Left shift and right shift in C

Introduction

Left shift and right shift move the bits of a number left or right. This helps multiply or divide numbers quickly by powers of two.

When you want to multiply a number by 2, 4, 8, etc., quickly.

When you want to divide a number by 2, 4, 8, etc., quickly (for positive numbers).

When you need to manipulate individual bits in low-level programming.

When optimizing code for speed in embedded systems or games.

Syntax

C

value << n;  // shifts bits of value to the left by n places
value >> n;  // shifts bits of value to the right by n places

Left shift (<<) moves bits to the left, adding zeros on the right.

Right shift (>>) moves bits to the right, discarding bits on the right.

Examples

Shifting 3 left by 2 bits multiplies it by 4.

C

int x = 3; // binary 00000011
int y = x << 2; // shifts left by 2 bits
// y is now 12 (00001100)

Shifting 16 right by 3 bits divides it by 8.

C

int x = 16; // binary 00010000
int y = x >> 3; // shifts right by 3 bits
// y is now 2 (00000010)

Sample Program

This program shows how shifting left multiplies by 2 and shifting right divides by 2.

C

#include <stdio.h>

int main() {
    int a = 5; // binary 00000101
    int left_shift = a << 1; // multiply by 2
    int right_shift = a >> 1; // divide by 2

    printf("Original value: %d\n", a);
    printf("After left shift by 1: %d\n", left_shift);
    printf("After right shift by 1: %d\n", right_shift);

    return 0;
}

Important Notes

Right shifting negative numbers may behave differently depending on the system (arithmetic vs logical shift).

Shifting by more bits than the size of the data type can cause undefined behavior.

Summary

Left shift (<<) moves bits left, multiplying by powers of two.

Right shift (>>) moves bits right, dividing by powers of two for positive numbers.

Useful for fast math and bit-level operations.

Practice

1. What does the left shift operator << do to an integer in C?

easy

A. Moves bits to the left, multiplying the number by powers of two

B. Moves bits to the right, dividing the number by powers of two

C. Reverses the bits of the number

D. Sets all bits to zero

Solution

1. Step 1: Understand left shift operation

   The left shift operator moves all bits in the number to the left by the specified count.

2. Step 2: Effect on the number's value

   Each left shift multiplies the number by 2, so shifting left by n multiplies by 2^n.

3. Final Answer:

   Moves bits to the left, multiplying the number by powers of two -> Option A

4. Quick Check:

   Left shift = multiply by 2^n [OK]

Hint: Left shift multiplies by 2 to the power of shift count [OK]

Common Mistakes:

• Confusing left shift with right shift
• Thinking left shift divides the number
• Assuming left shift reverses bits

2. Which of the following is the correct syntax to shift the integer variable x right by 3 bits in C?

easy

A. x >>> 3;

B. x >> 3;

C. x << 3;

D. x >>3

Solution

1. Step 1: Identify the right shift operator in C

   The right shift operator is >> and requires spaces or no spaces around it.

2. Step 2: Check syntax correctness

   x >> 3; is correct syntax. x << 3; is left shift, x >>> 3; is invalid in C, and x >>3 is missing a semicolon.

3. Final Answer:

   x >> 3; -> Option B

4. Quick Check:

   Right shift syntax = x >> n; [OK]

Hint: Right shift uses >> with semicolon to end statement [OK]

Common Mistakes:

• Using >>> which is not valid in C
• Confusing left shift << with right shift >>
• Omitting semicolon at end

3. What is the output of the following C code?

int x = 4;
int y = x << 2;
printf("%d", y);

medium

A. 0

B. 1

C. 8

D. 16

Solution

1. Step 1: Understand the initial value of x

   x is 4, which in binary is 00000100.

2. Step 2: Apply left shift by 2 bits

   Shifting left by 2 bits multiplies 4 by 2^2 = 4 * 4 = 16.

3. Final Answer:

   16 -> Option D

4. Quick Check:

   4 << 2 = 16 [OK]

Hint: Left shift by n multiplies by 2^n [OK]

Common Mistakes:

• Calculating 4 << 2 as 8 instead of 16
• Confusing left shift with right shift
• Ignoring the shift count

4. Find the error in this C code snippet:

int a = 32;
int b = a >> 2
printf("%d", b);

medium

A. Wrong operator used for right shift

B. Incorrect printf format specifier

C. Missing semicolon after right shift operation

D. Variable b is not declared

Solution

1. Step 1: Check syntax line by line

   The line int b = a >> 2 is missing a semicolon at the end.

2. Step 2: Verify other parts

   The operator >> is correct, printf uses %d for int, and b is declared properly.

3. Final Answer:

   Missing semicolon after right shift operation -> Option C

4. Quick Check:

   Every statement needs semicolon [OK]

Hint: Check for missing semicolons after shift operations [OK]

Common Mistakes:

• Assuming >> is invalid operator
• Thinking printf format is wrong
• Ignoring missing semicolon errors

5. Given int x = 1;, what is the value of y after this operation?

int y = (x << 3) >> 1;

hard

A. 4

B. 8

C. 16

D. 2

Solution

1. Step 1: Calculate left shift (x << 3)

   Left shifting 1 by 3 bits multiplies 1 by 2^3 = 8.

2. Step 2: Calculate right shift of result by 1

   Right shifting 8 by 1 bit divides 8 by 2^1 = 2, so 8 / 2 = 4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   (1 << 3) >> 1 = 8 >> 1 = 4 [OK]

Hint: Left shift then right shift: multiply then divide by powers of two [OK]

Common Mistakes:

• Calculating (1 << 3) as 16
• Ignoring right shift effect
• Mixing up shift directions