Set Clear Toggle a Specific Bit in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after setting the 3rd bit?

Given the initial integer value 8 (binary 1000), what is the output after setting the 3rd bit (0-based from right)?

DSA C

int x = 8; // binary 1000
int bit = 3;
x = x | (1 << bit);
printf("%d\n", x);

A 8

B 12

C 16

D 24

💡 Hint

Remember that bits are counted from 0 starting at the rightmost bit.

Initial value 8 (binary 1000). Bit positions (0-based from right): bit 0=0, bit 1=0, bit 2=0, bit 3=1. Setting bit 3: 1 << 3 = 8 (binary 1000). 1000 | 1000 = 1000 (decimal 8).

❓ Predict Output

intermediate

What is the output after clearing the 2nd bit?

Given the integer value 15 (binary 1111), what is the output after clearing the 2nd bit (0-based from right)?

DSA C

int x = 15; // binary 1111
int bit = 2;
x = x & ~(1 << bit);
printf("%d\n", x);

A 7

B 15

C 11

D 8

💡 Hint

Clearing a bit means setting it to 0 using AND with the negation of the bit mask.

Clearing bit 2 means turning off the bit at position 2. 1 << 2 is 4 (binary 0100). Negation is 1011. 1111 & 1011 = 1011 (decimal 11).

❓ Predict Output

advanced

What is the output after toggling the 1st bit?

Given the integer value 10 (binary 1010), what is the output after toggling the 1st bit (0-based from right)?

DSA C

int x = 10; // binary 1010
int bit = 1;
x = x ^ (1 << bit);
printf("%d\n", x);

A 14

B 8

C 10

D 12

💡 Hint

Toggling a bit flips it: if it was 1, it becomes 0; if 0, it becomes 1.

Toggling bit 1 means XOR with 1 << 1 (2). 10 (1010) XOR 2 (0010) = 8 (1000).

🧠 Conceptual

advanced

Which operation clears a specific bit in an integer?

You want to clear (set to 0) the bit at position k in an integer x. Which of the following operations correctly does this?

A x = x | (1 << k);

B x = x & (1 << k);

C x = x ^ (1 << k);

D x = x & ~(1 << k);

💡 Hint

Clearing a bit means making sure that bit is 0 while keeping others unchanged.

Using AND with the negation of the bit mask clears the bit at position k.

🔧 Debug

expert

Why does this code fail to toggle the 4th bit?

Consider this code snippet intended to toggle the 4th bit of x: int x = 20; int bit = 4; x = x ^ 1 << bit; printf("%d\n", x); Why might this code not work as expected?

DSA C

int x = 20;
int bit = 4;
x = x ^ 1 << bit;
printf("%d\n", x);

A Operator precedence causes the XOR to apply before the shift, so the wrong bit is toggled.

B The bit index 4 is out of range for an int, causing undefined behavior.

C The code is missing parentheses around the shift operation, causing a syntax error.

D The variable x is not initialized before use.

💡 Hint

Check operator precedence between ^ and << in C.

In C, << has higher precedence than ^, so x ^ 1 << bit is evaluated as (x ^ 1) << bit, which is not intended. Parentheses are needed: x ^ (1 << bit).

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