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Hash key composition with a comma?

by mpersico (Scribe)
on Mar 16, 2018 at 20:09 UTC ( #1211078=perlquestion: print w/replies, xml ) Need Help??

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

Just when I thought I had seen everything:
my %hash; my $foo = 'hello'; my $bar = 'world'; my $baz = 'folk'; $hash{ $foo, $bar, $baz } = '##eek what magic is this?'; print $hash{"$foo\x{1c}$bar\x{1c}$baz"};
##eek what magic is this?

By what rule are multiple variables, separated by a comma, turned into a string with a File Separator between the pieces? I mean if I squint hard enough, that expression for the key is a list, and then the list is "stringified" to be a key, in which case I would have expected a space (\x{20}) separated expression. Perl 5.16 and Perl 5.26.1

Replies are listed 'Best First'.
Re: Hash key composition with a comma?
by LanX (Cardinal) on Mar 16, 2018 at 20:18 UTC
Re: Hash key composition with a comma?
by pryrt (Monsignor) on Mar 16, 2018 at 20:16 UTC
Re: Hash key composition with a comma?
by pwagyi (Scribe) on Mar 17, 2018 at 08:00 UTC

    It is mostly used in legacy code as answered before.

    If you are familiar with awk, it is the *way* to emulate multi-dimensional hash in awk/ and perl without reference

    I admit I've used a bit in some places where the use is well hidden and localized. You should prefer proper multi-dimensional hash (if it's really what you need). imagine to get keys of hash{foo} in emulated hash version, it'd be very difficult to do so.

    # with proper multi-dimensional hash keys(%{$hash{foo}}); # get keys of hash{foo}
Re: Hash key composition with a comma?
by Anonymous Monk on Mar 16, 2018 at 23:27 UTC
Re: Hash key composition with a comma?
by Anonymous Monk on Mar 17, 2018 at 03:21 UTC
    It may have been a nice idea back then but you really should eliminate all such things from your code today.   Because, sooner or sooner, your actual-data is going to include an instance of that "separator" and ... congratulations, you've just been waked-up at three o'clock in the morning when your pager went off, and ushered into Debugging Triage Hell. When you encounter such logic, it should be promptly refactored into a HoH = Hash of Hashes re-implementation.

      Perhaps you could show how such a legacy code trap might work. Some code to demonstrate the issue.

        It's not at all hard to see it an example is not necessary. The perldoc quoted in the first reply says everything: the trick works by means of join(), such that it would have been far better if the feature had not been a part of the language and programmers had to type a few extra letters into their 300-baud teletypes. If your data contains the separator-character, the code breaks. Whereas, if you (today) use hash-of-hashes and auto-vivification, you can quickly construct real multi-dimensional data structures without concern for what the hash-key values might be, and the code is no more cumbersome. Some of the "older" ideas in Perl were really good ones. But this is not one of them.
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