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Re^2: do/redo or for(;;): what's Kosher?

by Aristotle (Chancellor)
on Jan 04, 2002 at 00:05 UTC ( #136060=note: print w/ replies, xml ) Need Help??


in reply to Re: do/redo or for(;;): what's Kosher?
in thread do/redo or for(;;): what's Kosher?

Absolutely agree. Personally I find: the Perl way of doing things is either while() or for (@list). This is for a variety of reasons.

First of all, if you have a loop, it should be obvious that it is a loop. (To take it to extremes, that's what BASIC's GOSUB did wrong - it is a subroutine call, but the called place doesn't look like a subroutine.)

Also, the for(;;) form is to be avoided; most of the time you can write it as for (@list). Perl understands the notion of a list natively so you should not concern yourself with index variables. This eliminates an entire class of coding errors - the fencepost errors which consist in mostly overlooking nasty special conditions that result in the loop executing once more or less than it should. for(;;) also is inefficient (internally, it is handled like a while () {} continue {} construct).

{ ... ; redo } has its place just like for(;;) is occasionally useful - but for the sake of your own sanity you should not make either of them a habit.


Comment on Re^2: do/redo or for(;;): what's Kosher?
Re: Re: Re: do/redo or for(;;): what's Kosher?
by Juerd (Abbot) on Jan 04, 2002 at 00:24 UTC
    And here's the benchmark to prove it:
    #!/usr/bin/perl -w use strict; use Benchmark; Benchmark::cmpthese(10_000, { 'C-style' => sub { for (my $i = 0; $i <= 1000; $i++){ undef } }, 'foreach' => sub { for my $i (0..1000) { undef } } }); __END__ Benchmark: timing 10000 iterations of C-style, foreach... C-style: 5 wallclock secs ( 5.20 usr + 0.00 sys = 5.20 CPU) @ 19 +23.08/s (n=10000) foreach: 4 wallclock secs ( 3.73 usr + 0.00 sys = 3.73 CPU) @ 26 +80.97/s (n=10000) Rate C-style foreach C-style 1923/s -- -28% foreach 2681/s 39% --

    By the way: does perl have absolutely _no_ optimisation for for (;;) without any expressions in its parens? I find that a bit hard to believe.

    2;0 juerd@ouranos:~$ perl -e'undef christmas' Segmentation fault 2;139 juerd@ouranos:~$

      $ perl -MO=Deparse -e "while(1){die}" for (;;) { die; } -e syntax OK
      which tells me that Perl considers while(1) and for(;;) to be the same, which means it does optimize for(;;) into a simple infinite loop.

              - tye (but my friends call me "Tye")

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