Re: cut columns with an array slice

[TGI]

This is where showing sample data is really important if you want people to be able to help you. I am going to make some guesses about your data files and attempt to answer your question.

First, I think your columns file looks like this:

1 3 4 6

If that's the case, then you are forgetting to split the columns data.

In any case, you are doing something strange when you access the data array in your while loop.

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

my $col_file  = shift;
my $data_file = shift;

open my $columns, '<', $col_file 
    or die "Could not open $col_file - $!\n";

open my $file, '<', $col_file
    or die "Could not open $data_file - $!\n");

my @columns = split /\s+/, <$columns>;
chomp(@columns);

# use a hash for quick lookup of columns to skip
# here I use a hash slice to assign undef values to the keys stored in
+ @columns
my %skip; @skip{@columns} = ();

while( my $line = <$file>) {
    chomp $line;
    my @raw_data = split /\s+/, $line;

    # Map and grep are a bit tough for newbies but they are important,
+ powerful tools that should be mastered.
    # Read from right to left:
    # Take the list of indexes in @raw_data
    # (grep) keep only those that do not exist as keys to %skip
   # (map) and look up the the item at that index in @raw_data
   # and finally print the resulting list.

    print join ' ', 
          map $raw_data[$_], 
          grep !exists $skip{$_}, 0..$#raw_data;
    print "\n";

}

Accessing a negative array index indexes from the end of the array.

Negating an array, causes the array to be evaluated in scalar context, yielding the number of members in the array. This value is then negated. So if @columns has 6 members, then -@columns is -6.

Finally, a few points.

• You can use a slice instead of map: print "@raw_data[ grep !exists $skip{$_}, 0..$#raw_data ]"; if you are strongly wedded to using an array slice.
• If you know the width of your data file you can build your keep list outside the loop. my @keep = grep !exists $skip{$_}, 0..$width; ... print "@raw_data[@keep]\n";

  Update I made some updates with a few little bug fixes, and some extra points on slices and moving the keep list calculation out of the while loop. If things are suddenly different on you, that's why.