Set Clear Toggle a Specific Bit in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the 3rd bit of variable x.

DSA C

x = x | (1 << [1]);

Drag options to blanks, or click blank then click option'

A2

B3

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 instead of 2 for the bit position.

Using bitwise AND instead of OR.

Bits are zero-indexed from the right. To set the 3rd bit, shift 1 by 2 positions.

2fill in blank

medium

Complete the code to clear the 4th bit of variable y.

DSA C

y = y & ~(1 << [1]);

Drag options to blanks, or click blank then click option'

A4

B3

C2

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 4 instead of 3 for the bit position.

Forgetting to invert the mask with ~.

To clear the 4th bit, shift 1 by 3 positions and invert the mask before AND.

3fill in blank

hard

Fix the error in toggling the 5th bit of variable z.

DSA C

z = z ^ (1 << [1]);

Drag options to blanks, or click blank then click option'

A3

B5

C4

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using 5 instead of 4 for the bit position.

Using OR or AND instead of XOR for toggling.

To toggle the 5th bit, shift 1 by 4 positions because bits start at 0.

4fill in blank

hard

Fill both blanks to set bit 1 and clear bit 3 of variable a.

DSA C

a = (a | (1 << [1])) & ~(1 << [2]);

Drag options to blanks, or click blank then click option'

A1

B2

C3

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up bit positions for set and clear.

Not negating the mask when clearing.

Set bit 1 by shifting 1 by 1, clear bit 3 by shifting 1 by 3 and negating.

5fill in blank

hard

Fill all three blanks to toggle bit 0, set bit 2, and clear bit 4 of variable b.

DSA C

b = (b ^ (1 << [1])) | (1 << [2]); b = b & ~(1 << [3]);

Drag options to blanks, or click blank then click option'

A0

B2

C4

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong bit positions for toggle, set, or clear.

Forgetting to negate the mask when clearing.

Toggle bit 0 by shifting 1 by 0, set bit 2 by shifting 1 by 2, clear bit 4 by shifting 1 by 4 and negating.

Practice

1. What does the operation num |= (1 << k) do to the integer num?

easy

A. Toggles the k-th bit of num

B. Clears the k-th bit of num to 0

C. Checks if the k-th bit of num is set

D. Sets the k-th bit of num to 1

Solution

1. Step 1: Understand the bit shift operation

   1 << k creates a number with only the k-th bit set to 1 and others 0.

2. Step 2: Apply bitwise OR to set the bit

   Using num |= (1 << k) sets the k-th bit of num to 1 without changing other bits.

3. Final Answer:

   Sets the k-th bit of num to 1 -> Option D

4. Quick Check:

   Set bit = OR with 1 shifted [OK]

Hint: Use OR with 1 shifted to set a bit [OK]

Common Mistakes:

• Confusing set with clear operation
• Using AND instead of OR to set bits
• Misunderstanding bit positions starting from 0

2. Which of the following C statements correctly clears the k-th bit of an integer num?

easy

A. num = num << k;

B. num &= ~(1 << k);

C. num ^= (1 << k);

D. num |= (1 << k);

Solution

1. Step 1: Understand clearing a bit

   Clearing means setting the bit to 0. We use AND with the complement of the bit mask.

2. Step 2: Analyze the options

   num &= ~(1 << k); clears the k-th bit by ANDing with all 1s except 0 at k-th bit.

3. Final Answer:

   num &= ~(1 << k); -> Option B

4. Quick Check:

   Clear bit = AND with NOT of shifted 1 [OK]

Hint: Use AND with NOT of shifted 1 to clear bit [OK]

Common Mistakes:

• Using OR instead of AND to clear bits
• Forgetting to negate the mask
• Using XOR which toggles instead of clears

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

int num = 10; // binary 1010
int k = 1;
num ^= (1 << k);
printf("%d", num);

medium

A. 12

B. 10

C. 8

D. 14

Solution

1. Step 1: Understand initial value and bit positions

   num = 10 (binary 1010), k = 1 means second bit from right (0-based).

2. Step 2: Apply XOR to toggle the k-th bit

   Toggle bit 1: bit at position 1 is 1 in 1010, toggling it changes to 0, so binary becomes 1000 which is 8.

3. Final Answer:

   8 -> Option C

4. Quick Check:

   Toggle bit 1 in 1010 = 1000 = 8 [OK]

Hint: XOR toggles bit: 1 becomes 0, 0 becomes 1 [OK]

Common Mistakes:

• Confusing toggle with set or clear
• Miscounting bit positions starting from 1 instead of 0
• Assuming XOR always sets bit

4. The following code is intended to clear the 3rd bit of num. What is the error?

int num = 15; // binary 1111
int k = 3;
num = num & (1 << k);

medium

A. It clears all bits except the 3rd bit

B. It sets the 3rd bit instead of clearing

C. It toggles the 3rd bit instead of clearing

D. No error, code works correctly

Solution

1. Step 1: Analyze the bitwise AND operation

   The code does num & (1 << k), which keeps only the k-th bit and clears others.

2. Step 2: Understand the intended operation

   To clear the k-th bit, we must AND with the complement of the bit mask, not the mask itself.

3. Final Answer:

   It clears all bits except the 3rd bit -> Option A

4. Quick Check:

   AND with mask keeps only that bit, not clears it [OK]

Hint: Clear bit = AND with NOT of mask, not mask alone [OK]

Common Mistakes:

• Using mask instead of its complement to clear bit
• Confusing AND with OR for clearing bits
• Assuming AND with mask clears bit

5. You want to toggle the 0th and 2nd bits of an integer num simultaneously. Which expression correctly does this in C?

hard

A. num ^= (1 << 0) | (1 << 2);

B. num |= (1 << 0) & (1 << 2);

C. num &= ~( (1 << 0) | (1 << 2) );

D. num = (num << 0) ^ (num << 2);

Solution

1. Step 1: Understand toggling multiple bits

   Toggling bits uses XOR. To toggle multiple bits, XOR with mask having those bits set.

2. Step 2: Create mask for bits 0 and 2

   Mask is (1 << 0) | (1 << 2), which sets bits 0 and 2 to 1.

3. Step 3: Apply XOR with mask

   Expression num ^= (1 << 0) | (1 << 2); toggles both bits simultaneously.

4. Final Answer:

   num ^= (1 << 0) | (1 << 2); -> Option A

5. Quick Check:

   XOR with combined mask toggles multiple bits [OK]

Hint: XOR with ORed shifted bits toggles multiple bits [OK]

Common Mistakes:

• Using AND or OR instead of XOR for toggling
• Incorrectly combining masks with AND
• Misusing shift operations in toggle