Passing strings as array to subroutine and return count of specific char

Question

I was trying to think in the right way to tackle this: -I would to pass say, n elements array as argument to a subroutine. And for each element match two char types S and T and print for each element, the count of these letters. So far I did this but I am locked and found some infinite loops in my code.

use strict; use warnings;

sub main {

my @array = @_;

while (@array) {
    my $s = ($_ = tr/S//);
    my $t = ($_ = tr/T//);
    print "ST are in total $s + $t\n";
    }
}

my @bunchOfdata = ("QQQRRRRSCCTTTS", "ZZZSTTKQSST", "ZBQLDKSSSS");
main(@bunchOfdata);

I would like the output to be:

Element 1 Counts of ST = 5
Element 2 Counts of ST = 6
Element 3 Counts of ST = 4

Any clue how to solve this?

Chris Charley · Accepted Answer · 2021-07-24 01:16:35Z

7

while (@array) will be an infinite loop since @array never gets smaller. You can't read into the default variable $_ this way. For this to work, use for (@array) which will read the array items into $_ one at a time until all have been read.

The tr transliteration operator is the right tool for your task.

The code needed to get your results could be:

#!/usr/bin/perl
use strict;
use warnings;

my @data = ("QQQRRRRSCCTTTS", "ZZZSTTKQSST", "ZBQLDKSSSS");

my $i = 1;
    
for (@data) {
    my $count = tr/ST//;
    print "Element $i Counts of ST = $count\n";
    $i++;
}

Also, note that my $count = tr/ST//; doesn't require the binding of the transliteration operator with $_. Perl assumes this when $_ holds the value to be counted here. Your code tried my $s = ($_ = tr/S//); which will give the results but the shorter way I've shown is the preferred way.

(Just noticed you had = instead of =~ in your statement. That is an error. Has to be $s = ($_ =~ tr/S//);)

You can combine the 2 sought letters as in my code. Its not necessary to do them separately.

I got the output you want.

Element 1 Counts of ST = 5
Element 2 Counts of ST = 6
Element 3 Counts of ST = 4

Also, you can't perform math operations in a quoted string like you had.

print "ST are in total $s + $t\n";

Instead, you would need to do:

print "ST are in total ", $s + $t, "\n";

where the operation is performed outside of the string.

edited Jul 24, 2021 at 1:16

answered Oct 14, 2018 at 2:37

Chris Charley

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4 Comments

tavalendo Over a year ago

Thanks @Chris. This explanation not only answers the question but also teaches me a lot of things. As a newbie to Perl, this is the best answer. Thank you.

ikegami Over a year ago

$s = ($_ = tr/S//) actually worked as it mean $s = $_ = ($_ =~ tr/S//). That said, it needlessly clobbered the contents of @array.

ysth Over a year ago

@ikegami but the assignment to $_ breaks the following tr/T//, doesn't it?

ikegami Over a year ago

Indeed, it does

Dave Cross · Accepted Answer · 2018-10-14 08:18:04Z

0

Don't use while to traverse an array - your array gets no smaller, so the condition is always true and you get an infinite loop. You should use for (or foreach) instead.

for (@array) {
  my $s = tr/S//; # No need for =~ as tr/// works on $_ by default
  my $t = tr/T//;
  print "ST are in total $s + $t\n";
}

answered Oct 14, 2018 at 8:18

Dave Cross

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Comments

ikegami · Accepted Answer · 2018-10-14 14:17:32Z

0

Why tr///??

sub main {
    my @array = @_;

    while (@array) {
        my $s = split(/S/, $_, -1) - 1;
        my $t = split(/T/, $_, -1) - 1;
        print "ST are in total $s + $t\n";
    }
}

edited Oct 14, 2018 at 14:17

ikegami

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answered Oct 14, 2018 at 1:08

k12l

1

3 Comments

Dave Cross Over a year ago

tr/// doesn't use the regex engine, so it's a little more efficient.

ikegami Over a year ago

@Dave Cross, Neither does split /S/ (or if it does, it's in some mode that's much faster than a normal match). Still, tr/S// should still be simpler and faster.

ysth Over a year ago

Split, in addition to being not as efficient, also returns one more than the number of S or Ts.

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