Ulam numbers
From Rosetta Code
Ulam numbers is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.
With the following initial conditions:
u1 = 1
u2 = 2
We define the nth Ulam number un as the smallest number that is greater than un-1 and
is the unique sum of two different Ulam numbers ui (<n) and uj (<n).
- Task
Write a function to generate the n-th Ulam number.
- References
Contents
11l[edit]
F ulam(n)
I n <= 2
R n
V mx = 1352000
V lst = [1, 2] [+] [0] * mx
V sums = [0] * (mx * 2 + 1)
sums[3] = 1
V size = 2
Int query
L size < n
query = lst[size - 1] + 1
L
I sums[query] == 1
L(i) 0 .< size
V sum = query + lst[i]
V t = sums[sum] + 1
I t <= 2
sums[sum] = t
(lst[size], size) = (query, size + 1)
L.break
query++
R query
L(p) 5
V n = 10 ^ p
print(‘The #.#. Ulam number is #.’.format(n, I n != 1 {‘th’} E ‘st’, ulam(n)))
- Output:
The 1st Ulam number is 1 The 10th Ulam number is 18 The 100th Ulam number is 690 The 1000th Ulam number is 12294 The 10000th Ulam number is 132788
AWK[edit]
# syntax: GAWK -f ULAM_NUMBERS.AWK
BEGIN {
u = split("1,2",ulam,",")
for (n=3; ; n++) {
count = 0
for (x=1; x<=u-1; x++) {
for (y=x+1; y<=u; y++) {
if (ulam[x] + ulam[y] == n) {
count++
}
}
}
if (count == 1) {
ulam[++u] = n
if (u ~ /^(10|50|100|500|1000)$/) {
printf("%6d %6d\n",u,n)
if (++shown >= 5) { break }
}
}
}
exit(0)
}
- Output:
10 18
50 253
100 690
500 5685
1000 12294
C[edit]
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
void fatal(const char* message) {
fprintf(stderr, "%s\n", message);
exit(1);
}
void* xmalloc(size_t n) {
void* ptr = malloc(n);
if (ptr == NULL)
fatal("Out of memory");
return ptr;
}
void* xrealloc(void* p, size_t n) {
void* ptr = realloc(p, n);
if (ptr == NULL)
fatal("Out of memory");
return ptr;
}
int* extend(int* array, int min_length, int* capacity) {
int new_capacity = *capacity;
if (new_capacity >= min_length)
return array;
while (new_capacity < min_length)
new_capacity *= 2;
array = xrealloc(array, new_capacity * sizeof(int));
memset(array + *capacity, 0, (new_capacity - *capacity) * sizeof(int));
*capacity = new_capacity;
return array;
}
int ulam(int n) {
int* ulams = xmalloc((n < 2 ? 2 : n) * sizeof(int));
ulams[0] = 1;
ulams[1] = 2;
int sieve_length = 2;
int sieve_capacity = 2;
int* sieve = xmalloc(sieve_capacity * sizeof(int));
sieve[0] = sieve[1] = 1;
for (int u = 2, ulen = 2; ulen < n; ) {
sieve_length = u + ulams[ulen - 2];
sieve = extend(sieve, sieve_length, &sieve_capacity);
for (int i = 0; i < ulen - 1; ++i)
++sieve[u + ulams[i] - 1];
for (int i = u; i < sieve_length; ++i) {
if (sieve[i] == 1) {
u = i + 1;
ulams[ulen++] = u;
break;
}
}
}
int result = ulams[n - 1];
free(ulams);
free(sieve);
return result;
}
int main() {
clock_t start = clock();
for (int n = 1; n <= 100000; n *= 10)
printf("Ulam(%d) = %d\n", n, ulam(n));
clock_t finish = clock();
printf("Elapsed time: %.3f seconds\n", (finish - start + 0.0)/CLOCKS_PER_SEC);
return 0;
}
- Output:
Ulam(1) = 1 Ulam(10) = 18 Ulam(100) = 690 Ulam(1000) = 12294 Ulam(10000) = 132788 Ulam(100000) = 1351223 Elapsed time: 8.630 seconds
C++[edit]
#include <algorithm>
#include <chrono>
#include <iostream>
#include <vector>
int ulam(int n) {
std::vector<int> ulams{1, 2};
std::vector<int> sieve{1, 1};
for (int u = 2; ulams.size() < n; ) {
sieve.resize(u + ulams[ulams.size() - 2], 0);
for (int i = 0; i < ulams.size() - 1; ++i)
++sieve[u + ulams[i] - 1];
auto it = std::find(sieve.begin() + u, sieve.end(), 1);
if (it == sieve.end())
return -1;
u = (it - sieve.begin()) + 1;
ulams.push_back(u);
}
return ulams[n - 1];
}
int main() {
auto start = std::chrono::high_resolution_clock::now();
for (int n = 1; n <= 100000; n *= 10)
std::cout << "Ulam(" << n << ") = " << ulam(n) << '\n';
auto end = std::chrono::high_resolution_clock::now();
std::chrono::duration<double> duration(end - start);
std::cout << "Elapsed time: " << duration.count() << " seconds\n";
}
- Output:
Ulam(1) = 1 Ulam(10) = 18 Ulam(100) = 690 Ulam(1000) = 12294 Ulam(10000) = 132788 Ulam(100000) = 1351223 Elapsed time: 9.09242 seconds
FreeBASIC[edit]
redim as uinteger ulam(1 to 2)
ulam(1) = 1 : ulam(2) = 2
function get_ulam( n as uinteger, ulam() as uinteger ) as uinteger
dim as uinteger nu = ubound(ulam), c, r, s, t, i, usr
if n <= nu then return ulam(n) 'if we have already calculated this one, just return it
'otherwise, calculate it and all intermediate terms
redim preserve ulam(1 to n)
for t = nu+1 to n
i = ulam(t-1)
while true
i += 1 : c = 0
for r = 1 to t-2
for s = r+1 to t-1
usr = ulam(s)+ulam(r)
if usr > i then exit for 'efficiency
if usr = i then
if c = 1 then continue while
c += 1
end if
next s
next r
if c = 0 then continue while 'I'm not 100% sure this is even possible...
ulam(t) = i
exit while
wend
next t
return ulam(n)
end function
for i as uinteger = 1 to 4
print 10^i, get_ulam(10^i, ulam())
next i
- Output:
10 18 100 690 1000 12294 10000 132788
Go[edit]
Version 1[edit]
package main
import "fmt"
func ulam(n int) int {
ulams := []int{1, 2}
set := map[int]bool{1: true, 2: true}
i := 3
for {
count := 0
for j := 0; j < len(ulams); j++ {
_, ok := set[i-ulams[j]]
if ok && ulams[j] != (i-ulams[j]) {
count++
if count > 2 {
break
}
}
}
if count == 2 {
ulams = append(ulams, i)
set[i] = true
if len(ulams) == n {
break
}
}
i++
}
return ulams[n-1]
}
func main() {
for n := 10; n <= 10000; n *= 10 {
fmt.Println("The", n, "\bth Ulam number is", ulam(n))
}
}
- Output:
The 10th Ulam number is 18 The 100th Ulam number is 690 The 1000th Ulam number is 12294 The 10000th Ulam number is 132788
Version 2[edit]
The above version is reasonably efficient and runs in about 2.9 seconds on my machine (Intel Core i7-8565U). The following version, which builds up a sieve as it goes along, is (astonishingly) about 40 times faster!
Although not shown here, the 100,000th Ulam number (1,351,223) is computed in about 13.5 seconds.
package main
import (
"fmt"
"time"
)
func ulam(n int) int {
ulams := []int{1, 2}
sieve := []int{1, 1}
u := 2
for len(ulams) < n {
s := u + ulams[len(ulams)-2]
t := s - len(sieve)
for i := 0; i < t; i++ {
sieve = append(sieve, 0)
}
for i := 1; i <= len(ulams)-1; i++ {
v := u + ulams[i-1] - 1
sieve[v]++
}
index := -1
for i, e := range sieve[u:] {
if e == 1 {
index = u + i
break
}
}
u = index + 1
ulams = append(ulams, u)
}
return ulams[n-1]
}
func commatize(n int) string {
s := fmt.Sprintf("%d", n)
if n < 0 {
s = s[1:]
}
le := len(s)
for i := le - 3; i >= 1; i -= 3 {
s = s[0:i] + "," + s[i:]
}
if n >= 0 {
return s
}
return "-" + s
}
func main() {
start := time.Now()
for n := 1; n <= 10000; n *= 10 {
s := "th"
if n == 1 {
s = "st"
}
fmt.Println("The", commatize(n), "\b"+s+" Ulam number is", commatize(ulam(n)))
}
fmt.Println("\nTook", time.Since(start))
}
- Output:
The 1st Ulam number is 1 The 10th Ulam number is 18 The 100th Ulam number is 690 The 1,000th Ulam number is 12,294 The 10,000th Ulam number is 132,788 Took 74.45373ms
Version 3[edit]
This version is even quicker than Version 2 and reduces the time needed to calculate the 10,000th and 100,000th Ulam numbers to about 40 milliseconds and 3.25 seconds respectively.
As mentioned in the Wren version 3 example, you need to know how much memory to allocate in advance.
package main
import (
"fmt"
"time"
)
func ulam(n int) int {
if n <= 2 {
return n
}
const MAX = 1_352_000
list := make([]int, MAX+1)
list[0], list[1] = 1, 2
sums := make([]byte, 2*MAX+1)
sums[3] = 1
size := 2
var query int
for {
query = list[size-1] + 1
for {
if sums[query] == 1 {
for i := 0; i < size; i++ {
sum := query + list[i]
t := sums[sum] + 1
if t <= 2 {
sums[sum] = t
}
}
list[size] = query
size++
break
}
query++
}
if size >= n {
break
}
}
return query
}
func commatize(n int) string {
s := fmt.Sprintf("%d", n)
if n < 0 {
s = s[1:]
}
le := len(s)
for i := le - 3; i >= 1; i -= 3 {
s = s[0:i] + "," + s[i:]
}
if n >= 0 {
return s
}
return "-" + s
}
func main() {
start := time.Now()
for n := 10; n <= 100000; n *= 10 {
fmt.Println("The", commatize(n), "\bth Ulam number is", commatize(ulam(n)))
}
fmt.Println("\nTook", time.Since(start))
}
- Output:
The 10th Ulam number is 18 The 100th Ulam number is 690 The 1,000th Ulam number is 12,294 The 10,000th Ulam number is 132,788 The 100,000th Ulam number is 1,351,223 Took 3.226255944s
Haskell[edit]
Lazy List[edit]
import Data.List
ulam
:: Integral i =>
Int -> i
ulam 1 = 1
ulam 2 = 2
ulam n
| n > 2 = ulams !! (n-1)
ulams
:: Integral n =>
[n]
ulams = 1:2:(nexts [2,1])
nexts us = u: (nexts (u:us))
where
n = length us
[u] = head . filter isSingleton . group . sort $
[v | i <- [0 .. n-2], j <- [i+1 .. n-1]
, let s = us !! i
, let t = us !! j
, let v = s+t
, v > head us
]
isSingleton :: [a] -> Bool
isSingleton as
| length as == 1 = True
| otherwise = False
Java[edit]
public class UlamNumbers {
public static void main(String[] args) {
long start = System.currentTimeMillis();
for (int n = 1; n <= 100000; n *= 10) {
System.out.printf("Ulam(%d) = %d\n", n, ulam(n));
}
long finish = System.currentTimeMillis();
System.out.printf("Elapsed time: %.3f seconds\n", (finish - start)/1000.0);
}
private static int ulam(int n) {
int[] ulams = new int[Math.max(n, 2)];
ulams[0] = 1;
ulams[1] = 2;
int sieveLength = 2;
int[] sieve = new int[sieveLength];
sieve[0] = sieve[1] = 1;
for (int u = 2, ulen = 2; ulen < n; ) {
sieveLength = u + ulams[ulen - 2];
sieve = extend(sieve, sieveLength);
for (int i = 0; i < ulen - 1; ++i)
++sieve[u + ulams[i] - 1];
for (int i = u; i < sieveLength; ++i) {
if (sieve[i] == 1) {
u = i + 1;
ulams[ulen++] = u;
break;
}
}
}
return ulams[n - 1];
}
private static int[] extend(int[] array, int minLength) {
if (minLength <= array.length)
return array;
int newLength = 2 * array.length;
while (newLength < minLength)
newLength *= 2;
int[] newArray = new int[newLength];
System.arraycopy(array, 0, newArray, 0, array.length);
return newArray;
}
}
- Output:
Ulam(1) = 1 Ulam(10) = 18 Ulam(100) = 690 Ulam(1000) = 12294 Ulam(10000) = 132788 Ulam(100000) = 1351223 Elapsed time: 9.098 seconds
Julia[edit]
function nthUlam(n)
ulams = [1, 2]
memoized = Set([1, 2])
i = 3
while true
count = 0
for j in 1:length(ulams)
if i - ulams[j] in memoized && ulams[j] != i - ulams[j]
(count += 1) > 2 && break
end
end
if count == 2
push!(ulams, i)
push!(memoized, i)
length(ulams) == n && break
end
i += 1
end
return ulams[n]
end
nthUlam(5)
for n in [10, 100, 1000, 10000]
@time println("The ", n, "th Ulam number is: ", nthUlam(n))
end
- Output:
The 10th Ulam number is: 18 0.000657 seconds (27 allocations: 1.422 KiB) The 100th Ulam number is: 690 0.000959 seconds (39 allocations: 7.094 KiB) The 1000th Ulam number is: 12294 0.027564 seconds (52 allocations: 72.188 KiB) The 10000th Ulam number is: 132788 3.076024 seconds (63 allocations: 473.125 KiB)
Lua[edit]
Implemented from scratch, but algorithmically equivalent to other solutions where a running count of number-of-ways-to-reach-sum is maintained in order to sieve candidate values.
function ulam(n)
local ulams, nways, i = { 1,2 }, { 0,0,1 }, 3
repeat
if nways[i] == 1 then
for j = 1, #ulams do
local sum = i + ulams[j]
nways[sum] = (nways[sum] or 0) + 1
end
ulams[#ulams+1] = i
end
i = i + 1
until #ulams == n
return ulams[#ulams]
end
for _,n in ipairs({10,100,1000,10000,100000}) do
local s, u, e = os.clock(), ulam(n), os.clock()
print(string.format("%dth is %d (%f seconds elapsed)", n, u, e-s))
end
- Output:
Times are Lua 5.4 on [email protected]
10th is 18 (0.000000 seconds elapsed) 100th is 690 (0.000000 seconds elapsed) 1000th is 12294 (0.020000 seconds elapsed) 10000th is 132788 (1.724000 seconds elapsed) 100000th is 1351223 (277.824000 seconds elapsed)
Pascal[edit]
like GO,PHIX who was firstprogram UlamNumbers;
{$IFDEF FPC}
{$MODE DELPHI}
{$Optimization On,All}
{$ENDIF}
uses
sysutils;
const
maxUlam = 100000;
Limit = 1351223+4000;
type
tCheck = Uint16;
tpCheck = pUint16;
var
Ulams : array of Uint32;
Check0 : array of tCheck;
Ulam_idx :NativeInt;
procedure init;
begin
setlength(Ulams,maxUlam);
Ulams[0] := 1;
Ulams[1] := 2;
Ulam_idx := 1;
setlength(check0,Limit);
check0[1]:=1;
check0[2]:=1;
end;
procedure OutData(idx,num:NativeInt);
Begin
writeln(' Ulam(',idx+1,')',#9#9,num:10);
end;
function findNext(i:nativeInt;pCh0:tpCheck):NativeInt;
begin
result := i;
repeat
if pCh0[result] = 1 then
break;
inc(result);
until false;
end;
procedure SumOne(idx:NativeUint;pCh:tpCheck;pUlams:pUint32);
//seperated speeds up a lot by reducing register pressure in main
Begin
For idx := idx downto 0 do
pCh[pUlams[idx]] +=1;
end;
var
pCh0,pCh : tpCheck;
pUlams : pUint32;
ul,idx,lmtidx :nativeInt;
Begin
Init;
lmtidx := 9;
pCh0:= @Check0[0];
pUlams := @Ulams[0];
OutData(0,pUlams[0]);
ul := pUlams[Ulam_idx];
pCh:= @pCh0[ul];
repeat
SumOne(Ulam_idx-1,pCh,pUlams);
ul := findNext(ul+1,pCh0);
inc(Ulam_idx);
pUlams[Ulam_idx] := ul;
pCh:= @pCh0[ul];
IF ul>Limit DIV 2 then
break;
if Ulam_idx=lmtIdx then
Begin
OutData(Ulam_idx,ul);
lmtidx := lmtidx*10+9;
end;
until Ulam_idx >= maxUlam-1;
idx := Ulam_idx-1;
//now reducing then the highest used summing idx
while Ulam_idx< maxUlam-1 do
begin
while ul+pUlams[idx] > limit do
dec(idx);
SumOne(idx,pCh,pUlams);
ul := findNext(ul+1,pCh0);
inc(Ulam_idx);
pUlams[Ulam_idx] := ul;
pCh:= @pCh0[ul];
end;
OutData(Ulam_idx,ul);
setlength(check0,0);
setlength(Ulams,0);
end.
- Output:
Ulam(1) 1 Ulam(10) 18 Ulam(100) 690 Ulam(1000) 12294 Ulam(10000) 132788 Ulam(100000) 1351223 // real 0m4,731s AMD 2200G //TIO.RUN Free Pascal Compiler version 3.0.4 [2018/07/13] for x86_64 Copyright (c) 1993-2017 by Florian Klaempfl and others Target OS: Linux for x86-64 Compiling .code.tio.pp Linking .bin.tio /usr/bin/ld: warning: link.res contains output sections; did you forget -T? 92 lines compiled, 0.2 sec Real time: 3.025 s
Perl[edit]
use strict;
use warnings;
use feature <say state>;
sub ulam {
my($n) = @_;
state %u = (1 => 1, 2 => 1);
state @ulams = <0 1 2>; # 0 a placeholder to shift indexing up one
return $ulams[$n] if $ulams[$n];
$n++;
my $i = 3;
while () {
my $count = 0;
$u{ $i - $ulams[$_] }
and $ulams[$_] != $i - $ulams[$_]
and $count++ > 2
and last
for 0..$#ulams;
$count == 2
and push(@ulams,$i)
and $u{$i} = 1
and @ulams == $n
and last;
$i++;
}
$ulams[$n-1];
}
printf "The %dth Ulam number is: %d\n", 10**$_, ulam(10**$_) for 1..4;
- Output:
The 10th Ulam number is: 18 The 100th Ulam number is: 690 The 1000th Ulam number is: 12294 The 10000th Ulam number is: 132788 The 10000th Ulam number is: 132788
Phix[edit]
function ulam(integer n)
sequence ulams = {1, 2},
sieve = {1, 1}
integer u := 2
while length(ulams)<n do
integer s = u+ulams[$-1], t
sieve &= repeat(0,s-length(sieve))
for i=1 to length(ulams)-1 do
s = u+ulams[i]
t = sieve[s]+1
if t<=2 then
sieve[s] = t
end if
end for
u = find(1,sieve,u+1)
ulams &= u
end while
return ulams[n]
end function
atom t0 = time()
for p=0 to 4 do
integer n = power(10,p)
printf(1,"The %,d%s Ulam number is %,d\n",{n,ord(n),ulam(n)})
end for
?elapsed(time()-t0)
- Output:
The 1st Ulam number is 1 The 10th Ulam number is 18 The 100th Ulam number is 690 The 1,000th Ulam number is 12,294 The 10,000th Ulam number is 132,788 "1.0s"
For comparison, Julia took 4.5s (9.3s on repl.it), Go took 4.9s, Wren (on tio) 27s,
Ring timed out (>60s) on tio before getting the 1,000th number,
REXX (also on tio) got to the 1,000th number in 12.3s but timed out before getting the 10,000th,
Raku (on repl.it) 9mins 50s,
FreeBASIC 17mins 44s, and I cancelled XPL0 (on EXPL32) after 53 minutes.
The Haskell entry does not compile for me on either tio or repl.it
The above algorithm can also yield "The 100,000th Ulam number is 1,351,223" in 1 minute and 40s, for me. (I fully expect translations of this better algorithm to run even faster, btw)
Python[edit]
import time
def ulam(n):
if n <= 2:
return n
mx = 1352000
lst = [1, 2] + [0] * mx
sums = [0] * (mx * 2 + 1)
sums[3] = 1
size = 2
while size < n:
query = lst[size-1] + 1
while True:
if sums[query] == 1:
for i in range(size):
sum = query + lst[i]
t = sums[sum] + 1
if t <= 2:
sums[sum] = t
lst[size], size = query, size + 1
break
query += 1
return query
t0 = time.time()
for p in range(5):
n = 10**p
print(f"The {n}{'th' if n!=1 else 'st'} Ulam number is {ulam(n)}")
print("\nElapsed time:", time.time() - t0)
- Output:
The 1st Ulam number is 1 The 10th Ulam number is 18 The 100th Ulam number is 690 The 1_000th Ulam number is 12_294 The 10_000th Ulam number is 132_788 (Elapsed time: 11.470759391784668 seconds)
- Extra
In [1]: %time ulam(100_000) Wall time: 23min 58s Out[1]: 1351223 In [37]:
Raku[edit]
my @ulams = 1, 2, &next-ulam … *;
sub next-ulam {
state $i = 1;
state @sums = 0,1,1;
my $last = @ulams[$i];
(^$i).map: { @sums[@ulams[$_] + $last]++ };
++$i;
quietly ($last ^.. *).first: { @sums[$_] == 1 };
}
for 1 .. 4 {
say "The {10**$_}th Ulam number is: ", @ulams[10**$_ - 1]
}
- Output:
The 10th Ulam number is: 18 The 100th Ulam number is: 690 The 1000th Ulam number is: 12294 The 10000th Ulam number is: 132788
REXX[edit]
This REXX version has several speed improvements.
/*REXX program finds & displays the Nth Ulam number (or any number of specified values).*/
parse arg $ /*obtain optional argument from the CL.*/
if $='' | $="," then $= 10 100 1000 10000 /*Not specified? Then use the defaults.*/
do k=1 for words($) /*process each of the specified values.*/
x= Ulam( word($, k) ) /*invoke Ulam to find a Ulam number. */
say 'the ' commas(#)th(#) ' Ulam number is: ' commas(x)
end /*k*/
exit 0 /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
commas: parse arg _; do jc=length(_)-3 to 1 by -3; _= insert(',', _, jc); end; return _
th: parse arg th; return word('th st nd rd', 1 + (th//10)*(th//100%10\==1)*(th//10<4))
/*──────────────────────────────────────────────────────────────────────────────────────*/
Ulam: parse arg n; @.1= 1; @.2=2; #= 2 /*1st two terms; #: sequence length. */
!.= 0; !.1= 1; !.2=1 /*semaphore for each term in sequence. */
z= 3 /*value of next possible term in seq. */
do until #==n
cnt= 0
do j=1 for #; _= z - @.j /*_: short circuit value. */
if !._ then if @.j\==_ then do; cnt= cnt + 1
if cnt>2 then leave
end
end /*j*/
if cnt==2 then do; #= # + 1 /*bump the number of terms. */
@.#= z; !.z= 1 /*add Z to sequence; bool.*/
end
z= z + 1 /*bump next possible term. */
end /*until*/
return @.#
- output when using the default input of: 10 100 1000 10000
the 10th Ulam number is: 18 the 100th Ulam number is: 690 the 1,000th Ulam number is: 12,294 the 10,000th Ulam number is: 132,788
- output (courtesy of Paul Kislanko's PC) when using the input of: 100000
the 100,000th Ulam number is: 1,351,223
Ring[edit]
load "stdlib.ring"
limit = 12500
Ulam = []
add(Ulam,1)
add(Ulam,2)
for n = 3 to limit
flag = 0
count = 0
len = len(Ulam)
for x = 1 to len-1
for y = x+1 to len
if Ulam[x] + Ulam[y] = n
flag = 1
count = count + 1
ok
next
next
if flag = 1 and count = 1
add(Ulam,n)
ln = len(Ulam)
if ln = 10
see "The 10th Ulam number is: " + n + nl
ok
if ln = 100
see "The 100th Ulam number is: " + n + nl
ok
if ln = 1000
see "The 1000th Ulam number is: " + n + nl
ok
if ln = 10000
see "The 10000th Ulam number is: " + n + nl
ok
ok
next
Output:
The 10th Ulam number is: 18 The 100th Ulam number is: 690 The 1000th Ulam number is: 12294 The 10000th Ulam number is: 132788
Rust[edit]
fn ulam(n: usize) -> usize {
let mut ulams = vec![1, 2];
let mut sieve = vec![1, 1];
let mut u = 2;
while ulams.len() < n {
sieve.resize(u + ulams[ulams.len() - 2], 0);
for i in 0..ulams.len() - 1 {
sieve[u + ulams[i] - 1] += 1;
}
for i in u..sieve.len() {
if sieve[i] == 1 {
u = i + 1;
ulams.push(u);
break;
}
}
}
ulams[n - 1]
}
fn main() {
use std::time::Instant;
let start = Instant::now();
let mut n = 1;
while n <= 100000 {
println!("Ulam({}) = {}", n, ulam(n));
n *= 10;
}
println!("Elapsed time: {:.2?}", start.elapsed());
}
- Output:
Ulam(1) = 1 Ulam(10) = 18 Ulam(100) = 690 Ulam(1000) = 12294 Ulam(10000) = 132788 Ulam(100000) = 1351223 Elapsed time: 10.68s
Wren[edit]
Version 1[edit]
import "/set" for Set
var ulam = Fn.new() { |n|
var ulams = [1, 2]
var set = Set.new(ulams)
var i = 3
while (true) {
var count = 0
for (j in 0...ulams.count) {
if (set.contains(i - ulams[j]) && ulams[j] != (i - ulams[j])) {
count = count + 1
if (count > 2) break
}
}
if (count == 2) {
ulams.add(i)
set.add(i)
if (ulams.count == n) break
}
i = i + 1
}
return ulams[-1]
}
var n = 1
while (true) {
n = n * 10
System.print("The %(n)th Ulam number is %(ulam.call(n))")
if (n == 10000) break
}
- Output:
The 10th Ulam number is 18 The 100th Ulam number is 690 The 1000th Ulam number is 12294 The 10000th Ulam number is 132788
Version 2[edit]
The above version is reasonably efficient and runs in about 21.6 seconds on my machine (Intel Core i7-8565U). The following version, which builds up a sieve as it goes along, is more than 3 times faster.
import "/seq" for Lst
import "/fmt" for Fmt
var ulam = Fn.new { |n|
var ulams = [1, 2]
var sieve = [1, 1]
var u = 2
while (ulams.count < n) {
var s = u + ulams[-2]
sieve = sieve + ([0] * (s - sieve.count))
for (i in 1..ulams.count - 1) {
var v = u + ulams[i-1] - 1
sieve[v] = sieve[v] + 1
}
u = Lst.indexOf(sieve, 1, u) + 1
ulams.add(u)
}
return ulams[n-1]
}
var start = System.clock
for (p in 0..4) {
var n = 10.pow(p)
Fmt.print("The $,r Ulam number is $,d", n, ulam.call(n))
}
System.print("\nTook %(System.clock - start) seconds.")
- Output:
The 1st Ulam number is 1 The 10th Ulam number is 18 The 100th Ulam number is 690 The 1,000th Ulam number is 12,294 The 10,000th Ulam number is 132,788 Took 6.366709 seconds.
Version 3[edit]
This version is even quicker than Version 2 and reduces the time needed to calculate the 10,000th Ulam number to about 3.65 seconds. It also makes the 100,000th Ulam number a viable proposition for the Wren interpreter coming in at about 6 minutes 50 seconds.
The only downside with this version is that you need to know how much memory to allocate in advance.
import "/fmt" for Fmt
var ulam = Fn.new { |n|
if (n <= 2) return n
var max = 1352000
var list = List.filled(max+1, 0)
list[0] = 1
list[1] = 2
var sums = List.filled(max*2+1, 0)
sums[3] = 1
var size = 2
var query
while (true) {
query = list[size-1] + 1
while (true) {
if (sums[query] == 1) {
for (i in 0..size-1) {
var sum = query + list[i]
var t = sums[sum] + 1
if (t <= 2) sums[sum] = t
}
list[size] = query
size = size + 1
break
}
query = query + 1
}
if (size >= n) break
}
return query
}
var start = System.clock
var n = 10
while (true) {
Fmt.print("The $,r Ulam number is $,d", n, ulam.call(n))
n = n * 10
if (n > 100000) break
}
System.print("\nTook %(System.clock - start) seconds.")
- Output:
The 10th Ulam number is 18 The 100th Ulam number is 690 The 1,000th Ulam number is 12,294 The 10,000th Ulam number is 132,788 The 100,000th Ulam number is 1,351,223 Took 409.990502 seconds.
XPL0[edit]
Seeing "set" in the Go version and "sieve" in Phix, etc. lit a little light bulb. This version exploits those ideas and finds the 100,000th Ulam number in 24.7 seconds on a Pi4.
func Ulam(N); \Return Nth Ulam number
int N;
def Max = 1_352_000; \enough for 100_000th Ulam number
int List(Max); \array of Ulam numbers
char Sums(Max*2); \array: 0, 1, or more ways to sum Ulams
int I, Size, Query, Sum, T;
[if N <= 2 then return N;
for I:= 0 to Max*2 do Sums(I):= 0;
List(0):= 1; List(1):= 2;
Sums(3):= 1; \only one way to sum Ulams: 1+2 = 3
Size:= 2; \start after first 2 Ulams
repeat Query:= List(Size-1)+1; \possible next Ulam no.
loop [if Sums(Query) = 1 then \sums 1 way so it's next
[for I:= 0 to Size-1 do \update Sums array with
[Sum:= Query + List(I); \ all combos of sums
T:= Sums(Sum)+1; \ but limit max count
if T <= 2 then Sums(Sum):= T;
];
List(Size):= Query; \add Query to List
Size:= Size+1;
quit;
];
Query:= Query+1; \possible next Ulam no.
];
until Size >= N;
return Query;
];
int N;
[N:= 10;
repeat Text(0, "The "); IntOut(0, N);
Text(0, "th Ulam number is ");
IntOut(0, Ulam(N)); CrLf(0);
N:= N*10;
until N > 100_000;
]
- Output:
The 10th Ulam number is 18 The 100th Ulam number is 690 The 1000th Ulam number is 12294 The 10000th Ulam number is 132788 The 100000th Ulam number is 1351223
