Hello smturner1,

I am trying to understand how this subroutine works and I just cannot workout the logic.

Despite the various missing pieces ($ARCHIVE, $SourceDir, @errors, ...), the general logic of the subroutine is fairly straightforward. The aim is to populate an array, @diffs, with details of the differences between the two directories $ARCHIVE and $SourceDir. First, the contents of $ARCHIVE are stored as keys in the hash %old_list, and the contents of $SourceDir are stored as keys in the hash %new_list. With this information available, it is then easy to determine which files are present in $ARCHIVE but not in $SourceDir:

for my $file (sort keys %old_list) {
    if ( !defined $new_list{$file} ) {
      push @diffs, "Old file not in new: $file";
    }
}

[download]

This tests each filename in %old_list and looks for it in %new_list. And this code reveals the reason for storing filenames as hash keys rather than as array elements: it is much simpler to test whether a key is present in a hash using defined than to search through an array. Incidentally, the call to sort is pointless here, and exists is usually preferable to defined in this situation.

In a similar manner, the next for loop identifies those files which are present in $SourceDir but absent from $ARCHIVE.

The specifically “Perlish” aspect to this code is the elegant use of hashes to search through lists. See, for example, How can I tell whether a certain element is contained in a list or array?

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

In reply to Re: Comparing files in directory by Athanasius
in thread Comparing files in directory by smturner1

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