Nikhil Learn Hub - Bit Manipulation Tricks in C

Essential Bit Manipulation Tricks

#	Trick	Description
1	Check if number is even/odd	Use (num & 1) - returns 1 for odd, 0 for even
2	Test if n-th bit is set	Use (num & (1 << n)) - non-zero means set
3	Set n-th bit	Use num \| (1 << n) to set specific bit
4	Unset n-th bit	Use num & ~(1 << n) to clear specific bit
5	Toggle n-th bit	Use num ^ (1 << n) to flip specific bit
6	Turn off rightmost set bit	Use num & (num - 1)
7	Isolate rightmost set bit	Use num & -num
8	Right propagate rightmost set bit	Use num \| (num - 1)
9	Count set bits (popcount)	Use __builtin_popcount() or manual counting
10	Swap two numbers	Use XOR: a ^= b; b ^= a; a ^= b;
11	Check power of 2	Use (num & (num - 1)) == 0
12	Absolute value without branching	Use mask = num >> 31; (num + mask) ^ mask;
13	Find log2 of a number	Count leading zeros or use bit scanning
14	Reverse bits of a number	Use lookup table or bit-by-bit reversal
15	Find next power of 2	Use --num; num \|= num >> 1; num \|= num >> 2; etc.

Implementation Examples

Basic Bit Operations

// Set, clear, toggle, check bits
unsigned int num = 0;

// Set 3rd bit
num |= (1 << 3);

// Clear 2nd bit
num &= ~(1 << 2);

// Toggle 5th bit
num ^= (1 << 5);

// Check if 4th bit is set
if (num & (1 << 4)) {
    printf("Bit 4 is set\n");
}

Count Set Bits

// Count number of 1 bits (popcount)
int countSetBits(unsigned int n) {
    int count = 0;
    while (n) {
        n &= (n - 1); // clears least significant set bit
        count++;
    }
    return count;
}

// Built-in version (GCC)
int builtinCount = __builtin_popcount(n);

Swap Without Temp

// Swap two numbers without temp
void swap(int *a, int *b) {
    *a ^= *b;
    *b ^= *a;
    *a ^= *b;
}

// Alternative method
void swapAdd(int *a, int *b) {
    *a = *a + *b;
    *b = *a - *b;
    *a = *a - *b;
}

Fast Power of 2 Check

// Check if number is power of 2
int isPowerOfTwo(unsigned int n) {
    return n && !(n & (n - 1));
}

// Get next power of 2
unsigned int nextPowerOf2(unsigned int n) {
    n--;
    n |= n >> 1;
    n |= n >> 2;
    n |= n >> 4;
    n |= n >> 8;
    n |= n >> 16;
    n++;
    return n;
}