for loop ?

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

Hello, I am absolutely perplexed by the following subroutine:

sub getPlatform {
    my $os = `uname`;
    chomp($os);

    for ($os) {
        /HP-UX/ and do {
            return('HP');
        };

        /Linux/ and do {
            my $hardware = `uname -m`;
            chomp($hardware);

            for ($hardware) {
                /ppc/    and return('PPC');
                /ppc64/  and return('PPC');
                /i686/   and return('INTEL');
                /x86_64/ and return('INTEL');

                errorMsg("Could not determine platform type for '$hard
+ware'");
            }
        };
    }
}
[download]

I think I know what the overall function of the subroutine is, it returns the platform. But we programmers are taught that 'for' is universally a looping construct, whereas here its intended use appears to resemble that of a switch statement, but I am not sure. Can a monk enlighten this newbie? Thanks

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Re: for loop ? by philipbailey (Curate) on Aug 10, 2011 at 19:43 UTC
Of course, for is indeed a looping construct, but in this case the for loops each have only one iteration. The programmer is using the "for" to set the default variable $_ to the value of $os or $hardware in each case, then performing a regular expression match (/.../), implicitly on $_, against various values. Older Perls (5.8 and prior) do not have a switch statement (well, given/when statement), so various other approaches have been used in the past to emulate one. I would probably not use this particular approach myself, but it seems to work well enough.	[reply]
Re: for loop ? by cdarke (Prior) on Aug 10, 2011 at 19:46 UTC
This use of 'for' is unusual, but early perl's did not really have a switch/case statement. Nowadays we would use `given` and `when`. The 'for' loop loops over each item in its list (the variables inside parentheses). In each for loop there is only one item in the list, so they would only go through one iteration.	[reply] [d/l] [select]
Re^2: for loop ? by DrHyde (Prior) on Aug 11, 2011 at 10:15 UTC
We did, however, have `if ... elsif ... else` in the "bad old days", which would be a better approach than abusing a looping construct. Using `for` like this just makes the code harder to read than it should be.	[reply] [d/l] [select]
Re^3: for loop ? by cdarke (Prior) on Aug 12, 2011 at 08:24 UTC
So far as I know we still have `if ... elsif ... else`. Personally I would not even think of using a `for` loop in this way, which means I learnt something. I'm all for trying different code tricks, some are dire and should be destroyed at birth, but some bring enlightenment. I'm not sure that the code is any harder to read than some of the attempts at switch/case using LABELS (as in the Perl Cookbook). I agree with your sentiment though, and grateful that we have been blessed with `given/when`.	[reply] [d/l] [select]
Re: for loop ? by ikegami (Patriarch) on Aug 10, 2011 at 19:47 UTC
`for` is being used as a topicaliser, aliasing `$_` to `$os`. It's still technically a (foreach) loop, but the body will unconditionally be executed once.	[reply] [d/l] [select]
Re: for loop ? by DrHyde (Prior) on Aug 11, 2011 at 10:18 UTC
Ignoring the confusing `for`, I'd also point out that spawning `uname` is completely unnecessary. Everything that code does can be done by `use`ing `Config.pm` and interrogating `%Config`. Maybe I should add architecture stuff to Devel::CheckOS.	[reply] [d/l] [select]
Re^2: for loop ? by Tanktalus (Canon) on Aug 11, 2011 at 21:13 UTC
That's not entirely true. You can compile Perl for Linux/ia32 and run it on Linux/amd64. %Config won't tell you that. You can compile Perl for HP/PA-RISC and run it on HP/ia64 (though that's not what the OP is doing). %Config won't tell you that, either. Yes, been there, done that. Though I'd still prefer POSIX::uname over `uname`.	[reply]
Re: for loop ? by kcott (Archbishop) on Aug 11, 2011 at 10:46 UTC
Take a look at the Perl v5.8.8 manpage for perlsyn (Basic BLOCKs and Switch Statements). Look for "A common idiom for a switch statement is ..." followed by a code example similar to what you've posted here. :-) -- Ken	[reply]