Need to open a file and replace multiple strings - string

Question

I have a really big xml file. It has certain incrementing numbers inside, which i would like to replace with a different incrementing number. I've looked and here is what someone suggested here before. Unfortunately i cant get it to work :(
In the code below all instances of 40960 should be replaced with 41984, all instances of 40961 with 41985 etc. Nothing happens. What am i doing wrong?
use strict;
use warnings;
my $old = 40960;
my $new = 41984;
my $string;
my $file = 'file.txt';
rename($file, $file.'.bak');
open(IN, '<'.$file.'.bak') or die $!;
open(OUT, '>'.$file) or die $!;
$old++;
$new++;
for (my $i = 0; $i < 42; $i++) {
while(<IN>) {
$_ =~ s/$old/$new/g;
print OUT $_;
}
}
close(IN);
close(OUT);

Answer 1

Other answers give you better solutions to your problem. Mine concentrates on explaining why your code didn't work.
The core of your code is here:
$old++;
$new++;
for (my $i = 0; $i < 42; $i++) {
while(<IN>) {
$_ =~ s/$old/$new/g;
print OUT $_;
}
}
You increment the values of $old and $new outside of your loops. And you never change those values again. So you're only making the same substitution (changing 40961 to 41985) 42 times. You never try to change any other numbers.
Also, look at the while loop that reads from IN. On your first iteration (when $i is 0) you read all of the data from IN and the file pointer is left at the end of the file. So when you go into the while loop again on your second iteration (and all subsequent iterations) you read no data at all from the file. You need to reset the file pointer to the start of your file at the end of each iteration.
Oh, and the basic logic is wrong. If you think about it, you'll end up writing each line to the output file 42 times. You need to do all possible substitutions before writing the line. So your inner loop needs to be the outer loop (and vice versa).
Putting those suggestions together, you need something like this:
my $old = 40960;
my $change = 1024;
while (<IN>) {
# Easier way to write your loop
for my $i ( 1 .. 42 ) {
my $new = $old + $change;
# Use \b to mark word boundaries
s/\b$old\b/$new/g;
$old++;
}
# Print each output line only once
print OUT $_;
}

Answer 2

Here's an example that works line by line, so the size of file is immaterial. The example assumes you want to replace things like "45678", but not "fred45678". The example also assumes that there is a range of numbers, and you want them replaced with a new range offset by a constant.
#!/usr/bin/perl
use strict;
use warnings;
use constant MIN => 40000;
use constant MAX => 90000;
use constant DIFF => +1024;
sub repl { $_[0] >= MIN && $_[0] <= MAX ? $_[0] + DIFF : $_[0] }
while (<>) {
s/\b(\d+)\b/repl($1)/eg;
print;
}
exit(0);
Invoked with the file you want to transform as an argument, it produces altered output on stdout. With the following input ...
foo bar 123
40000 50000 60000 99999
fred60000
fred 60000 fred
... it produces this output.
foo bar 123
41024 51024 61024 99999
fred60000
fred 61024 fred
There are a couple of classic Perlisms here, but the example shouldn't be hard to follow if you RTFM appropriately.

Answer 3

Here is an alternative way which reads the input file into a string and does all the substitutions at once:
use strict;
use warnings;
{
my $old = 40960;
my $new = 41984;
my ($regexp) = map { qr/$_/ } join '|', map { $old + $_ } 0..41;
my $file = 'file.txt';
rename($file, $file.'.bak');
open(IN, '<'.$file.'.bak') or die $!;
my $str = do {local $/; <IN>};
close IN;
$str =~ s/($regexp)/do_subst($1, $old, $new)/ge;
open(OUT, '>'.$file) or die $!;
print OUT $str;
close OUT;
}
sub do_subst {
my ( $old, $old_base, $new_base ) = #_;
my $i = $old - $old_base;
my $new = $new_base + $i;
return $new;
}
Note: Can probably be made more efficient by using Regexp::Assemble