How to copy and append strings in 2-line chunks using perl - perl

Question

while ($line = <IN>){
...
print OUT "$line";
print OUT1 "$line";
}
As far as I know my while loop only reads from my input file one line at a time. How can I adjust this so that it reads 2 lines at a time?
Suppose a 2-line chunk looks like this
%line1
THISISLINE2
I want my while loop to copy the first line and paste it after the second line (but replace % with #). I also want to add a line of 11 characters of A as line 4. Essentially I want the output to be
%line1
THISISLINE2
#line1
AAAAAAAAAAA
How can I write a while loop to do this?

Answer 1

I am going to make a guess that you've got multi-line records like this:
%line1
something something line1
%lineB
something to do with lineb
I would suggest in this scenario - rather than reading two lines at a time, you instead set your record separator via $/.
E.g.:
#!/usr/bin/env perl;
use strict;
use warnings;
local $/ = "%";
while (<DATA>) {
chomp;
my #lines = split "\n";
next unless #lines;
print '%', join( "\n", #lines ), "\n";
print $lines[0] =~ s/^/\#/r, "\n";
print "Something else to do with record $.\n";
print "---END---\n";
}
__DATA__
%line1
something something line1
%lineB
something to do with lineb
This means that each iteration of the while loop - it reads until the next % symbol. As a result, the first iteration is empty, but subsequent records will work fine.
This prints:
%line1
something something line1
#line1
Something else to do with record 2
---END---
%lineB
something to do with lineb
#lineB
Something else to do with record 3
---END---

Answer 2

Here is one option for a loop that gets two lines at once:
my $l1;
my $l2;
while (defined($l1=<DATA>) and defined($l2=<DATA>))
{
print "line 1: $l1\n";
print "line 2: $l2\n";
}
__DATA__
line1
line2
line3
line4
line5
This does not require reading the whole file into an array first.
It also ignores a single line at the end of the file (but you could change that by switching to or).

Answer 3

#!/usr/bin/perl
use strict;
use warnings;
open (my $fh, "<", "test.txt") or die $!;
open (my $op, ">", "output.txt") or die $!;
my #slurp = <$fh>;
while(my #lines = splice(#slurp, 0, 2)){
my ($line1, $line2) = #lines;
print $op $line1;
print $op $line2;
if($line1 =~ s/%/#/){
print $op $line1;
if($line2 =~ tr/.*/A/c){
print $op $line2."\n";
}
}
}

Answer 4

can you use a for loop instead of while? remember that for requires the whole file to be read into memory. but unless you have very high performance standards and a very big datafile it shouldn't a problem.
open IN,"<",$file;
my #lines = <IN>;
for (my $i = 0;$i le $#lines; $i = $i+2) {
my $first_line = $lines[$i];
my $second_line = $lines[$i+1];
}

Answer 5

Your question looks like a possible use case for a simple finite-state machine:
use strict;
use warnings;
my $state = 1;
my $first_line;
while (<>) {
if ($state == 1) {
$first_line = $_;
$state = 2;
} elsif ($state == 2) {
# do whatever you want with $_ and $first_line
$state = 1;
} else {
die "Unknown state '$state', not sure how we got here";
};
};