How to Use Left Shift Operator in C: Syntax and Examples

In C, the left shift operator << shifts the bits of an integer to the left by a specified number of positions, effectively multiplying the number by 2 for each shift. Use it as value << number_of_positions to move bits left and fill with zeros on the right.

Syntax

The left shift operator in C is written as <<. It takes two operands: the value to shift and the number of bit positions to shift.

• value: An integer whose bits you want to shift.
• number_of_positions: How many bits to move to the left.

The bits shifted out on the left are discarded, and zeros fill in from the right.

c

result = value << number_of_positions;

Example

This example shows how left shifting a number by 1 doubles it, and shifting by 3 multiplies it by 8.

c

#include <stdio.h>

int main() {
    int value = 5; // binary 00000101
    int shifted1 = value << 1; // shift left by 1 bit
    int shifted3 = value << 3; // shift left by 3 bits

    printf("Original value: %d\n", value);
    printf("Value << 1: %d\n", shifted1);
    printf("Value << 3: %d\n", shifted3);

    return 0;
}

Output

Original value: 5
Value << 1: 10
Value << 3: 40

Common Pitfalls

Common mistakes when using left shift in C include:

• Shifting by a number greater or equal to the bit width of the type (e.g., 32 or more for 32-bit int) causes undefined behavior.
• Using left shift on negative numbers can lead to unexpected results because of how signed integers are stored.
• Assuming left shift always multiplies by powers of two without considering overflow.

c

#include <stdio.h>

int main() {
    int x = 1;
    // Wrong: shifting by 32 on a 32-bit int is undefined
    // int y = x << 32;

    // Correct: shift less than bit width
    int z = x << 4; // shifts by 4 bits

    printf("Shifted by 4: %d\n", z);
    return 0;
}

Output

Shifted by 4: 16

Quick Reference

Operation	Description	Example
value << n	Shift bits of value left by n positions	5 << 1 = 10 (5 * 2)
Bits shifted out	Discarded from left	N/A
New bits	Filled with zeros on right	N/A
Shift limit	Must be less than bit width	int: 0 to 31 on 32-bit system
Effect on negative	May cause undefined or unexpected results	Avoid shifting negative numbers

Key Takeaways

Use << to shift bits left, multiplying by 2 for each shift.

Never shift by a number equal or larger than the bit width of the type to avoid undefined behavior.

Left shifting negative numbers can cause unexpected results; use with caution.

Bits shifted out on the left are lost; zeros fill in from the right.

Left shift is a fast way to multiply by powers of two but watch for overflow.