backwards if and while looping

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

Replies are listed 'Best First'.
Re: backwards if and while looping by Grygonos (Chaplain) on Oct 01, 2004 at 19:51 UTC
Putting it on one line will illuminate the problem. `while ($a < $b) if not $error{$a++};` It thinks error is a hash since immediately following it (and owe to perls ignoring whitespaces) is a opening curly a var and a closing curly.. effectively referencing the $a+1 key of the hash `%error`. personally I don't see a better way than `if(!$error) { while($a < $b) { $a++; } }` [download] Grygonos	[reply] [d/l] [select]
Re: backwards if and while looping by blokhead (Monsignor) on Oct 01, 2004 at 20:35 UTC
I'm surprised no one has suggested this (I'll also assume your simple `$a++` in the loop is just for demonstration purposes): `while (not $error and $a < $b) { $a++; }` [download] blokhead	[reply] [d/l] [select]
Re^2: backwards if and while looping by Forsaken (Friar) on Oct 02, 2004 at 10:20 UTC
this will test $error in every instance of the loop, as opposed to only once before initiating the loop. it works, but it requires more cpu time...	[reply]
Re: backwards if and while looping by ikegami (Patriarch) on Oct 01, 2004 at 20:40 UTC
Some other alternatives not yet mentioned: `while (not $error and $a < $b) { $a++; }` [download] and `do { while ($a < $b) { $a++; } } if (not $error);` [download] and `do { $a++ while ($a < $b); } if (not $error);` [download]	[reply] [d/l] [select]
Re: backwards if and while looping by periapt (Hermit) on Oct 01, 2004 at 19:55 UTC
The line `while ($a < $b) if not $error { $a++; }` [download] is more clearly interpreted by perl to be `while ($a < $b) if not $error{ $a++; }`. Perl thinks $error{ $a++ } is a hash reference to %error. A construct that would DWYM would be (parenthesis not strictly necessary) `(!$error) && ($a++ while ($a < $b))` [download] PJ use strict; use warnings; use diagnostics; (if needed)	[reply] [d/l] [select]
Re^2: backwards if and while looping by jcpunk (Friar) on Oct 01, 2004 at 20:20 UTC
My dumbed down example seems to not be as excellent as I initially thought..... `(!$error) && (print "$data" while ($data = <input>))` [download] is erroring out on me with the print... Is this a fatal flaw in my plan? jcpunk all code is tested, and doesn't work so there :p (varient on common PM sig for my own ammusment)	[reply] [d/l]
Re^3: backwards if and while looping by BrowserUk (Patriarch) on Oct 01, 2004 at 22:44 UTC
`$error or print $data while $data = <input>;` [download] 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]
Re: backwards if and while looping by cLive ;-) (Prior) on Oct 01, 2004 at 19:58 UTC
Not really sure what you're trying to do, but 1) avoid $a and $b outside of sort subs and 2) if $a and $b are integers, you can simplify to: `$error or $a = $a < $b ? $b : $a;` [download] cLive ;-)	[reply] [d/l]
Re: backwards if and while looping by !1 (Hermit) on Oct 01, 2004 at 22:16 UTC
For all of these "why not" people, why not just do: `$a = $b unless $error;` [download] I know it doesn't work correctly if $a > $b. In my defense, some of the other solutions don't either. Anyway, `while ($a < $b) if not $error { $a++; }` [download] looks pretty disgusting if you ask me. Whereas `$a++ while ! $error && $a < $b; if ( ! $error ) { $a++ while $a < $b; } do { $a++ while $a < $b } if ! $error;` [download] all look a bit more elegant. Also `$a++ while not $error \|\| $a >= $b; $a = $b unless $error or $a >= $b;` [download] are cool, IMHO.	[reply] [d/l] [select]
Re^2: backwards if and while looping by Zed_Lopez (Chaplain) on Oct 01, 2004 at 23:18 UTC
There's another case to account for... if $a or $b isn't integer, and their fractional parts differ (i.e. ($a - int $a) != ($b - int $b)) and $a < $b, then `$a = $b unless $error or $a >= $b;` (or similar examples in the thread) has a different result than the OP's code. I think it muddies the water to test $error every time through the loop, as some of these solutions do, when you only need to test it once. If $a or $b might be non-integer, here's one way to do it: `if ($a < $b and !$error) { $a = int $b + ($a - int $a); $a++ if $a < $b; }` [download] I left out doing the floating point comparisons correctly... Updated: Fixed a dumb error in the code.	[reply] [d/l] [select]
Re^3: backwards if and while looping by !1 (Hermit) on Oct 02, 2004 at 00:28 UTC
Yeah, but who uses real numbers? ;-P	[reply]
Re^4: backwards if and while looping by Zed_Lopez (Chaplain) on Oct 02, 2004 at 05:27 UTC
Re: backwards if and while looping by johnnywang (Priest) on Oct 01, 2004 at 19:56 UTC
`while ($a < $b) if not $error { $a++; }` [download] is the same as `while ($a < $b) if not $error{ $a++; }` [download] so the parser thinks you are asking for %error with key "$a++;". Why not: `(not $error) and $a = $b;` [download]	[reply] [d/l] [select]
Re: backwards if and while looping by talexb (Chancellor) on Oct 01, 2004 at 21:39 UTC
Sorry, have I missed something? .. Can't you just write `if ( not $error ) { $a = $b }` [download] and be done with it? Why do you have to increment `$a` up to `$b` until they're equal? Alex / talexb / Toronto "Groklaw is the open-source mentality applied to legal research" ~ Linus Torvalds	[reply] [d/l] [select]

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