How many different ways are there to concatenate two files line by line using Perl?

Question

Suppose file1 looks like this:

bye bye
hello
thank you

And file2 looks like this:

chao
hola
gracias

The desired output is this:

Answer 1

If I were a golfing man, I could rewrite @FM's answer as:

($,,$\)=(' ',"\n");@_=@ARGV;open $_,$_ for @_;print
map{chomp($a=<$_>);$a} @_=grep{!eof $_} @_ while @_

which you might be able to turn into a one-liner but that is just evil. ;-)

Well, here it is, under 100 characters:

C:\Temp> perl -le "$,=' ';@_=@ARGV;open $_,$_ for @_;print map{chomp($a =<$_>);$a} @_=grep{!eof $_ }@_ while @_" file1 file2

If it is OK to slurp (and why the heck not — we are looking for different ways), I think I have discovered the path the insanity:

@_=@ARGV;chomp($x[$.-1]{$ARGV}=$_) && eof
and $.=0 while<>;print "@$_{@_}\n" for @x

C:\Temp> perl -e "@_=@ARGV;chomp($x[$.-1]{$ARGV}=$_) && eof and $.=0 while<>;print qq{@$_{@_}\n} for @x" file1 file2

Output:

bye bye chao
hello hola
thank you gracias

Answer 2

An easier alternative to your Code 5 which allows for an arbitrary number of lines and does not care if files have different numbers of lines (hat tip @FM):

#!/usr/bin/perl

use strict; use warnings;

use File::Slurp;
use List::AllUtils qw( each_arrayref );

my @lines = map [ read_file $_ ], @ARGV;

my $it = each_arrayref @lines;

while ( my @lines = grep { defined and chomp and length } $it->() ) {
    print join(' ', @lines), "\n";
}

And, without using any external modules:

#!perl
use autodie; use warnings; use strict;

my ($file1, $file2) = @ARGV;

open my $file1_h,'<', $file1;
my @file1 = grep { chomp; length } <$file1_h>;

open my $file2_h,'<', $file2;
my @file2 =  grep { chomp; length } <$file2_h>;

my $n_lines = @file1 > @file2 ? @file1 : @file2;

for my $i (0 .. $n_lines - 1) {
    my ($line1, $line2) = map {
        defined $_ ? $_ : ''
    } $file1[$i], $file2[$i];
    print $line1, ' ', $line2, "\n";
}

If you want to concatenate only the lines that appear in both files:

#!perl
use autodie; use warnings; use strict;

my ($file1, $file2) = @ARGV;

open my $file1_h,'<', $file1;
my @file1 = grep { chomp; length } <$file1_h>;

open my $file2_h,'<', $file2;
my @file2 =  grep { chomp; length } <$file2_h>;

my $n_lines = @file1 < @file2 ? @file1 : @file2;

for my $i (0 .. $n_lines - 1) {
    print $file1[$i], ' ', $file2[$i], "\n";
}

Answer 3

An easy one with minimal error checking:

#!/usr/bin/perl -w

use strict;

open FILE1, '<file1.txt';
open FILE2, '<file2.txt';

while (defined(my $one = <FILE1>) or defined(my $twotemp = <FILE2>)){
    my $two = $twotemp ? $twotemp : <FILE2>;
    chomp $one if ($one);
    chomp $two if ($two);
    print ''.($one ? "$one " : '').($two ? $two : '')."\n";
}

And no, you can't run two loops simultaneous within the same thread, you'd have to fork, but that would not be guaranteed to run synchronously.

Answer 4

This works for any number of files:

use strict;
use warnings;
use autodie;

my @handles = map { open my $h, '<', $_; $h } @ARGV;

while (@handles){
    @handles = grep { ! eof $_ } @handles;
    my @lines = map { my $v = <$_>; chomp $v; $v } @handles;
    print join(' ', @lines), "\n";
}

close $_ for @handles;

Answer 5

The most elegant way doesn't involve perl at all:

paste -d' ' file1 file2