Tutorials - XOR Challenge
Solution to XOR Variable Swapping
So you have two variables, a and b; we'll assign a to be 5, and b to be 10.
We want to store the value of a in b, and the value of b in a, but without
using a temporary variable.
int a, b;
a = 5;
b = 10;
We can do this using bitwise XOR remembering the principle that two
applications of XOR to a variable will "cancel out". Here's a function that
will take pointers to two variables and flip them. For clarity, I'll refer to
the original values stored in a and b as a_orig and b_orig. Originally, *a
and *b will be a_orig and b_orig, but this will change over time.
void flip(int *a, int *b)
{
*a = *a ^ *b;
// Now, we can recover *a_orig by applying *a XOR *b_orig
*b = *a ^ *b;
// So we have the following situation:
// The value originally stored in *a, a_orig, is now in *b
// and *a still stores a_orig ^ b_orig
// This means that we can recover the value of b_orig by applying
// the XOR operation to *a and a_orig. Since *b stores a_orig...
*a = *a ^ *b
}
Starting out
