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My fellow monks,
I have an interesting text processing task before (Not homework). What I need to do is open a file, skip the first 4 lines, then on all the remaining lines, duplicate each character except for the '^' and '#' characters, and rewrite the file.

On an input file of:
andromeda:davidj perl_test > cat f.txt ^this^ ^is^ ^a^ ^test^ ^david#jenkins^ ^ cinea#jenkins ^
the output should be:
andromeda:davidj perl_test > cat out.txt ^this^ ^is^ ^a^ ^test^ ^ddaavviidd#jjeennkkiinnss^ ^ cciinneeaa#jjeennkkiinnss ^
I currently have the following code which works perfectly well:
#!/usr/bin/perl use strict; open(FILE, "<f.txt"); open(OUT, ">out.txt"); while(<FILE>) { my $str = ""; chomp $_; if( 1 .. 4 ) { print OUT "$_\n"; next; } while( $_ =~ m/(.)/g ) { if( $1 =~ m/(\^|\#)/ ) { $str .= "$1"; } else { $str .= "$1$1"; } } print "$str\n"; print OUT "$str\n"; } close(FILE); close(OUT);
I didn't like the idea of creating a temporary string, so I have the following which modifies the text as it is processing it, and also works perfectly well:
#!/usr/bin/perl use strict; open(FILE, "<f.txt"); open(OUT, ">out.txt"); while(<FILE>) { chomp $_; if( 1 .. 4 ) { print OUT "$_\n"; next; } for( my $i = 0; $i < length($_); $i++ ) { if( substr($_, $i, 1) =~ m/(\^|\#)/ ) { substr($_, $i, 1) = "$1"; } elsif( substr($_, $i, 1) =~ m/(.)/ ) { substr($_, $i, 1) = "$1$1"; $i++; } } print OUT "$_\n"; } close(FILE); close(OUT);
I don't like this solution because it breaks the cardinal rule of not modifying a for loop counter inside the loop. (Not that I'm any kind of coding purist, mind you :)

Benchmarking the solutions indicates that (not surprisingly) using a temporary string is quicker. The following results are on 250000 iterations of a file with 1750 lines, each line no more than 50 characters.
andromeda:davidj perl_test > perl Rate 2nd string In place 2nd string 28969/s -- -17% In place 35112/s 21% --
Now to my curiosity: Both of these solutions work and I am satisfied with using either of them. What I'd like to have, purely for the educational value, is a more "Perlish" way of doing this, and/or a more efficient way.

as always thank you for your assistance,


In reply to modifying a string in place by davidj

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