Left shift and right shift in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of left shift operation

What is the output of this C code snippet?

C

int main() {
    unsigned int x = 5; // binary: 0000 0101
    unsigned int y = x << 2;
    printf("%u", y);
    return 0;
}

A 40

B 10

C 5

D 20

💡 Hint

Left shift by 2 moves bits two places to the left, multiplying by 4.

5 in binary is 00000101. Shifting left by 2 bits gives 00010100, which is 20 in decimal.

❓ Predict Output

intermediate

Output of right shift operation

What is the output of this C code snippet?

C

int main() {
    unsigned int x = 32; // binary: 0010 0000
    unsigned int y = x >> 3;
    printf("%u", y);
    return 0;
}

A 4

B 8

C 16

D 64

💡 Hint

Right shift by 3 moves bits three places to the right, dividing by 8.

32 in binary is 00100000. Shifting right by 3 bits gives 00000100, which is 4 in decimal.

🧠 Conceptual

advanced

Effect of left shift on signed integers

Consider the following code. What is the output and why?

C

int main() {
    int x = 1073741824; // 2^30
    int y = x << 2;
    printf("%d", y);
    return 0;
}

A -1073741824

B 1073741824

C 0

D 4294967296

💡 Hint

Left shift by 2 shifts bit 30 to bit 32, which is discarded in 32-bit int.

2^30 (0x40000000, bit 30 set) shifted left by 2 moves the bit to position 32 (discarded), resulting in a negative number due to overflow (two's complement).

🔧 Debug

advanced

Identify the error in right shift usage

What error will this code produce when compiled and run?

C

int main() {
    int x = -16;
    unsigned int y = x >> 2;
    printf("%u", y);
    return 0;
}

A Prints -4

B Compilation error due to mixing signed and unsigned types

C Prints a large positive number due to sign extension and conversion

D Runtime error: undefined behavior

💡 Hint

Right shifting a negative signed int keeps the sign bit, then converting to unsigned prints a large number.

Right shifting negative signed int keeps sign bits (arithmetic shift). Assigning to unsigned int prints the large unsigned value of that bit pattern.

🚀 Application

expert

Calculate the number of bits set after shifts

Given the code below, what is the number of bits set to 1 in variable z after execution?

C

int main() {
    unsigned int x = 0b1011; // decimal 11
    unsigned int y = x << 3;  // shift left by 3
    unsigned int z = y >> 2;  // shift right by 2
    int count = 0;
    for (int i = 0; i < 32; i++) {
        if ((z & (1 << i)) != 0) count++;
    }
    printf("%d", count);
    return 0;
}

A 2

B 3

C 4

D 1

💡 Hint

Track the bits step by step after each shift.

x = 0b1011 (bits set: 3). Left shift by 3: 0b1011000 (bits set: 3). Right shift by 2: 0b10110 (bits set: 3). So count is 3.

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