Re^2: Parse a list of path strings into a nested hash

[Jim]

Can you please briefly explain paths2TreeGF to us, especially the tricky bit? It looks slick. I just can't get my head around it.

    $scan = $scan->{shift @parts} ||= {} while @parts;

Thanks.

[GrandFather]

Sure. For each path split the path into parts:

        my @parts = split /\\/, $path;

Then for each part in the path starting at the 'root' (drive specifier for example) add it to the parent hash. %pTree is the parent hash for the whole tree so we start out by setting $scan to reference that:

        my $scan = \%pTree;

then the 'tricky' bit adds the parts. shift @parts removes the left most part of the path. $scan->{shift @parts} ||= {} will create a new hash element if there isn't one already. In any case it returns the hash ref for the current part of the path and that becomes the new parent $scan = ... while @parts;. Taken all together the 'tricky' bit walks down the tree generating entries as required:

        $scan = $scan->{shift @parts} ||= {} while @parts;

A few important tricks:

1. $scan->{shift @parts} ||= {} autovivifies (creates) an entry in the hash if it doesn't exist yet.
2. ||= {} only assigns a new (empty) hash if a new entry has been created (or in the nasty special case that the directory name is 0).
3. assignment operators (=, ||=, +=, ...) 'return' the value assigned which is how $scan gets updated.

Note that with Perl 5.10 and later you can use //= in place of ||= and it fixes the 0 issue mentioned in item 2.