How can I count characters in Perl?

问题:

I have the following Perl script counting the number of Fs and Ts in a string:

my $str = "GGGFFEEIIEETTGGG";
my $ft_count = 0;
$ft_count++ while($str =~ m/[FT]/g);
print "$ft_count\n";

Is there a more concise way to get the count (in other words, to combine line 2 and 3)?

回答1:

my $ft_count = $str =~ tr/FT//;

See perlop.

If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. This latter is useful for counting characters in a class …

  $cnt = $sky =~ tr/*/*/;     # count the stars in $sky
  $cnt = tr/0-9//;            # count the digits in $_

Here's a benchmark:

use strict; use warnings;

use Benchmark qw( cmpthese );

my ($x, $y) = ("GGGFFEEIIEETTGGG" x 1000) x 2;

cmpthese -5, {
    'tr' => sub {
        my $cnt = $x =~ tr/FT//;
    },
    'm' => sub {
        my $cnt = ()= $y =~ m/[FT]/g;
    },
};

        Rate     tr      m
     Rate     m    tr
m   108/s    --  -99%
tr 8118/s 7440%    --

With ActiveState Perl 5.10.1.1006 on 32 Windows XP.

The difference seems to be starker with

C:\Temp> c:\opt\strawberry-5.12.1\perl\bin\perl.exe t.pl
      Rate      m     tr
m   88.8/s     --  -100%
tr 25507/s 28631%     --

回答2:

Yes, you can use the CountOf secret operator:

my $ft_count = ()= $str =~ m/[FT]/g;

回答3:

When the "m" operator has the /g flag AND is executed in list context, it returns a list of matching substrings. So another way to do this would be:

my @ft_matches = $str =~ m/[FT]/g;
my $ft_count = @ft_matches; # count elements of array

But that's still two lines. Another weirder trick that can make it shorter:

my $ft_count = () = $str =~ m/[FT]/g;

The "() =" forces the "m" to be in list context. Assigning a list with N elements to a list of zero variables doesn't actually do anything. But then when this assignment expression is used in a scalar context ($ft_count = ...), the right "=" operator returns the number of elements from its right-hand side - exactly what you want.

This is incredibly weird when first encountered, but the "=()=" idiom is a useful Perl trick to know, for "evaluate in list context, then get size of list".

Note: I have no data on which of these are more efficient when dealing with large strings. In fact, I suspect your original code might be best in that case.

回答4:

You can combine line 2, 3 and 4 into one like so:

my $str = "GGGFFEEIIEETTGGG";
print $str =~ s/[FT]//g; #Output 4;