G'day PilotinControl,

Firstly, I think you need to step back from the question of one or two loops and consider what data you're looping over.

Currently, you're opening a file, reading all its data into an array, closing the file, then reading all the same data again from the array. Beyond the waste of CPU cycles to read the data twice, you've now copied all the data from disk into memory: with a large file this could be problematic.

A better approach is to read and process each line from the file without ever creating an intermediate array. Here's one way to do it.

#!/usr/bin/env perl -l

use strict;
use warnings;

my $format = "| %-15s | %-15s |\n";
my $format_width = 37;

print '=' x $format_width;
printf $format => 'Inbound Track 1', 'Inbound Track 2';
print '-' x $format_width;

my ($file1, $file2) = qw{pm_1126327_inbndtrk1.txt pm_1126327_inbndtrk2
+.txt};
open my $fh1, '<', $file1 or die "Can't open '$file1' for reading: $!"
+;
open my $fh2, '<', $file2 or die "Can't open '$file2' for reading: $!"
+;

while (<$fh1>) {
    printf $format => get_data($_), get_data(scalar <$fh2>);
}

close $fh1;
close $fh2;

print '=' x $format_width;

sub get_data {
    chomp(my $line = shift);
    join ' ' => (split /:/ => $line)[1,2];
}
[download]

Output:

$ pm_1126327_combine_file_data.pl
=====================================
| Inbound Track 1 | Inbound Track 2 |
-------------------------------------
| B&O 101         | CSXT 1001       |
| B&O 102         | CSXT 1002       |
=====================================
[download]

Notes:

• Your printf formats look fine; although, note that you can put common text into the format, e.g. the pipe separators in "| %-15s | %-15s |\n". However, the syntax is printf FORMAT, LIST; your first three printf statements use printf FORMAT, STRING! Had you used warnings, you would have been advised of this problem: "Missing argument in printf at ..."
• The call to get_data() provides a list context for its arguments. <$fh2> in list context will read the entire file; scalar <$fh2> reads just one record (which is what we want). See perlintro: Files and I/O.

The data you show is ideal, in that both files are the same length, all records have data, no data is corrupt. Real world data can rarely be relied upon to be ideal. In a while (1) {...} loop, you can code whatever loop exit conditions you want. In the following example, I've simply checked for an end-of-file (eof) condition in one file; in your real-world code, you'd probably want a lot more checking than this.

while (1) {
    last if eof $fh1;
    printf $format => get_data(scalar <$fh1>), get_data(scalar <$fh2>)
+;
}
[download]

You can replace the while loop in my first script with this one and get exactly the same output.

Update: I had three instances of the literal '37' hard-coded. This is not good! What the hell does '37' refer to? You'll now find $format_width instead of '37': much better.

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