
DECLARE REPEAT CHAR(1);
DECLARE RESULT VARCHAR(100);
DECLARE TIMES SMALLINT;

SET REPEAT = SUBSTR(NMBR, 1, 1);
SET NMBR = SUBSTR(NMBR, 2) || ' ';
SET TIMES = 1;
SET I = 1;
SET SIZE = LENGTH(NMBR);

WHILE (I <= SIZE) DO
SET ACTUAL = SUBSTR(NMBR, I, 1);
IF (ACTUAL <> REPEAT) THEN
SET RESULT = COALESCE(RESULT, '') || TIMES || '' || REPEAT;
SET TIMES = 1;
SET REPEAT = ACTUAL;
ELSE
SET TIMES = TIMES + 1;
END IF;
SET I = I + 1;
END WHILE;

CALL DBMS_OUTPUT.PUT_LINE(RESULT);
SET NMBR = RESULT;
END ;
SET J = J + 1;
END WHILE;
END @

Output:

db2 => BEGIN
...
db2 (cont.) => END @
DB20000I  The SQL command completed successfully.

1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

## Swift

Translation of: Rust
func lookAndSay(_ seq: [Int]) -> [Int] {
var result = [Int]()
var cur = seq[0]
var curRunLength = 1

for i in seq.dropFirst() {
if cur == i {
curRunLength += 1
} else {
result.append(curRunLength)
result.append(cur)
curRunLength = 1
cur = i
}
}

result.append(curRunLength)
result.append(cur)

return result
}

var seq = [1]

for i in 0..<10 {
print("Seq \(i): \(seq)")
seq = lookAndSay(seq)
}
Output:
Seq 0: [1]
Seq 1: [1, 1]
Seq 2: [2, 1]
Seq 3: [1, 2, 1, 1]
Seq 4: [1, 1, 1, 2, 2, 1]
Seq 5: [3, 1, 2, 2, 1, 1]
Seq 6: [1, 3, 1, 1, 2, 2, 2, 1]
Seq 7: [1, 1, 1, 3, 2, 1, 3, 2, 1, 1]
Seq 8: [3, 1, 1, 3, 1, 2, 1, 1, 1, 3, 1, 2, 2, 1]
Seq 9: [1, 3, 2, 1, 1, 3, 1, 1, 1, 2, 3, 1, 1, 3, 1, 1, 2, 2, 1, 1]

## Tcl

proc lookandsay n {
set new ""
while {[string length \$n] > 0} {
set char [string index \$n 0]
for {set count 1} {[string index \$n \$count] eq \$char} {incr count} {}
append new \$count \$char
set n [string range \$n \$count end]
}
interp alias {} next_lookandsay {} lookandsay \$new
return \$new
}

puts 1 ;# ==> 1
puts [lookandsay 1] ;# ==> 11
puts [next_lookandsay] ;# ==> 21
puts [next_lookandsay] ;# ==> 1211
puts [next_lookandsay] ;# ==> 111221
puts [next_lookandsay] ;# ==> 312211

Alternatively, with coroutines:

Works with: Tcl version 8.6
proc seq_lookandsay {n {coroName next_lookandsay}} {
coroutine \$coroName apply {n {
for {} {[yield \$n] ne "stop"} {set n \$new} {
set new ""
foreach subseq [regexp -all -inline {0+|1+|2+|3+|4+|5+|6+|7+|8+|9+} \$n] {
append new [string length \$subseq] [string index \$subseq 0]
}
}
}} \$n
}

puts [seq_lookandsay 1]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
puts [next_lookandsay]
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

## TUSCRIPT

\$\$ MODE TUSCRIPT,{}
num=1,say=""
LOOP look
digits=STRINGS (num," ? ")
digitgrouped=ACCUMULATE (digits,howmany)
LOOP/CLEAR h=howmany,digit=digitgrouped
say=JOIN (say,"",h,digit)
ENDLOOP
PRINT say
num=VALUE(say),say=""
IF (look==14) EXIT
ENDLOOP

Output:
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

## UNIX Shell

Works with: bash
lookandsay() {
local num=\$1 char seq i
for ((i=0; i<=\${#num}; i++)); do
char=\${num:i:1}
if [[ \$char == \${seq:0:1} ]]; then
seq+=\$char
else
[[ -n \$seq ]] && printf "%d%s" \${#seq} \${seq:0:1}
seq=\$char
fi
done
}

for ((num=1, i=1; i<=10; i++)); do
echo \$num
num=\$( lookandsay \$num )
done
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

## Ursala

The look_and_say function returns the first n results by iterating the function that maps a given sequence to its successor.

#import std
#import nat

look_and_say "n" = ~&H\'1' next"n" rlc~&E; *= ^lhPrT\~&hNC %nP+ length

#show+

main = look_and_say 10
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211

## VBA

Public Sub LookAndSay(Optional Niter As Integer = 10)
'generate "Niter" members of the look-and-say sequence
'(argument is optional; default is 10)

Dim s As String 'look-and-say number
Dim news As String 'next number in sequence
Dim curdigit As String 'current digit in s
Dim newdigit As String 'next digit in s
Dim curlength As Integer 'length of current run
Dim p As Integer 'position in s
Dim L As Integer 'length of s

On Error GoTo Oops 'to catch overflow, i.e. number too long

s = "1"
For i = 1 To Niter
'initialise
L = Len(s)
p = 1
curdigit = Left\$(s, 1)
curlength = 1
news = ""
For p = 2 To L
'check next digit in s
newdigit = Mid\$(s, p, 1)
If curdigit = newdigit Then 'extend current run
curlength = curlength + 1
Else ' "output" run and start new run
news = news & CStr(curlength) & curdigit
curdigit = newdigit
curlength = 1
End If
Next p
' "output" last run
news = news & CStr(curlength) & curdigit
Debug.Print news
s = news
Next i
Exit Sub

Oops:
Debug.Print
If Err.Number = 6 Then 'overflow
Debug.Print "Oops - number too long!"
Else
Debug.Print "Error: "; Err.Number, Err.Description
End If
End Sub

Output:
LookAndSay 7
11
21
1211
111221
312211
13112221
1113213211

(Note: overflow occurs at 38th iteration!)

## Vedit macro language

This implementation generates look-and-say sequence starting from the sequence on cursor line in edit buffer. Each new sequence is inserted as a new line. 10 sequences are created in this example.

Repeat(10) {
BOL
Reg_Empty(20)
While (!At_EOL) {
Num_Str(Chars_Matched, 20, LEFT+APPEND)
Reg_Copy_Block(20, CP-1, CP, APPEND)
}
Ins_Newline Reg_Ins(20)
}
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221

## VBScript

##### Implementation
function looksay( n )
dim i
dim accum
dim res
dim c
res = vbnullstring
do
if n = vbnullstring then exit do
accum = 0
c = left( n,1 )
do while left( n, 1 ) = c
accum = accum + 1
n = mid(n,2)
loop
if accum > 0 then
res = res & accum & c
end if
loop
looksay = res
end function
##### Invocation
dim m
m = 1
for i = 0 to 13
m = looksay(m)
wscript.echo m
next
Output:
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

## Yorick

func looksay(input) {
// Special case: one digit
if(strlen(input) == 1)
return "1" + input;
// Convert string into an array of digits
digits = strchar(input)(:-1);
// Find indices where each run starts
w = where(digits(dif));
start = numberof(w) ? grow(1, w+1) : [1];
// Find length of each run
len = grow(start, numberof(digits)+1)(dif);
// Find digits for each run
run = digits(start);
// Construct output array
result = array(string, numberof(start)*2);
// Fill in lengths
result(1::2) = swrite(format="%d", len);
// Fill in digits; first must add trailing nulls to coerce single string
// into an array of strings.
run = transpose([run, array(char(0), numberof(run))])(*);
result(2::2) = strchar(run);
// Merge string array into single string
return result(sum);
}

val = "1";
do {
write, val;
val = looksay(val);
} while(strlen(val) < 80);
Output:
1
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221
3113112221232112111312211312113211
1321132132111213122112311311222113111221131221
11131221131211131231121113112221121321132132211331222113112211
311311222113111231131112132112311321322112111312211312111322212311322113212221

## zkl

Treating the task as a string manipulation problem.

Translation of: Scala
fcn lookAndSay(seed){ // numeric String --> numeric String
len,c:=[1..seed.len()-1].reduce(fcn([(len,c)]lc,index,s,sb){
if(c!=s[index]) { sb.write(len); sb.write(c); lc.clear(1,s[index]) }
else lc.clear(len+1,c);
},L(1,seed[0]), seed,sb:=Sink(String));
sb.write(len); sb.write(c);
sb.close();
}
Output:
(0).reduce(10,fcn(seed,_){ lookAndSay(seed).println() },"1");
11
21
1211
111221
312211
13112221
1113213211
31131211131221
13211311123113112211
11131221133112132113212221