How to substring a string with several position with Perl? - perl

Question

I have several places where I want to cut my string in several parts.
For example:
$string= "AACCAAGTAA";
#cut_places= {0,4, 8 };
My $string should look like this: AACC AAGT AA;
How can I do that?

Answer 1

To populate an array, use round parentheses, not curly brackets (they're used for hash references).
One possible way is to use substr where the first argument is the position, so you can use the array elements. You just need to compute the length by subtracting the position from the following one; and to be able to compute the last length, you need the length of the whole string, too:
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
my $string = 'AACCAAGTAA';
my #cut_places = (0, 4, 8);
push #cut_places, length $string;
my #parts = map {
substr $string, $cut_places[$_], $cut_places[$_+1] - $cut_places[$_]
} 0 .. $#cut_places - 1;
say for #parts;
If the original array contained lengths instead of positions, the code would be much easier.
#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
my $string = 'AACCAAGTAA';
my #lengths = (4, 4, 2); # 4, 4, 4 would work, too
my #parts = unpack join("", map "A$_", #lengths), $string;
say for #parts;
See unpack for details.

Answer 2

Here's a solution that starts by calculating the forward differences in the list of positions. The length of the string is first appended to the end of the list of it doesn't already span the full string
The differences are then used to build an unpack format string, which is used to build the required sequence of substrings.
I have written the functionality as a do block, which would be simple to convert to a subroutine if desired.
use strict;
use warnings 'all';
use feature 'say';
my $string = 'AACCAAGTAA';
my #cut_places = ( 0, 4, 8 );
my #parts = do {
my #places = #cut_places;
my $len = length $string;
push #places, $len unless $places[-1] >= $len;
my #w = map { $places[$_]-$places[$_-1] } 1 .. $#places;
my $patt = join ' ', map { "A$_" } #w;
unpack $patt, $string;
};
say "#parts";
output
AACC AAGT AA

Answer 3

Work out the lengths of needed parts first, then all methods are easier. Here regex is used
use warnings;
use strict;
use feature 'say';
my $string = 'AACCAAGTAA';
my #pos = (0, 4, 8);
my #lens = do {
my $prev = shift #pos;
"$prev", map { my $e = $_ - $prev; $prev = $_; $e } #pos;
};
my $patt = join '', map { '(.{'.$_.'})' } #lens;
my $re = qr/$patt/;
my #parts = grep { /./ } $string =~ /$re(.*)/g;
say for #parts;
The lengths #lens are computed by subtracting the successive positions, 2-1, 3-2 (etc). I use do merely so that the #prev variable, unneeded elsewhere, doesn't "pollute" the rest of the code.
The "$prev" is quoted so that it is evaluated first, before it changes in map.
The matches returned by regex are passed through grep to filter out empty string(s) due to the 0 position (or whenever successive positions are the same).
This works for position arrays of any lengths, as long as positions are consistent with a string.