extract multiples columns from txt file perl

Question

I have a txt file like this:

#Genera columnA columnB columnC columnD columnN
x1       1       3       7      0.9      2
x2       5       3       13     7            


        

Answer 1

This program does as you ask. It expects the path to the input file as a parameter on the command line, which can then be read using the empty "diamond operator" <> without explicitly opening it

Each non-blank line of the file is split into fields, and the header line is identified by the first starting with a hash symbol #

A call to map converts the @wanted_fields array into a list of indexes into @fields where those column headers appear and stores it in array @idx

This array is then used to slice the wanted columns from @fields for every line of input. The fields are printed, separated by tabs

use strict;
use warnings 'all';

use List::Util 'first';

my @wanted_fields = qw/ columnA columnC columnN /;

my @idx;

while ( <> ) {
    next unless /\S/;

    my @fields = split;

    if ( $fields[0] =~ /^#/ ) {

        @idx = ( 0, map {
            my $wanted = $_;
            first { $fields[$_] eq $wanted } 0 .. $#fields;
        } @wanted_fields );
    }

    print join( "\t", @fields[@idx] ), "\n" if @idx;
}

output

#Genera columnA columnC columnN
x1  1   7   2
x2  5   13  5
x3  0.1 7   0.4