Bit hacks

Learn fast bit hacks and tricks.

Swap with XOR

You can swap two variables without a temporary using three XOR operations. This is known as the XOR swap.

Example: a ^= b; b ^= a; a ^= b;. After three steps, a and b exchange values. In the first step a becomes a ^ b. In the second step b gets the old value of a. In the third step a gets the old value of b. Though cool, on modern CPUs swapping with a temporary is faster; XOR swap is more of a curiosity and an embedded trick.

min with ternary

Bitwise operations are often combined with the ternary operator (?:). It is a short way to assign a conditional value in one line.

Example: int min = (a < b) ? a : b;. Here, if a < b is true, min becomes a; otherwise b. This is shorter than writing if-else. Combined with bit hacks it is a powerful tool; for instance, after an XOR swap you can find the smaller value in one line. Ternary is ideal for producing conditional values in expression context and in macros.

Exercise Write XOR swap and min

Declare int a = 7, b = 3;. Swap with a ^= b; b ^= a; a ^= b;. Print with printf("a=%d b=%d\n", a, b);. Compute int min = (a < b) ? a : b;. Print with printf("min=%d\n", min);.

Show example solution

#include <stdio.h>

int main(void) {
    int a = 7, b = 3;
    a ^= b; b ^= a; a ^= b;
    printf("a=%d b=%d\n", a, b);
    int min = (a < b) ? a : b;
    printf("min=%d\n", min);
    return 0;
}

Test yourself

How many steps does the XOR swap take?
- One
- Two
- Three
- Four
Show answer

Three — The XOR swap uses three ^= operations.
Match the bit hacks

a ^= b

XOR swap step
(a<b)?a:b

Smaller value
^

Bitwise XOR
?:

Ternary

Complete the XOR swap

a ^= b; b ^= a; a ^= ____;

Show answer

a ^= b; b ^= a; a ^= b;

Next lesson: Pack and unpack

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