Swapping numbers using XOR operator

[achuth]

Sir,
For swapping two numbers, say x and y, the following single statement will help in C.
x^=y^=x^=y;

But when I implemented this in a shell script, it is not working.
I used the following code in my shell script
$((x^=y^=x^=y))

When I echoed the variables to the screen, one number gets the swapped value but the other always gets assigned a value 0.

Please give me a working version of the above single statement in a shell script.

[Vaphell]

when you split this expression to pieces it works fine.


$ x=15; y=403
$ (( x^=y, y^=x, x^=y ))
$ echo x=$x y=$y
x=403 y=15

[Arndt]

Sir,
For swapping two numbers, say x and y, the following single statement will help in C.
x^=y^=x^=y;

But when I implemented this in a shell script, it is not working.
I used the following code in my shell script
$((x^=y^=x^=y))

When I echoed the variables to the screen, one number gets the swapped value but the other always gets assigned a value 0.

Please give me a working version of the above single statement in a shell script.

What you are doing in C is actually not well-defined, even though it happens to give the result you expect with your compiler. With warnings turned on, gcc reports this:


$ gcc -Wall -c xorr.c
xorr.c: In function ‘main’:
xorr.c:12: warning: operation on ‘x’ may be undefined

Where did you see this expression?

[Vaphell]

yup
http://en.wikipedia.org/wiki/XOR_swap_algorithm


Code example

A C function that implements the XOR swap algorithm:


void xorSwap (int *x, int *y) {
if (x != y) {
*x ^= *y;
*y ^= *x;
*x ^= *y;
}
}

Note that the code does not swap the integers passed immediately, but first checks if their addresses are distinct. This is because, if the addresses are equal, the algorithm will fold to a triple *x ^= *x resulting in zero.

The body of this function is sometimes seen incorrectly shortened to if (x != y) *x^=*y^=*x^=*y;. This code has undefined behavior, since it modifies the lvalue *x twice without an intervening sequence point.

[achuth]

What you mean by "intervening sequence point"?. Care to explain?

[MadCow108]

A sequence point in imperative programming defines any point in a computer program's execution at which it is guaranteed that all side effects of previous evaluations will have been performed, and no side effects from subsequent evaluations have yet been performed

http://en.wikipedia.org/wiki/Sequence_point

it boils down to:
the compiler is allowed to execute that expression in any order it wishes too and that may produce a result you did not intend.
the more classical example of this:

i = i++;
which value i has in the end is undefined. due to the lack of sequence points