Merge two files based on the starting of the line - perl

Question

I want to merge two files into one using perl. Below are the sample files.
***FILE 1***
XDC123
XDC456
XDC678
BB987
BB654
*** FILE 2 ***
XDC876
XDC234
XDC789
BB456
BB678
And I want the merged file to look like:
***MERGED FILE***
XDC123
XDC456
XDC678
XDC876
XDC234
XDC789
BB987
BB654
BB456
BB678
For the above functionality I have written the below perl script snippet:
#!/usr/bin/env perl;
use strict;
use warnings;
my $file1 = 'C:/File1';
my $file2 = 'C:/File2';
my $file3 = 'C:/File3';
open( FILEONE, '<$file1' );
open( FILETWO, '<$file2' );
open( FILETHREE, '>$file3' );
while (<FILEONE>) {
if (/^XDC/) {
print FILETHREE;
}
if (/^BB/) {
last;
}
}
while (<FILETWO>) {
if (/^XDC/) {
print FILETHREE;
}
if (/^BB/) {
last;
}
}
while (<FILEONE>) {
if (/^BB/) {
print FILETHREE;
}
}
while (<FILETWO>) {
if (/^BB/) {
print FILETHREE;
}
}
close($file1);
close($file2);
close($file3);
But the merged file that is generated from the above code looks like:
***FILE 3***
XDC123
XDC456
XDC678
XDC876
XDC234
XDC789
BB654
BB678
The first line that starts from BB is missed out from both the files. Any help on this will be appreciated. Thank you.

Answer 1

The problem is, you iterate each file to the end, but never 'rewind' for if you're wanting to start over.
So your while ( <FILEONE> ) { line consumes (and discards) the first line that matches m/^BB/ - the last exits the "while" loop, but only after it's already read the line.
However that's assuming you get your open statements right, because:
open( FILEONE, '>$file1' );
Actually empties it, it doesn't read from it. So I am assuming you've transposed your code, and introduced new errors whilst doing so.
As a style point - you should really use 3 argument open, with lexical filehandles.
So instead:
#!/usr/bin/env perl;
use strict;
use warnings;
my $file1 = 'C:/File1';
my $file2 = 'C:/File2';
my $file3 = 'C:/File3';
my #lines;
foreach my $file ( $file1, $file2 ) {
open( my $input, '<', $file ) or die $!;
push( #lines, <$input> );
close($input);
}
open( my $output, '>', $file3 ) or die $!;
print {$output} sort #lines;
close($output)
(Although as noted in the comments - if that's all you want to do, the unix sort utility is probably sufficient).
However, if you need to preserve the numeric ordering, whilst sorting on the alphabetical, you need a slightly different data structure:
#!/usr/bin/env perl;
use strict;
use warnings;
my $file1 = 'C:/File1';
my $file2 = 'C:/File2';
my $file3 = 'C:/File3';
my %lines;
foreach my $file ( $file1, $file2 ) {
open( my $input, '<', $file ) or die $!;
while ( my $line = <$file> ) {
my ( $key ) = $line =~ m/^(\D+)/;
push %{$lines{$key}}, $line;
}
close($input);
}
open( my $output, '>', $file3 ) or die $!;
foreach my $key ( sort keys %lines ) {
print {$output} #{$lines{$key}};
}
close($output)