PHP comparison nature - adaptive or oblivious?

Question

What is the nature of PHP in terms of logical operators? Suppose a and b are expressions:

if (a || b) { body }

Which of the following is true?

PHP always evaluates truthfulness of a and b. (Oblivious nature), OR
PHP doesn't evaluate truthfulness of b if a is true (Adaptive nature and memory optimized: PHP evaluates truthfulness of b only if a is false).

It would take you 10 seconds to answer your own question *if only* you tested it. — user703016, Jan 07 '12 at 22:11
This question isn't about comparisons. It's about boolean operators. — Fred Foo, Jan 07 '12 at 22:12
You can't test: `if (a() || b())` - well, then look at least into the [PHP manual](http://php.net/manual/en/language.operators.logical.php). — hakre, Jan 07 '12 at 22:16
possible duplicate of [PHP short circuit lazy evaluation, where is it in the php.net manual?](http://stackoverflow.com/questions/3220919/php-short-circuit-lazy-evaluation-where-is-it-in-the-php-net-manual) — hakre, Jan 07 '12 at 22:17
No, it's not. This has to do with short-circuit evaluation of the boolean (logical) operators. Are you asking if PHP somehow caches the result of the conditional? How would that ever work then, they are variables?! — Jonathon Reinhart, Jan 07 '12 at 22:18
**How is this not testable?** An example of how to prove this is *right in the PHP manual!* — Jonathon Reinhart, Jan 07 '12 at 22:19
possible duplicate of [PHP Short-Circuit Evaluation](http://stackoverflow.com/questions/5694733/php-short-circuit-evaluation) — outis, Jan 07 '12 at 22:31
@Praveen: your use of "adaptive" and "oblivious" to describe the behavior of logical operators is not one I've seen before. Please cite your source. — outis, Jan 07 '12 at 22:32
@outis, http://www.youtube.com/watch?v=r7bgwYkgC4c, at minute 36.45. — P K, Jan 07 '12 at 22:34
@Praveen: In written form please, not some audio, preferable a published book. — hakre, Jan 07 '12 at 22:39
@harke, MIT.Introduction.to.Algorithms.3rd.Edition Page 227. "An oblivious compare-exchange algorithm operates solely by a sequence of prespecified compare-exchange operations".i may be using term 'oblivious'wrongly for a simple comparison, but for sorting algortihms this type of blind comparison is told as oblivious. — P K, Jan 07 '12 at 22:44
Wow, I still don't know what you are talking about. Compare-exchange sounds like the x86 CMPXCHG instruction? Either way I feel like you are getting topics gravely mixed up. Someone correct me if I'm mistaken. — Jonathon Reinhart, Jan 08 '12 at 04:46
@Jonathon.. this is not a math theorem that u want me to prove, sorry i can't help you.SEARCH meaning of Oblivious nature in dictionary or read the complete book that i pointed out. — P K, Jan 09 '12 at 07:11

score 12 · Accepted Answer · answered Jan 07 '12 at 22:17

12

<?php

if (print_r("Why don't you try?") || print_r("It's not that hard")) { }

?>

answered Jan 07 '12 at 22:17

user703016

37,307
8
87
112

score 1 · Answer 2 · edited May 23 '17 at 10:24

This is also known as Short-circuit evaluation, which PHP uses for logical operators.

For those not wanting to click the link above, this is adapted from the PHP manual:

// --------------------
// foo() will never get called as those operators are short-circuit

$a = (false && foo());
$b = (true  || foo());
$c = (false and foo());
$d = (true  or  foo());

function foo() { echo "PHP.net Lies!"; }

There are numerous SO questions on this as well:

Also, this has negligible impact on memory usage, provided nothing is being allocated in the code that is short-circuited.

I don't know what you mean by adaptive vs oblivious. Please cite something that explains. — Jonathon Reinhart, Jan 07 '12 at 22:23

PHP comparison nature - adaptive or oblivious?

2 Answers2