There are also some snippets laying around in old nodes, for taking slices from 2-d arrays, you could just take slices and use it to convert from rows to columns or vice-versa.

Here is one I have handy( use Super Search)

#!/usr/bin/perl
# 2-d array-slice

my @arr = (['a','b','c'],
          ['h','i','j'],
          ['x','y','z']);

print map $_->[1], @arr;
print "\n";
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Yes, I discovered pretty quickly that PDL is not the right tool.

However, the Transpose a matrix of chars node was exactly what I needed.

The other one you mentioned I did eventually find but it seemed more suited to numbers and the theory/explanation was way beyond my meager understanding of that stuff.

Thanks again for the rapid and spot-on reply.

#!/usr/bin/perl
# maybe look at  Math::Matrix

# by Tachyon
# The main requirement is that you have as much free disk space 
# for the temp files as the total file size. You will be limited 
# in the number of columns you can transpose by the number of open 
# file descriptors your Perl will let you have. It is very easy to 
# hack the logic to do N colunms per pass at the expense of 1 full 
# read of the input file per extra pass. Alternatively you could DBM 
# or tie a hash to a file and use the keys as pseudo file handles 
# and just append data to the values.

# It should be really fast as we make a single pass through the input 
# data and then effectively just write it out (each temp file has one 
# full line in it).
# transpose.pl

#works but the temp files seems wasteful to me

use strict;
transpose90( "data.txt", "data-transpose.txt" );

sub transpose90 {
    my ( $infile, $outfile, $tmp ) = @_;
    $tmp ||= 'temp';
    open IN, $infile or die "Can't read $infile $!\n";
    # find number of columns and open a temp file for each
    local $_ = <IN>;
    chomp;
    my @data = split ' ';
    my $num_cols = $#data;
    my @fhs;
    open $fhs[$_], ">$tmp$_.txt" or die "Can't create temp file $tmp$_
+ $!\n"
        for 0..$num_cols;
    print {$fhs[$_]} $data[$_], "\t" for 0..$num_cols;
    while( <IN> ) {
        chomp;
        @data = split ' ';
        print {$fhs[$_]} $data[$_], "\t" for 0..$num_cols;
    }
    close IN;
    close $fhs[$_] for 0..$num_cols;
    open OUT, ">$outfile" or die "Can't write $outfile $!\n";
    for ( 0.. $num_cols ) {
        open IN, "$tmp$_.txt" or die "Can't read temp file $tmp$_ $!\n
+";
        print OUT scalar(<IN>), "\n";
        close IN;
        unlink "$tmp$_.txt"
    }
    close OUT;
}
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Thank you very much for the code; I will consider if it can be used to suite my needs. If not, I will hang on to it for a time when I can.

I'd read it into a (tied) 2 dimensional array:

a b
c d

And write it out exchanging inner and outer loop:

a c
b d
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