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Re^2: Why/how are these different?

by perl-diddler (Hermit)
on Jan 31, 2013 at 04:40 UTC ( #1016204=note: print w/ replies, xml ) Need Help??


in reply to Re: Why/how are these different?
in thread Why/how are these different?

Wow...(warnings...duh!)....(doi!))

I didn't know they were different like that. How perverse! Too bad they didn't limit it to 'foreach' and created the confusion. I can't think of any other places where this could arise, off hand, but are there?

I noted that if I put parens around the args of the 'for', as in the prefix'd form, I get errors.

Hmmm...So even when you are clearly trying to use a postfix form, it throws an error instead of allowing it (though I understand why). I just can't figure out why it would have been designed that way.

And to think I stayed away from 'foreach' specifically because many books recommend doing so because it uses more memory (and I'm working with large arrays)...


Comment on Re^2: Why/how are these different?
Re^3: Why/how are these different?
by Athanasius (Prior) on Jan 31, 2013 at 06:46 UTC
    How perverse! Too bad they didn't limit it to 'foreach' and created the confusion.

    From the Camel Book (4th Edition, 2012, page 142):

    For historical reasons, the foreach keyword is a synonym for the for keyword, so you can use for and foreach interchangeably, whichever you think is more readable in a given situation. We tend to prefer for because we are lazy and because it is more readable, especially with the my. (Don’t worry—Perl can easily distinguish for (@ARGV) from for ($i=0; $i<$#ARGV; $i++) because the latter contains semicolons.)
    And to think I stayed away from 'foreach' specifically because many books recommend doing so because it uses more memory (and I'm working with large arrays)...

    I could be wrong, but I think this advice is outdated (if it was ever correct). Again from the Camel Book (page 143):

    If LIST consists of assignable values (meaning variables, generally, not enumerated constants), you can modify each of those variables by modifying VAR inside the loop. That’s because the loop variable becomes an implicit alias for each item in the list that you’re looping over.

    If I’m reading that correctly, no copying is needed, so there’s no reason a foreach-style loop would use more memory than a C-style for loop.

    And there are significant benefits. See Foreach Loops in perlsyn, where a C-style for loop is rewritten as a foreach-style loop in idiomatic Perl, and the latter is stated to be “cleaner, safer, and faster.” And the Camel Book has this in its cosideration of “Time Efficiency” (pages 691–2):

    Avoid subscripting when a foreach or list operator will do.... There’s often a better way to do it. Consider using foreach, shift, and splice operations.

    Hope that helps,

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

      Ok, I gotta love this part:

      <quote> For historical reasons, the foreach keyword is a synonym for the for keyword, so you can use for and foreach interchangeably, whichever you think is more readable in a given situation. We tend to prefer for because we are lazy and because it is more readable, especially with the my. </quote>

      1) For historical reasons we regard the world as flat. It remains so for compatibility purposes! *ahem*.

      <quote> (Don’t worry—Perl can easily distinguish for (@ARGV) from for ($i=0; $i<$#ARGV; $i++) because the latter contains semicolons.) </quote>

      2) Eh? Now this sounds like a bug .. implementation not following design. Mine DID contain semicolons which were ignored and cast off as warnings or errors (w/parens). That seems a case of implementation not following intent of design, no?

      p.s.- for compatibility reasons <quote> is not? supported?

Re^3: Why/how are these different?
by AnomalousMonk (Monsignor) on Jan 31, 2013 at 08:51 UTC
    [Use of warnings] often gives an insight into what Perl understands your code to be. (Athanasius)

    perl-diddler: Another path to insight is to have Perl deparse your code for you (see B::Deparse and O):

    >perl -wMstrict -MO=Deparse,-p -le "print qq{\$_ '$_'} for $_ = 3; $_-- >0; " Useless use of numeric gt (>) in void context at -e line 1. BEGIN { $^W = 1; } BEGIN { $/ = "\n"; $\ = "\n"; } use strict 'refs'; print("\$_ '${_}'") foreach (($_ = 3)); (($_--) > 0); -e syntax OK

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