Inspired by today's how to count the no of repeats in a string, I came up with an admittedly artificial but possibly very interesting problem: that of counting for a string the repetition count of each substring that is actually repeated. For example, given the string

my $str = 'aabcdabcabcecdecd';

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I see that -limiting ourselves to substrings of length 2 or more- we have the following counts:

(abc => 3, bc => 3, cd => 3, ecd => 2)

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One possible way to get those counts is as follows -I'm casting the thing in the form of a sub-:

sub count1 {
    local $_=shift;
    my %count;
    
    for my $pos (0..(length)-1) {
        pos=$pos;
        @count{ /(.{2,}).*\1/g } = ();
    }
    
    for my $k (keys %count) {
        $count{$k} =()= /$k/g;
    }

    \%count;
}

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In fact, print Dumper count1 $str; gives me:

$VAR1 = {
          'cd' => 3,
          'ecd' => 2,
          'abc' => 3,
          'bc' => 3
        };

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To my unpleasant surprise, if I change .{2,} to .+ I do not get all substrings (of length 1):

$VAR1 = {
          'cd' => 3,
          'c' => 5,
          'ecd' => 2,
          'a' => 4,
          'abc' => 3,
          'd' => 3,
          'bc' => 3
        };

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And I can't understand why...

I personally believe that map based solutions are often very cute and so I rolled my own too:

sub count2 {
    my $s=shift;
    my %saw;
    my %count = map {
        map {
            $saw{$_}++ ? () : 
              $_ => scalar(()= $s =~ /$_/g);
        } do { pos=$_; $s =~ /(.{2,}).*\1/g };
    } 0..length($s)-1;
    \%count;
}

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But the latter

  1. is clumsier than the non-map version and;
  2. what's worst, is not correct at all: 
    $VAR1 = {
          'ecd' => 2,
          'abc' => 3,
          '3' => 2
        };

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And I can't understand why, either.

What's your take on the problem?

In reply to how to count the number of repeats in a string (really!) by blazar

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