Find the first un-repeated character in a string

Question

What is the quickest way to find the first character which only appears once in a string?

Answer 1

Here's an implementation in Perl (version >=5.10) that doesn't care whether the repeated characters are consecutive or not:

use strict;
use warnings;

foreach my $word(@ARGV)
{
  my @distinct_chars;
  my %char_counts;

  my @chars=split(//,$word);

  foreach (@chars)
  {
    push @distinct_chars,$_ unless $_~~@distinct_chars;
    $char_counts{$_}++;
  }

  my $first_non_repeated="";

  foreach(@distinct_chars)
  {
    if($char_counts{$_}==1)
    {
      $first_non_repeated=$_;
      last;
    }
  }

  if(length($first_non_repeated))
  {
    print "For \"$word\", the first non-repeated character is '$first_non_repeated'.\n";
  }
  else
  {
    print "All characters in \"$word\" are repeated.\n";
  }
}

Storing this code in a script (which I named non_repeated.pl) and running it on a few inputs produces:

jmaney> perl non_repeated.pl aabccd "a huge string in which some characters repeat" abcabc
For "aabccd", the first non-repeated character is 'b'.
For "a huge string in which some characters repeat", the first non-repeated character is 'u'.
All characters in "abcabc" are repeated.