Logical Operators AND OR vs. && ||

halojoy

Why do not
all these work

<?php

//Notice: Undefined variable: y .. probably in test()
$y = run($x) || $do && test($y); // does not work

$y = run($x) OR $do && test($y); // works
$y = run($x) OR $do AND test($y); // works

?>

Can you explain to me?
Thanks

laserlight

How does it not work? In fact, it looks like all three logical expressions are equivalent.

NogDog

"&&" and "||" have a higher precedence than "=", which has a higher precedence than "and" and "or". Therefore these two statements are equivalent:

$y = run($x) || $do && test($y);
$y = (run($x) or $do and test($y));

While these two would be the same:

($y = run($x)) || $do && test($y);
$y = run($x) or $do and test($y);

This is why "or" is used with the common idiom, as you want the mysql_query() result should be assigned to $result:

$result = mysql_query($sql) or die(mysql_error());

halojoy

//Notice: Undefined variable: y
.. probably in test() something
$y = run($x) || $do && test($y); // does not work
..... which indicates $y has no value when test() is run.

But in the other cases, $y has got its value by time test() is run

I think it can have to do with Precedence of OR/AND vs. ||/&&

http://phpbuilder.com/manual/en/language.operators.logical.php :

The reason for the two different variations of "and" and "or" operators
is that they operate at different precedences. (See Operator Precedence.)

Precendence ranking:
http://phpbuilder.com/manual/en/language.operators.php
But still I dont understand.
The AND/OR have same mutual relational precende as ||/&&

laserlight

NogDog wrote:
"&&" and "||" have a higher precedence than "=", which has a higher precedence than "and" and "or".

Oh dear, I forgot about the copy assignment operator.

halojoy wrote:
I think it can have to do with Precedence of OR/AND vs. ||/&&
(...)
The AND/OR have same mutual relational precende as ||/&&

It does, but as NogDog explained, it really has to do with =. Indeed, when you presented your example I stated that the logical expressions are equivalent, and they certainly are, if you exclude the assignment.

By the way, next time please provide the smallest and simplest script that demonstrates the problem, and also state what exactly is the problem. Although it is often possible to reason out the problem by inspection even when its description is incomplete, being able to run the script to confirm or deny the observation helps.