Hi,
I wonder how PHP handles true/false comparison internally. I understand that true is defined as 1 and false is defined as 0. When I do if("a"){ echo "true";} it echos "true". How does PHP recognize "a" as 1 ?
+2
A:
Zero is false, nonzero is true.
In php you can test more explicitly using the === operator.
if (0==false)
echo "works"; // will echo works
if (0===false)
echo "works"; // will not echo anything
Byron Whitlock
2010-03-04 20:30:10
+12
A:
This is covered in the PHP documentation for boolean.
When converting to boolean, the following values are considered FALSE:
- the boolean FALSE itself
- the integer 0 (zero)
- the float 0.0 (zero)
- the empty string, and the string "0"
- an array with zero elements
- an object with zero member variables (PHP 4 only)
- the special type NULL (including unset variables)
- SimpleXML objects created from empty tags
Every other value is considered TRUE.
Mark Byers
2010-03-04 20:31:38
Actually, there are numerous values that convert to false, including empty arrays and null. Or were you referring just to strings?
sidereal
2010-03-04 20:34:09
@sidereal: I posted the whole extract now. Press refresh. ;-)
Mark Byers
2010-03-04 20:34:59
@Mark I blame ajax!
sidereal
2010-03-04 20:35:53
it's worth noting that these rules only apply when you *cast* to a boolean, not when you *compare* to it.
stereofrog
2010-03-04 21:27:55
@stereofrog, OK I've extended the quote to include that too.
Mark Byers
2010-03-04 21:45:19
A:
PHP uses weak typing (which it calls 'type juggling'), which is a bad idea (though that's a conversation for another time). When you try to use a variable in a context that requires a boolean, it will convert whatever your variable is into a boolean, according to some mostly arbitrary rules available here: http://www.php.net/manual/en/language.types.boolean.php#language.types.boolean.casting
sidereal
2010-03-04 20:32:34
That "bad idea" is purely subjective : once you know how this works, it is absolutly not a problem, in my opinion ;-) ;; and the conversion rules mostly don't seem quite un-logical to me ;-)
Pascal MARTIN
2010-03-04 20:36:05
Don't make me post a stackoverflow question about how bad an idea weak typing is ;). Consider that php manages to make a basic syllogism fail, because implicit type conversions make php equality non-transitive:echo ("true" == true)."\n"; //trueecho (true == 1)."\n"; // trueecho ("true" == 1)."\n"; // false :P
sidereal
2010-03-04 21:12:24
Yeah... it's a (potentially) very dangerous trick for new players, but not necessarily a bad idea. I'm not going to defend the robustness of code here, but sometimes it is a nice shortcut. Besides, if you're doing much in the way of larger scale work you'll probably use equality methods most of the time instead.
Keith Humm
2010-03-04 22:04:06
A:
think of operator as unary function: is_false(type value) which returns true or false, depending on the exact implementation for specific type and value. Consider if statement to invoke such function implicitly, via syntactic sugar.
other possibility is that type has cast operator, which turns type into another type implicitly, in this case string to Boolean.
PHP does not expose such details, but C++ allows operator overloading which exposes fine details of operator implementation.
aaa
2010-03-04 20:33:35
+1
A:
The best operator for strict checking is
if($foo === true){}
That way, you're really checking if its true, and not 1 or simply just set.
Citizen
2010-03-04 22:52:35