• open the file
• Read the lines of the file in a while loop
• Figure out if this line is a multiple of 25 (hint $.)
• If it isn't, print it
• close the file
• underpants Profit!

Using a one-liner:

perl -i.bk -ne 'print if ($. % 25 != 0)' file
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Hope this helps

citromatik

Failing that:

• Open the exsting file and a new file.
• For each line in the file
  • BLACK MAGIC! What do you think you should do?
• Close both files.

Here's a one-liner you can use from the command line:

perl -ne 'next if 0 == $. % 25; print' filename
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Here's a somewhat clearer version:

#!/usr/local/bin/perl --
use strict;
use warnings;

my $remove_lines_modulo = 25;

while ( <> ) {
    next if 0 == $. % $remove_lines_modulo;
    print;
}
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Or one that accepts an argument for which lines to remove:

#!/usr/local/bin/perl --
use strict;
use warnings;

my $remove_lines_modulo = shift @ARGV;
die "Cannot handle zero as an argument.\n" if 0 == $remove_lines_modul
+o;

while ( <> ) {
    next if 0 == $. % $remove_lines_modulo;
    print;
}
[download]

Please remember that if this is a homework assignment, it's proper form to say so. Asking for completed work instead of help when it's a school assignment doesn't do any good in the long run and isn't fair to students who actually did the work.

thank you for your help! no it isn't homework, it is just part of some larger scripts for which it took me long time to program, bc I am just a starter in perl. This way I can save some precious time.:)

TIMTOWTDI: Sed is probably the fastest way here ;-)

Others will probably show fance Perl golf examples (hopefully).

Others have given solutions with extensions in gnu sed or tricky perl commands. There's however a very easy way to solve this with the unix toolkit that should work everywhere. No need for perl or awk or sed or anything. Suppose the input file is named inp. Then run the following shell commands.

( cat inp; echo ) > gre
rm spl.*
split -l 25 gre spl.
head -qn-1 spl.* > out
rm gre spl.*
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Update 2010-03-07: See Re^2: Joining two files on common field for a list of other nodes where unix textutils is suggested.