Bitwise operations on boolean values
From my knowledge, bitwise operators perform a check on all corresponding bits, as in:
echo 64 | 32; //prints 96
echo 'a' & 'b'; //prints `
While conditional && and || operators perform operations on boolean values:
echo (int)(true && false); //prints: 0
echo (int)(true || false); //prints: 1
When I (in my head) want to predict the result of a bitwise operation, I first convert the values into their binary representation (which depends on the datatype). After this, I compare it bit-for-bit, and convert the result into the suitable output type (which I suppose is determined by the operands). Although at one point, I tried doing the same with boolean values, which (from my knowledge) only consists of one bit in memory, making true corresponding to 1₂ , and making false corresponding to 0₂ (in binary). Performing bitwise operations on these values should therefore produce a similar result as with && and || , right? To show you what I mean:
true & false => 1₂ & 0₂ => 0₂ => false
true | false => 1₂ | 0₂ => 1₂ => true
~true => ~1₂ => 0₂ => false
(Not including xor , as there's no corresponding conditional boolean operator.)
To me, it looks like the behaviour should be really equivalent to conditional operators:
true && false => false
true || false => true
!true => false
So therefore, I set this code up to test it:
echo "true AND false: " . ((true && false) ? "1" : "0") . "<br />n";
echo "true OR false: " . ((true || false) ? "1" : "0") . "<br />n";
echo "NOT true: " . ((!true) ? "1" : "0") . "<br />n";
echo "<br />n";
echo "true BITAND false: " . ((true & false) ? "1" : "0") . "<br />n";
echo "true BITOR false: " . ((true | false) ? "1" : "0") . "<br />n";
echo "BITNOT true: " . ((~true) ? "1" : "0") . "<br />n";
It gave me the following output:
true AND false: 0
true OR false: 1
NOT true: 0
true BITAND false: 0
true BITOR false: 1
Fatal error: Unsupported operand types in C:Abyss Web Serverhtdocshandler.php on line 21
So from this, I have two questions:
&& and || , if we (as it seems) can use & and | on boolean values instead? ~true (or with other words, why aren't boolean values supported)? To me, it sounds logical that ~true returns false . I did come up with one thing, being that && and || will (sometimes) cast the values into bool and thereafter return the correct results, if we (by mistake) would happen to pass a value that's not of type bool . But to solve that, can't we just do a cast first? Such as:
if ((bool)$foo & (bool)$bar) { ...
It feels like I'm missing a major piece here that changes everything... But just in case I didn't, I included as much info as I could. Could someone make me understand this a bit better, by answering my two questions? I'm pretty confused at this point, and I've been thinking about this for quite some time.
Answer 1
Parts of a boolean expressions ( || , && , ! , ...) are only evaluated if needed (from left to right):
if ($a | func()) { } // func is always called
if ($a || func()) { } // func is not called if $a is true,
// because expression is true whatever func will return
if ($a && func()) { } // func is not called if $a is false,
// because expression is false whatever func will return
func() || exit(); // exit() will be called if func() returns false
Take a look at the documentation: http://php.net/manual/en/language.operators.logical.php
Answer 2
~true seems not to be meaningful: true is 0x00...01 and ~true will be 0xff...fe and not false 0x000...0 :
var_dump(~((int)true)); // prints: int(-2)
echo dechex(~((int)true)); // prints: fffffffffffffffe
Use ! -operator instead:
var_dump(!true); // prints: bool(false)
Résumé
Use bitwise operators only if you need to change bits.
true and false boolean flags, albeit being stored in memory as 32-bit or 64-bit values, should be considered just as two-states boolean values. You'll end up using it only on brach guards so you should not make arithmetic over them. && and || evaluates as boolean flags, bitwise operators & and | evaluates as the same of the operands (eg int & int evaluates as int , int && int evaluates as boolean).
the story is: use && and || when you have to take decisions over some conditions. Use & and | to perform boolean arithmetic over values.
that's C++
C doesn't have any built-in boolean values so any non zero value is a true , and zero is false .
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