Swapping Values

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Swapping values with subtraction and addition

 #define SWAP(a, b) ((&(a) == &(b)) || \
                    (((a) -= (b)), ((b) += (a)), ((a) = (b) - (a))))

This swaps the values of a and b without using a temporary variable. The initial check for a and b being the same location in memory may be omitted when you know this can't happen. (The compiler may omit it anyway as an optimization.) If you enable overflows exceptions, then pass unsigned values so an exception isn't thrown. The XOR method that follows may be slightly faster on some machines. Don't use this with floating-point numbers (unless you operate on their raw integer representations).

Swapping values with XOR

 #define SWAP(a, b) (((a) ^= (b)), ((b) ^= (a)), ((a) ^= (b)))

This is an old trick to exchange the values of the variables a and b without using extra space for a temporary variable.

Swapping individual bits with XOR

 unsigned int i, j; // positions of bit sequences to swap
 unsigned int n;    // number of consecutive bits in each sequence
 unsigned int b;    // bits to swap reside in b
 unsigned int r;    // bit-swapped result goes here
 unsigned int x = ((b >> i) ^ (b >> j)) & ((1U << n) - 1); // XOR temporary
 r = b ^ ((x << i) | (x << j));

As an example of swapping ranges of bits suppose we have have b = 00101111 (expressed in binary) and we want to swap the n = 3 consecutive bits starting at i = 1 (the second bit from the right) with the 3 consecutive bits starting at j = 5; the result would be r = 11100011 (binary).

This method of swapping is similar to the general purpose XOR swap trick, but intended for operating on individual bits. The variable x stores the result of XORing the pairs of bit values we want to swap, and then the bits are set to the result of themselves XORed with x. Of course, the result is undefined if the sequences overlap.

References