Re^4: fall through switch/case in perl

The eval is necessary if you want to fully emulate the C semantic of not dieing if there is no matching case. Really, the full-on computed goto method (I should have just written this into my top-level response) is this (I think):

{
eval { goto "CASE$var" } or goto DEFAULT;

    CASE10: print "a";
    CASE9:  print "b";
    CASE8:  print "c";
    CASE7:  print "d";
    CASE6:  print "e";
    CASE5:  print "f";
    CASE4:  print "g";
    CASE3:  print "h";
    CASE2:  print "i";
    CASE1:  print "j";
    DEFAULT: 
}
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Which, admittedly, is not very different from Aristotle's. The only real difference being the outter braces. The purpose of them is to avoid polluting the label name-space. (Also, I prefer the "goto X or goto DEFAULT" over the "goto X; goto DEFAULT" purely for aesthetic reasons. :-D)

I sort of wonder why this isn't given as one of the ways of achieving C-like switch statement behavior in the perl docs? Oh, well... it's probably horribly inefficient or something (apart from just being too C-ish or something).

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Re^5: fall through switch/case in perl by BrowserUk (Patriarch) on Sep 07, 2004 at 04:10 UTC
Hmm. I think what your saying is that the eval is necessary to avoid a warning if `$var` doesn't contain a value for which there is a label defined? If so, then it still doesn't handle the case where `$var` is undefined. That can be handled by using a do BLOCK construct to disable warnings and adding a `CASE0:` `for my $var ( undef, 1 .. 10, 'fred' ) { goto 'CASE' . do{ local $^W; $var + 0 }; CASE10: print "a"; CASE9: print "b"; CASE8: print "c"; CASE7: print "d"; CASE6: print "e"; CASE5: print "f"; CASE4: print "g"; CASE3: print "h"; CASE2: print "i"; CASE1: print "j"; print "\n"; CASE0: }` [download] I sort of wonder why this isn't given as one of the ways of achieving C-like switch statement behavior in the perl docs? Oh, well... it's probably horribly inefficient or something (apart from just being to C-ish or something). C-ish it may be (though that's not a good reason in my book if it is clearer), but inefficient it is not: `P:\test>388915 ?> 1..10 Testing for vals [1..10] Rate if_cascade and_cascade goto_do goto_eval + goto if_cascade 11395/s -- -2% -11% -26% + -26% and_cascade 11627/s 2% -- -9% -24% + -25% goto_do 12845/s 13% 10% -- -17% + -17% goto_eval 15389/s 35% 32% 20% -- + -0% goto 15423/s 35% 33% 20% 0% + -- ?>` [download] Benchmark/results Read more... (10 kB) Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail "Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon	[reply] [d/l] [select]
Re^6: fall through switch/case in perl by etcshadow (Priest) on Sep 07, 2004 at 05:44 UTC
Hmm. I think what your saying is that the eval is necessary to avoid a warning if $var doesn't contain a value for which there is a label defined? No, I'm saying that `goto $label` dies if the label has not been defined. That is not how it works in C. In C, if you have not provided a `case` that matches your switch value, it jumps to the default case. That's why there's an eval, to prevent the die (not warning). `[sstone@granite sstone]$ perl -le 'print 1; goto "FOO"; print 2' 1 Can't find label FOO at -e line 1. [sstone@granite sstone]$` [download] As far as preventing a warning on undef, well... I'd imagine that that should fit into whatever scheme you are using around undefs: warn if you have undef warnings on, else not. And there's nothing about stringifying $var that changes that semantic, so all seems well and good with my method. `------------ :Wq Not an editor command: Wq` [download]	[reply] [d/l] [select]
Re^7: fall through switch/case in perl by BrowserUk (Patriarch) on Sep 07, 2004 at 05:57 UTC
Fair enough, but in TimToady's original, he had `'CASE'.($var+0)` which avoids dieing. `P:\test>perl -le" print 1; $var = 'foo'; goto 'CASE'.($var+0); CASE0: +print 2;" 1 2` [download] And that allows `CASE0:` to become the default. The only situation this doesn't cover is when $var is undef'd; but then none of the other solutions handle that (except the `do{ local $^W; $var + 0 };` I suggested). Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail "Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon	[reply] [d/l] [select]
Re^8: fall through switch/case in perl by etcshadow (Priest) on Sep 07, 2004 at 06:34 UTC
Re^9: fall through switch/case in perl by BrowserUk (Patriarch) on Sep 07, 2004 at 06:53 UTC
Some notes below your chosen depth have not been shown here
Re^7: ** fall through switch/case in perl by Anonymous Monk on Sep 07, 2004 at 08:43 UTC
`goto qw [CASE1 CASE2 CASE3 CASE4 CASE5 CASE6 CASE7 CASE8 CASE9 CASE10] [$var - 1] \|\| "default"; CASE10: print "a"; CASE9: print "b"; CASE8: print "c"; CASE7: print "d"; CASE6: print "e"; CASE5: print "f"; CASE4: print "g"; CASE3: print "h"; CASE2: print "i"; CASE1: print "j"; print "\n"; default:` [download]	[reply] [d/l]
Re^8: ** fall through switch/case in perl by Beechbone (Friar) on Sep 08, 2004 at 13:59 UTC