http://www.perlmonks.org?node_id=129653

polr has asked for the wisdom of the Perl Monks concerning the following question:

I am reading in a text file line by line. If the current line meets certain criteria, I want to modify the previous line.

Originally posted as a Categorized Question.

  • Comment on How to (conditionally) modify the previous line based on contents of current line?

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Re: How to (conditionally) modify the previous line based on contents of current line?
by nothingmuch (Priest) on Oct 19, 2002 at 21:03 UTC
    Since it's easy to do this with an array — simply look at element at index $i-1 — you could load the file into an array, process the array, then write the array back out to the file. Of course, this doesn't scale with the size of the file well. For a generally application solution, you could map the lines of the file into an array using the module Tie::File:
    use Tie::File; tie my @array, 'Tie::File', "file.txt" or die "$!"; for ( my $i = 1; $i <= $#array; $i++ ) { if ( check( $array[$i] ) ) # the "current line" { modify( $array[$i-1] ); } }
    In this example, I started at index 1 (the second line) because even if the first line (index 0) passes the check, there is no "previous line" to modify.