C Program to Set nth Bit of a Number

To set the nth bit of a number in C, use the bitwise OR operator with a mask: number |= (1 << n); where n is the bit position starting from 0.

Examples

Inputnumber = 8, n = 1

Output10

Inputnumber = 0, n = 3

Output8

Inputnumber = 15, n = 4

Output31

How to Think About It

To set the nth bit of a number, think of the number in binary form. You create a mask that has only the nth bit set to 1 by shifting 1 to the left n times using 1 << n. Then, combine this mask with the original number using the bitwise OR operator |=. This ensures the nth bit is set to 1 without changing other bits.

Algorithm

1

Get the input number and the bit position n.

2

Create a mask by shifting 1 to the left by n positions.

3

Use bitwise OR to set the nth bit of the number with the mask.

4

Return or print the updated number.

Code

c

#include <stdio.h>

int setNthBit(int number, int n) {
    return number | (1 << n);
}

int main() {
    int number = 8, n = 1;
    int result = setNthBit(number, n);
    printf("Number after setting %dth bit: %d\n", n, result);
    return 0;
}

Output

Number after setting 1th bit: 10

Dry Run

Let's trace setting the 1st bit of number 8 through the code.

1

Initial values

number = 8 (binary 1000), n = 1

2

Create mask

mask = 1 << 1 = 2 (binary 0010)

3

Set nth bit

result = number | mask = 8 | 2 = 10 (binary 1010)

Step	Operation	Value
1	number	8 (1000)
2	mask = 1 << n	2 (0010)
3	result = number | mask	10 (1010)

Why This Works

Step 1: Create mask with nth bit set

Using 1 << n shifts 1 left by n bits, creating a mask with only the nth bit set to 1.

Step 2: Combine mask with number

The bitwise OR | sets the nth bit of the number to 1 without changing other bits.

Step 3: Return updated number

The result is the original number with the nth bit guaranteed to be set.

Alternative Approaches

Using a function with pointer to modify in place

c

#include <stdio.h>

void setNthBitInPlace(int *number, int n) {
    *number |= (1 << n);
}

int main() {
    int num = 8;
    setNthBitInPlace(&num, 1);
    printf("Number after setting %dth bit: %d\n", 1, num);
    return 0;
}

This modifies the original number directly using a pointer, useful when you want to change the variable without returning a new value.

Using inline macro for setting nth bit

c

#include <stdio.h>
#define SET_NTH_BIT(num, n) ((num) | (1 << (n)))

int main() {
    int number = 8;
    int result = SET_NTH_BIT(number, 1);
    printf("Number after setting %dth bit: %d\n", 1, result);
    return 0;
}

Using a macro can make the code concise but may reduce type safety and debugging clarity.

Complexity: O(1) time, O(1) space

Time Complexity

Setting a bit uses a single bitwise operation, so it runs in constant time regardless of input size.

Space Complexity

No extra memory is needed beyond a few variables, so space complexity is constant.

Which Approach is Fastest?

All approaches use simple bitwise operations with constant time and space; using a macro or inline function may be slightly faster due to inlining.

Approach	Time	Space	Best For
Function returning new value	O(1)	O(1)	Clear and reusable code
Function modifying in place	O(1)	O(1)	When original variable must be changed
Macro	O(1)	O(1)	Concise code, inline expansion