Find common directory path
You are encouraged to solve this task according to the task description, using any language you may know.
Create a routine that, given a set of strings representing directory paths and a single character directory separator, will return a string representing that part of the directory tree that is common to all the directories.
Test your routine using the forward slash '/' character as the directory separator and the following three strings as input paths:
'/home/user1/tmp/coverage/test' '/home/user1/tmp/covert/operator' '/home/user1/tmp/coven/members'
Note: The resultant path should be the valid directory '/home/user1/tmp' and not the longest common string '/home/user1/tmp/cove'.
If your language has a routine that performs this function (even if it does not have a changeable separator character, then mention it as part of the task)
Ada
<lang ada> with Ada.Text_IO; use Ada.Text_IO;
procedure Test_Common_Path is
function "rem" (A, B : String) return String is
Slash : Integer := A'First; -- At the last slash seen in A
At_A : Integer := A'first;
At_B : Integer := B'first;
begin
loop
if At_A > A'Last then
if At_B > B'Last or else B (At_B) = '/' then
return A;
else
return A (A'First..Slash - 1);
end if;
elsif At_B > B'Last then
if A (At_A) = '/' then -- A cannot be shorter than B here
return B;
else
return A (A'First..Slash - 1);
end if;
elsif A (At_A) /= B (At_B) then
return A (A'First..Slash - 1);
elsif A (At_A) = '/' then
Slash := At_A;
end if;
At_A := At_A + 1;
At_B := At_B + 1;
end loop;
end "rem";
begin
Put_Line
( "/home/user1/tmp/coverage/test" rem
"/home/user1/tmp/covert/operator" rem
"/home/user1/tmp/coven/members"
);
end Test_Common_Path; </lang> Output:
/home/user1/tmp
ALGOL 68
<lang algol68># Utilities code #
CHAR dir sep = "/"; # Assume POSIX #
PROC dir name = (STRING dir)STRING: (
STRING out;
FOR pos FROM UPB dir BY -1 TO LWB dir DO
IF dir[pos] = dir sep THEN
out := dir[:pos-1];
GO TO out exit
FI
OD;
# else: # out:="";
out exit: out
);
PROC shortest = ([]STRING string list)STRING: (
INT min := max int;
INT min key := LWB string list - 1;
FOR key FROM LWB string list TO UPB string list DO
IF UPB string list[key][@1] < min THEN
min := UPB string list[key][@1];
min key := key
FI
OD;
string list[min key]
);
- Actual code #
PROC common prefix = ([]STRING strings)STRING: (
IF LWB strings EQ UPB strings THEN
# exit: # strings[LWB strings]
ELSE
STRING prefix := shortest(strings);
FOR pos FROM LWB prefix TO UPB prefix DO
CHAR first = prefix[pos];
FOR key FROM LWB strings+1 TO UPB strings DO
IF strings[key][@1][pos] NE first THEN
prefix := prefix[:pos-1];
GO TO prefix exit
FI
OD
OD;
prefix exit: prefix
FI
);
- Test code #
test:(
[]STRING dir list = ( "/home/user1/tmp/coverage/test", "/home/user1/tmp/covert/operator", "/home/user1/tmp/coven/members" ); print((dir name(common prefix(dir list)), new line))
)</lang> Output:
/home/user1/tmp
AutoHotkey
<lang autohotkey>Dir1 := "/home/user1/tmp/coverage/test" Dir2 := "/home/user1/tmp/covert/operator" Dir3 := "/home/user1/tmp/coven/members"
StringSplit, Dir1_, Dir1, / StringSplit, Dir2_, Dir2, / StringSplit, Dir3_, Dir3, /
Loop
If (Dir1_%A_Index% = Dir2_%A_Index%)
And (Dir1_%A_Index% = Dir3_%A_Index%)
Result .= (A_Index=1 ? "" : "/") Dir1_%A_Index%
Else Break
MsgBox, % Result</lang> Message box shows:
/home/user1/tmp
C
- This code specimen retains the original ALGOL 68 coding style.
note: This code uses the gcc block expression C language extension: ({ ~ }) <lang c>#include <stdlib.h>
- include <stdio.h>
- include <string.h>
- include <limits.h>
/* Utilities code */
char dir_sep = '/'; /* Assume POSIX */
- define LWB(s) 0 /* default C lower bound */
- define STRUPB(s) (strlen(s)) /* NUL terminated char* upper bound */
/* find upper bound of a NULL terminated list - ({ ~ }) gcc extension */
- define LISTUPB(list) ({ int i; for(i=0; list[i]!=NULL; i++); i-1; })
char *str_slice(char *s, int lwb, int upb){
char *new = malloc(upb-lwb+2); strncpy(new, s+lwb, upb-lwb+2); new[upb+1] = '\0'; return new;
}
char *dir_name(char *dir){
char *out;
int pos;
for(pos = STRUPB(dir); pos>=LWB(dir); pos--){
if(dir[pos] == dir_sep){
out = str_slice(dir,LWB(dir),pos-1);
return out;
}
}
return "";
}
char *shortest (char *string_list[]){
int min = INT_MAX;
int min_key = LWB(string_list) - 1;
int key;
for(key = LWB(string_list); string_list[key]; key++){
if(STRUPB(string_list[key]) < min){
min = STRUPB(string_list[key]) - LWB(string_list[key]);
min_key = key;
}
}
return string_list[min_key];
}
/* Actual code */
char *common_prefix(char *strings[]){
if(LWB(strings) == LISTUPB(strings)){
/* exit: */ return strings[LWB(strings)];
} else {
char *prefix = shortest(strings);
int pos;
for(pos = LWB(prefix); prefix[pos]; pos++){
char first = prefix[pos];
int key;
for(key = LWB(strings)+1; strings[key]; key++){
if(strings[key][pos] != first){
prefix = str_slice(prefix, LWB(prefix), pos-1);
return prefix;
}
}
}
return prefix ;
}
}
/* Test code */
main(){
char *dir_list[] = {
"/home/user1/tmp/coverage/test",
"/home/user1/tmp/covert/operator",
"/home/user1/tmp/coven/members",
NULL
};
printf("%s\n",dir_name(common_prefix(dir_list)));
}</lang> Output:
/home/user1/tmp
C++
<lang C++>#include <algorithm>
- include <iostream>
- include <string>
- include <vector>
std::string longestPath( const std::vector<std::string> & , char ) ;
int main( ) {
std::string dirs[ ] = { "/home/user1/tmp/coverage/test" , "/home/user1/tmp/covert/operator" ,
"/home/user1/tmp/coven/members" } ;
std::vector<std::string> myDirs ( dirs , dirs + 3 ) ;
std::cout << "The longest common path of the given directories is " << longestPath( myDirs ,
'/' ) << "!\n" ;
return 0 ;
}
std::string longestPath( const std::vector<std::string> & dirs , char separator ) {
std::vector<std::string>::const_iterator vsi = dirs.begin( ) ;
int maxCharactersCommon = vsi->length( ) ;
std::string compareString = *vsi ;
for ( vsi = dirs.begin( ) + 1 ; vsi != dirs.end( ) ; vsi++ ) {
std::pair<std::string::const_iterator , std::string::const_iterator> p =
std::mismatch( compareString.begin( ) , compareString.end( ) , vsi->begin( ) ) ;
if (( p.first - compareString.begin( ) ) < maxCharactersCommon )
maxCharactersCommon = p.first - compareString.begin( ) ;
} std::string::size_type found = compareString.rfind( separator , maxCharactersCommon ) ; return compareString.substr( 0 , found ) ;
}</lang> Output:
The longest common path of the given directories is /home/user1/tmp!
Clojure
<lang clojure>(use '[clojure.string :only [join,split]])
(defn common-prefix [sep paths]
(let [parts-per-path (map #(split % (re-pattern sep)) paths)
parts-per-position (apply map vector parts-per-path)]
(join sep
(for [parts parts-per-position :while (apply = parts)]
(first parts)))))
(println
(common-prefix "/"
["/home/user1/tmp/coverage/test"
"/home/user1/tmp/covert/operator"
"/home/user1/tmp/coven/members"]))</lang>
C#
<lang csharp> using System; using System.Collections.Generic; using System.Linq; using System.Text;
namespace RosettaCodeTasks {
class Program { static void Main ( string[ ] args ) { FindCommonDirectoryPath.Test ( ); }
}
class FindCommonDirectoryPath { public static void Test ( ) { Console.WriteLine ( "Find Common Directory Path" ); Console.WriteLine ( ); List<string> PathSet1 = new List<string> ( ); PathSet1.Add ( "/home/user1/tmp/coverage/test" ); PathSet1.Add ( "/home/user1/tmp/covert/operator" ); PathSet1.Add ( "/home/user1/tmp/coven/members" ); Console.WriteLine("Path Set 1 (All Absolute Paths):"); foreach ( string path in PathSet1 ) { Console.WriteLine ( path ); } Console.WriteLine ( "Path Set 1 Common Path: {0}", FindCommonPath ( "/", PathSet1 ) ); } public static string FindCommonPath ( string Separator, List<string> Paths ) { string CommonPath = String.Empty; List<string> SeparatedPath = Paths .First ( str => str.Length == Paths.Max ( st2 => st2.Length ) ) .Split ( new string[ ] { Separator }, StringSplitOptions.RemoveEmptyEntries ) .ToList ( );
foreach ( string PathSegment in SeparatedPath.AsEnumerable ( ) ) { if ( CommonPath.Length == 0 && Paths.All ( str => str.StartsWith ( PathSegment ) ) ) { CommonPath = PathSegment; } else if ( Paths.All ( str => str.StartsWith ( CommonPath + Separator + PathSegment ) ) ) { CommonPath += Separator + PathSegment; } else { break; } }
return CommonPath; } } }
</lang>
Factor
<lang factor>: take-shorter ( seq1 seq2 -- shorter )
[ shorter? ] 2keep ? ;
- common-head ( seq1 seq2 -- head )
2dup mismatch [ nip head ] [ take-shorter ] if* ;
- common-prefix-1 ( file1 file2 separator -- prefix )
[ common-head ] dip '[ _ = not ] trim-tail ;
- common-prefix ( seq separator -- prefix )
[ ] swap '[ _ common-prefix-1 ] map-reduce ;</lang>
( scratchpad ) {
"/home/user1/tmp/coverage/test"
"/home/user1/tmp/covert/operator"
"/home/user1/tmp/coven/members"
} CHAR: / common-prefix .
"/home/user1/tmp/"
Haskell
<lang haskell> import Data.List
-- Return the common prefix of two lists. commonPrefix2 (x:xs) (y:ys) | x == y = x : commonPrefix2 xs ys commonPrefix2 _ _ = []
-- Return the common prefix of zero or more lists. commonPrefix (xs:xss) = foldr commonPrefix2 xs xss commonPrefix _ = []
-- Split a string into path components. splitPath = groupBy (\_ c -> c /= '/')
-- Return the common prefix of zero or more paths. -- Note that '/' by itself is not considered a path component, -- so "/" and "/foo" are treated as having nothing in common. commonDirPath = concat . commonPrefix . map splitPath
main = putStrLn $ commonDirPath [
"/home/user1/tmp/coverage/test",
"/home/user1/tmp/covert/operator",
"/home/user1/tmp/coven/members"
]
</lang>
HicEst
<lang HicEst>CHARACTER a='/home/user1/tmp/coverage/test', b='/home/user1/tmp/covert/operator', c='/home/user1/tmp/coven/members'
minLength = MIN( LEN(a), LEN(b), LEN(c) ) lastSlash = 0
DO i = 1, minLength
IF( (a(i) == b(i)) * (b(i) == c(i)) ) THEN IF(a(i) == "/") lastSlash = i ELSEIF( lastSlash ) THEN WRITE(Messagebox) "Common Directory = ", a(1:lastSlash-1) ELSE WRITE(Messagebox, Name) "No common directory for", a, b, c ENDIF
ENDDO</lang>
Icon and Unicon
<lang Icon>procedure main() write(lcdsubstr(["/home/user1/tmp/coverage/test","/home/user1/tmp/covert/operator","/home/user1/tmp/coven/members"])) end
procedure lcdsubstr(sL,d) #: return the longest common sub-string of strings in the list sL delimited by d local ss
/d := "/" reverse(sL[1]) ? {
if tab(find(d)+*d) || allmatch(ss := reverse(tab(0)),sL) then
return ss
}
end
procedure allmatch(s,L) #: retrun s if it matches all strings in L local x every x := !L do
if not match(s,x) then fail
return s end</lang>
J
Solution: <lang j>parseDirs =: = <;.2 ] getCommonPrefix =: {. ;@{.~ 0 i.~ *./@(="1 {.)
getCommonDirPath=: [: getCommonPrefix parseDirs&></lang>
Example: <lang j> paths=: '/home/user1/tmp/coverage/test';'/home/user1/tmp/covert/operator';'/home/user1/tmp/coven/members'
getCommonPrefix >paths
/home/user1/tmp/cove
'/' getCommonDirPath paths
/home/user1/tmp/</lang>
Note: This alternative formulation of parseDirs provides cross-platform support, without the need to specify the path separator. <lang j>parseDirs =: (PATHSEP_j_&= <;.2 ])@jhostpath</lang>
Liberty BASIC
<lang lb>path$(1) = "/home/user1/tmp/coverage/test" path$(2) = "/home/user1/tmp/covert/operator" path$(3) = "/home/user1/tmp/coven/members"
print samepath$(3,"/")
end
function samepath$(paths,sep$)
d = 1 'directory depth
n = 2 'path$(number)
while 1
if word$(path$(1),d,sep$) <> word$(path$(n),d,sep$) or word$(path$(1),d,sep$) = "" then exit while
n = n + 1
if n > paths then
if right$(samepath$,1) <> sep$ and d<>1 then
samepath$ = samepath$ + sep$ + word$(path$(1),d,sep$)
else
samepath$ = samepath$ + word$(path$(1),d,sep$)
end if
n = 2
d = d + 1
end if
wend
end function</lang>
MUMPS
<lang MUMPS>FCD
NEW D,SEP,EQ,LONG,DONE,I,J,K,RETURN SET D(1)="/home/user1/tmp/coverage/test" SET D(2)="/home/user1/tmp/covert/operator" SET D(3)="/home/user1/tmp/coven/members" SET SEP="/" SET LONG=D(1) SET DONE=0 FOR I=1:1:$LENGTH(LONG,SEP) QUIT:DONE SET EQ(I)=1 FOR J=2:1:3 SET EQ(I)=($PIECE(D(J),SEP,I)=$PIECE(LONG,SEP,I))&EQ(I) SET DONE='EQ(I) QUIT:'EQ(I) SET RETURN="" FOR K=1:1:I-1 Q:'EQ(K) SET:EQ(K) $PIECE(RETURN,SEP,K)=$PIECE(LONG,SEP,K) WRITE !,"For the paths:" FOR I=1:1:3 WRITE !,D(I) WRITE !,"The longest common directory is: ",RETURN KILL D,SEP,EQ,LONG,DONE,I,J,K,RETURN QUIT</lang>
Usage:
USER>D FCD^ROSETTA For the paths: /home/user1/tmp/coverage/test /home/user1/tmp/covert/operator /home/user1/tmp/coven/members The longest common directory is: /home/user1/tmp
OCaml
<lang ocaml>let rec aux acc paths =
if List.mem [] paths then (List.rev acc) else let heads = List.map List.hd paths in let item = List.hd heads in let all_the_same = List.for_all ((=) item) (List.tl heads) in if all_the_same then aux (item::acc) (List.map List.tl paths) else (List.rev acc)
let common_prefix sep = function
| [] -> invalid_arg "common_prefix"
| dirs ->
let paths = List.map (Str.split (Str.regexp_string sep)) dirs in
let res = aux [] paths in
(sep ^ (String.concat sep res))
let () =
let dirs = [ "/home/user1/tmp/coverage/test"; "/home/user1/tmp/covert/operator"; "/home/user1/tmp/coven/members"; ] in print_endline (common_prefix "/" dirs);
- </lang>
(uses the module Str, str.cma)
Oz
With a few helper functions, we can express the solution like this in Oz: <lang oz>declare
fun {CommonPrefix Sep Paths}
fun {GetParts P} {String.tokens P Sep} end
Parts = {ZipN {Map Paths GetParts}}
EqualParts = {List.takeWhile Parts fun {$ X|Xr} {All Xr {Equals X}} end}
in
{Join Sep {Map EqualParts Head}}
end
fun {ZipN Xs}
if {Some Xs {Equals nil}} then nil
else
{Map Xs Head} | {ZipN {Map Xs Tail}}
end
end
fun {Join Sep Xs}
{FoldR Xs fun {$ X Z} {Append X Sep|Z} end nil}
end
fun {Equals C}
fun {$ X} X == C end
end
fun {Head X|_} X end
fun {Tail _|Xr} Xr end
in
{System.showInfo {CommonPrefix &/
["/home/user1/tmp/coverage/test"
"/home/user1/tmp/covert/operator"
"/home/user1/tmp/coven/members"]}}</lang>
Perl
Assuming all paths begin at root.
<lang Perl>sub compath {
my ($sep, @paths, %hash) = @_;
# Tokenize and tally subpaths
foreach (@paths) {
my @tok = split $sep, substr($_,1);
++$hash{join $sep, @tok[0..$_]} for (0..$#tok); }
# Return max length subpath or null
my $max = (reverse sort values %hash)[0];
return unless $max == @paths;
my @res = grep {$hash{$_} == $max} keys %hash;
return $sep . (sort {length($b) <=> length($a)} @res)[0];
}
- Test and display
my @paths = qw(/home/user1/tmp/coverage/test
/home/user1/tmp/covert/operator
/home/user1/tmp/coven/members);
print compath('/', @paths), "\n";</lang>
Output:
/home/user1/tmp
Perl 6
<lang Perl 6>my $sep = '/'; my @dirs = </home/user1/tmp/coverage/test
/home/user1/tmp/covert/operator
/home/user1/tmp/coven/members>;
my @comps = @dirs.map: { [ .comb(/ $sep [ <!before $sep> . ]* /) ] };
my $prefix = ;
while all(@comps[*]»[0]) eq @comps[0][0] {
$prefix ~= @comps[0][0] // last; @comps».shift;
}
say "The longest common path is $prefix"; </lang> Output:
The longest common path is /home/user1/tmp
If you'd prefer a pure FP solution without side effects, you can use this: <lang perl6>my $sep := '/'; my @dirs := </home/user1/tmp/coverage/test
/home/user1/tmp/covert/operator
/home/user1/tmp/coven/members>;
my @comps := @dirs.map: { [ .comb(/ $sep [ <!before $sep> . ]* /) ] };
say "The longest common path is ",
gather for 0..* -> $column {
last unless all(@comps[*]»[$column]) eq @comps[0][$column];
take @comps[0][$column] // last;
}</lang>
PHP
<lang php><?php
function common_path($dirList) {
$dirList = array_unique($dirList);
while (1 !== count($dirList)) {
$dirList = array_map('dirname',$dirList);
$dirList = array_unique($dirList);
}
reset($dirList);
return current($dirList);
}
/* TEST */
$dirs = array(
'/home/user1/tmp/coverage/test', '/home/user1/tmp/covert/operator', '/home/user1/tmp/coven/members',
);
if('/home/user1/tmp' !== common_path($dirs))
{
echo 'test fail';
} else {
echo 'test success';
}
?></lang>
PicoLisp
<lang PicoLisp>(de commonPath (Lst Chr)
(glue Chr
(make
(apply find
(mapcar '((L) (split (chop L) Chr)) Lst)
'(@ (or (pass <>) (nil (link (next))))) ) ) ) )</lang>
Output:
(commonPath
(quote
"/home/user1/tmp/coverage/test"
"/home/user1/tmp/covert/operator"
"/home/user1/tmp/coven/members" )
"/" )
-> "/home/user1/tmp"
PowerShell
<lang PowerShell># Find Common Directory Path
- Input is an array of strings holding the paths
function Get-CommonDirectoryPath($paths){
# Convert each path into array of tokens (i.e. convert array into jagged array)
for($i=0; $i -lt $paths.Count; $i++) {
$paths[$i] = ($paths[$i].TrimStart('/').Split('/'))
}
# Loop through tokens
$c = -1
$found = $false
do { # Do Until loop used to handle paths with different number of directories
$t = $paths[0][++$c]
for($r = 1; $r -lt $paths.Count; $r++) {
if ($t -ne $paths[$r][$c]) { $found=$true; break }
}
} until ($found)
# Return the answer
for($i=0; $i -lt $c; $i++) {$s += "/"+$paths[0][$i]}
return $s
}
- Main Entry Point
"The common directory path is " + (Get-CommonDirectoryPath ("/home/user1/tmp/coverage/test", "/home/user1/tmp/covert/operator", "/home/user1/tmp/coven/members"))</lang>
Output: <lang>The common directory path is /home/user1/tmp</lang>
PureBasic
PureBasic don't have a path comparator directly but instead have powerful string tools.
Simply by checking the catalog names until they mismatch and add up the correct parts, the task is accomplished. <lang PureBasic>Procedure.s CommonPath(Array InPaths.s(1),separator.s="/")
Protected SOut$=""
Protected i, j, toggle
If ArraySize(InPaths())=0
ProcedureReturn InPaths(0) ; Special case, only one path
EndIf
Repeat
i+1
toggle=#False
For j=1 To ArraySize(InPaths())
If (StringField(InPaths(j-1),i,separator)=StringField(InPaths(j),i,separator))
If Not toggle
SOut$+StringField(InPaths(j-1),i,separator)+separator
toggle=#True
EndIf
Else
ProcedureReturn SOut$
EndIf
Next
ForEver
EndProcedure</lang>
Example of implementation <lang PureBasic>Dim t.s(2) t(0)="/home/user1/tmp/coverage/test" t(1)="/home/user1/tmp/covert/operator" t(2)="/home/user1/tmp/coven/members"
Debug CommonPath(t(),"/"))</lang>
Python
The Python os.path.commonprefix function is broken as it returns common characters that may not form a valid directory path: <lang python>>>> import os >>> os.path.commonprefix(['/home/user1/tmp/coverage/test',
'/home/user1/tmp/covert/operator', '/home/user1/tmp/coven/members'])
'/home/user1/tmp/cove'</lang>
This result can be fixed: <lang python>>>> def commonprefix(*args, sep='/'): return os.path.commonprefix(*args).rpartition(sep)[0]
>>> commonprefix(['/home/user1/tmp/coverage/test',
'/home/user1/tmp/covert/operator', '/home/user1/tmp/coven/members'])
'/home/user1/tmp'</lang>
But it may be better to not rely on the faulty implementation at all: <lang python>>>> from itertools import takewhile >>> def allnamesequal(name): return all(n==name[0] for n in name[1:])
>>> def commonprefix(paths, sep='/'): bydirectorylevels = zip(*[p.split(sep) for p in paths]) return sep.join(x[0] for x in takewhile(allnamesequal, bydirectorylevels))
>>> commonprefix(['/home/user1/tmp/coverage/test',
'/home/user1/tmp/covert/operator', '/home/user1/tmp/coven/members'])
'/home/user1/tmp'</lang>
R
<lang r> get_common_dir <- function(paths, delim = "/") {
path_chunks <- strsplit(paths, delim)
i <- 1
repeat({
current_chunk <- sapply(path_chunks, function(x) x[i])
if(any(current_chunk != current_chunk[1])) break
i <- i + 1
})
paste(path_chunks1[seq_len(i - 1)], collapse = delim)
}
- Example Usage:
paths <- c(
'/home/user1/tmp/coverage/test', '/home/user1/tmp/covert/operator', '/home/user1/tmp/coven/members')
get_common_dir(paths) # "/home/user1/tmp"
</lang>
Ruby
Uses the standard library abbrev module: Given a set of strings, calculate the set of unambiguous abbreviations for those strings, and return a hash where the keys are all the possible abbreviations and the values are the full strings.
<lang ruby>require 'abbrev'
dirs = %w( /home/user1/tmp/coverage/test /home/user1/tmp/covert/operator /home/user1/tmp/coven/members )
common_prefix = dirs.abbrev.keys.min_by {|key| key.length}.chop # => "/home/user1/tmp/cove" common_directory = common_prefix.sub(%r{/[^/]*$}, ) # => "/home/user1/tmp"</lang>
Implementing without that module: <lang ruby>separator = '/' paths = dirs.collect {|dir| dir.split(separator)} uncommon_idx = paths.transpose.each_with_index.find {|dirnames, idx| dirnames.uniq.length > 1}.last common_directory = paths[0][0 ... uncommon_idx].join(separator) # => "/home/user1/tmp"</lang>
Tcl
<lang tcl>package require Tcl 8.5 proc pop {varname} {
upvar 1 $varname var set var [lassign $var head] return $head
}
proc common_prefix {dirs {separator "/"}} {
set parts [split [pop dirs] $separator]
while {[llength $dirs]} {
set r {}
foreach cmp $parts elt [split [pop dirs] $separator] {
if {$cmp ne $elt} break
lappend r $cmp
}
set parts $r
}
return [join $parts $separator]
}</lang>
% common_prefix {/home/user1/tmp/coverage/test /home/user1/tmp/covert/operator /home/user1/tmp/coven/members}
/home/user1/tmp
Ursala
The algorithm is to lex the paths into component directory names, and then find the greatest common prefix of those. <lang Ursala>#import std
comdir"s" "p" = mat"s" reduce(gcp,0) (map sep "s") "p"</lang>
where "s" is a dummy variable representing the separator, "p" is a dummy variable representing the list of paths, and
sepis second order function in the standard library that takes a separator character and returns a lexer mapping a string containing the separator to a list of the substrings found between occurrences of itmapis the conventional mapping combinator, which takes a function operating on items of a list to a function operating pointwise on a whole listgcpis a polymorphic greatest-common-prefix library function working on pairs of strings or lists of any typereduceis the standard functional programming reduction combinator, which cumulatively applies a binary operator to a list of operands given the operator and the vacuous case resultmatis a second order function in the standard library that takes a separator character and returns a function that flattens a list of strings into a single string with copies of the separator inserted between them
Here is a version using operators instead of mnemonics for map and reduce.
<lang Ursala>comdir"s" "p" = mat"s" gcp:-0 sep"s"* "p"</lang>
Here is one in partly point-free form, using the composition operator (+).
<lang Ursala>comdir"s" = mat"s"+ gcp:-0+ sep"s"*</lang>
Here it is in point-free form.
<lang Ursala>comdir = +^/mat gcp:-0++ *+ sep</lang>
test program:
<lang Ursala>#cast %s
test =
comdir`/ <
'/home/user1/tmp/coverage/test', '/home/user1/tmp/covert/operator', '/home/user1/tmp/coven/members'></lang>
output:
'/home/user1/tmp'