I'm a little confused when I see the output of following code:
$x = "a";
$y = "b";
$x ^= $y;
$y ^= $x;
$x ^= $y;
echo $x; //Got b
echo $y; //Got a
How does the operator ^ work here?
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Peter Mortensen
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Young
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3Answered in http://stackoverflow.com/questions/249423/how-does-xor-variable-swapping-work – Sebastian Paaske Tørholm Apr 20 '10 at 12:17
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Are you asking how the operator works or how the swap works? – SLaks Apr 20 '10 at 12:21
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@ Sebastian P,I've got it.Thanks. – Young Apr 20 '10 at 12:24
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3FYI: strings get truncated if they contain different number of characters – Geo Jan 09 '13 at 16:01
6 Answers
20
^ is the "exclusive or" bitwise operator. It reads in English as "either or". The result is 1 if and only if both bits differ:
1 ^ 0 = 1
1 ^ 1 = 0
0 ^ 0 = 0
Simplifying the example a bit so (and using Pseudo code):
$x = 0011 //binary
$y = 0010
$x = $x xor $y
//Result: x = 0001
//x = 0001
//y = 0010
$y = $y xor $x
//Result: y = 0011
//x = 0001
//y = 0011
$x = $x xor $y
//Result: x = 0010
All that PHP has done is treat the string "a" and "b" as their integer equivalents.
Yacoby
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This looks like swapping a value using XOR. Though I am not sure about the strings in PHP (normally you use it for ints or something). For a truth table of XOR you can look here.
The interesting thing about XOR is that it is reversable: A XOR B XOR B == A ... that is not working with AND or OR. Because of this fact, it can be used as in your example to swap two values:
$x ^= $y;
$y ^= $x;
$x ^= $y;
means:
$x = $x ^ $y
$y = $y ^ ($x ^ $y) // = $x
$x = ($x ^ $y) ^ ($y ^ ($x ^ $y)) // = $y
Peter Mortensen
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tanascius
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5PHP is dynamically typed and hates strings - it will convert them to int or double whenever an opportunity presents itself. – Michael Borgwardt Apr 20 '10 at 12:22
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@Michael: Thanks for pointing this out - I didn't know it and just assumed that something like this happens ^^ – tanascius Apr 20 '10 at 12:26
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In this example, when you're using ^ characters, they are casted to integers. So
"a" ^ "b"
is the same as:
ord("a") ^ ord ("b")
with one exception. In the first example, the result was casted back to a string. For example:
"a" ^ "6" == "W"
because of:
ord("a") ^ ord("6") == 87
and
chr(87) == "W"
Peter Mortensen
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Denis Bazhenov
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Th ^ operator is a bitwise operator, meaning that it operates on every bit of its operands.
It returns a value in which each bit is 1 if the two corresponding bits in the operands are unequal, and 0 if they're equal.
For example:
100110110 ^ 010001100 = 110111010
SLaks
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In the example code for the question the operands are *strings*. What actually happens? For instance, are the ASCII values used? What if strings are longer than one? – Peter Mortensen Dec 28 '15 at 16:03
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The ^ operator performs an XOR on the bit values of each variable. XOR does the following:
a = 1100
b = 1010
xor = 0110
x is the result of the XOR operation. If the bits are equal the result is 0 if they are different the result is 1.
In your example the ^= performs XOR and assignment, and you swap the bits around between the two variables $x and $y.
Read more here http://en.wikipedia.org/wiki/Xor_swap_algorithm
Justin
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XOR or the exclusive or is based on logic and circuits. It indicates that, for example, A ^= B where A is 0111 and B is 0101 can be either 1 or 0 at each corresponding bit but not both. Therefore
A = 0111
B = 0101
_____
^= 0010
To understand this better the rules of binary math apply except that there are no carry overs. So in binary math 1 + 0 = 1, 0 + 0 = 0, 0 + 1 = 1 and 1 + 1 = 0 (where a 1 is carried over to the next more significant position in binary math, but the XOR rules bypass this).
Note: That the XOR rules, therefore, allow you to take the result of A ^= B in the example above and add A to it to get B or add B to it to get A (referencing the swap ability mentioned above.
