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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after setting a bit?

Given the following C# code, what is the output printed to the console?

DSA C#

int number = 8; // binary 1000
int position = 1;
int result = number | (1 << position);
Console.WriteLine(result);

A 10

B 9

C 12

D 8

💡 Hint

Remember that setting a bit means turning that bit to 1 using OR operation.

The original number is 8 (binary 1000). Setting bit at position 1 (counting from 0) means turning the second bit from right to 1. 1 << 1 is 2 (binary 0010). OR operation 1000 | 0010 = 1010 which is 10 in decimal.

❓ Predict Output

intermediate

What is the output after clearing a bit?

What will be printed after clearing the bit at position 3 in the following code?

DSA C#

int number = 15; // binary 1111
int position = 3;
int result = number & ~(1 << position);
Console.WriteLine(result);

A 15

B 11

C 7

D 8

💡 Hint

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

Number 15 is 1111 in binary. Clearing bit at position 3 (the leftmost bit) means turning that bit to 0. 1 << 3 is 8 (1000). Negation is 0111 (7). AND operation 1111 & 0111 = 0111 which is 7 in decimal.

❓ Predict Output

advanced

What is the output after toggling a bit?

What will be printed after toggling the bit at position 2 in the following code?

DSA C#

int number = 5; // binary 0101
int position = 2;
int result = number ^ (1 << position);
Console.WriteLine(result);

A 1

B 3

C 5

D 7

💡 Hint

Toggling a bit flips it: 0 becomes 1, 1 becomes 0 using XOR.

Number 5 is 0101 in binary. Toggling bit at position 2 (third bit from right) flips it. 1 << 2 is 4 (0100). XOR 0101 ^ 0100 = 0001 which is 1 in decimal. But careful: position 2 bit is currently 1, toggling flips it to 0, so result is 1.

❓ Predict Output

advanced

What is the output after toggling a bit twice?

What will be printed after toggling the bit at position 0 twice in the following code?

DSA C#

int number = 2; // binary 0010
int position = 0;
int result = number ^ (1 << position);
result = result ^ (1 << position);
Console.WriteLine(result);

A 3

B 1

C 0

D 2

💡 Hint

Toggling the same bit twice returns the original number.

Number 2 is 0010. Toggling bit 0 flips the last bit: 0010 ^ 0001 = 0011 (3). Toggling again flips it back: 0011 ^ 0001 = 0010 (2). So the final result is 2.

🧠 Conceptual

expert

Which operation correctly clears the bit at position k?

Which of the following C# expressions correctly clears the bit at position k in integer n?

A ~n & (1 << k)

B n & ~(1 << k)

C n ^ (1 << k)

D n | (1 << k)

💡 Hint

Clearing a bit means setting it to 0 without changing other bits.

To clear a bit, we AND the number with the negation of the bit mask at position k. This sets that bit to 0 and leaves others unchanged. Option B does this correctly.

Practice

1. What does the operation number |= (1 << position) do to the bit at position in number?

easy

A. Toggles the bit at position

B. Clears the bit at position to 0

C. Sets the bit at position to 1

D. Checks if the bit at position is set

Solution

1. Step 1: Understand the bitwise OR with shifted 1

   The expression 1 << position creates a number with only the bit at position set to 1. Using OR (|=) with this sets that bit in number to 1 without changing other bits.

2. Step 2: Effect on the bit at position

   Since OR with 1 always results in 1, the bit at position is set to 1 regardless of its previous value.

3. Final Answer:

   Sets the bit at position to 1 -> Option C

4. Quick Check:

   OR with 1 sets bit [OK]

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

Common Mistakes:

• Confusing OR with XOR for toggling
• Thinking it clears the bit
• Assuming it checks the bit instead

2. Which of the following C# statements correctly clears (sets to 0) the bit at position pos in an integer num?

easy

A. num &= ~(1 << pos);

B. num |= (1 << pos);

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

D. num = num << pos;

Solution

1. Step 1: Understand clearing a bit

   To clear a bit, we use AND with the complement of a mask that has 1 at the target bit. The mask is 1 << pos, so complement is ~(1 << pos).

2. Step 2: Apply AND with complemented mask

   Using num &= ~(1 << pos); clears the bit at pos while leaving others unchanged.

3. Final Answer:

   num &= ~(1 << pos); -> Option A

4. Quick Check:

   AND with complement clears bit [OK]

Hint: Use AND with NOT mask to clear a bit [OK]

Common Mistakes:

• Using OR instead of AND to clear
• Using XOR which toggles instead of clears
• Using left shift alone which changes number

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

int num = 10; // binary 1010
int pos = 1;
num ^= (1 << pos);
Console.WriteLine(num);

medium

A. 10

B. 14

C. 12

D. 8

Solution

1. Step 1: Understand initial number and bit position

   Number 10 in binary is 1010. Position 1 means the second bit from right (0-based). Bit at position 1 is 1.

2. Step 2: Apply XOR to toggle bit at position 1

   XOR toggles the bit. Since bit at position 1 is 1, toggling it changes it to 0. New binary is 1000 which is decimal 8.

3. Final Answer:

   8 -> Option D

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 XOR with OR or AND
• Miscounting bit positions (0-based)
• Assuming no change if bit is 1

4. The following code is intended to toggle the bit at position pos in num. What is the error?

int num = 5; // binary 0101
int pos = 2;
num = num & (1 << pos);
Console.WriteLine(num);

medium

A. It clears all bits except the one at pos

B. It toggles the bit correctly

C. It sets the bit at pos to 1

D. It causes a syntax error

Solution

1. Step 1: Analyze the operation used

   The code uses AND (&) with 1 << pos. This keeps only the bit at pos if it was set, clearing all others.

2. Step 2: Compare with toggle operation

   Toggling requires XOR (^), not AND. So this code does not toggle but masks all bits except the one at pos.

3. Final Answer:

   It clears all bits except the one at pos -> Option A

4. Quick Check:

   AND with mask keeps only that bit [OK]

Hint: Use XOR to toggle, AND masks bits [OK]

Common Mistakes:

• Using AND instead of XOR for toggle
• Expecting toggle but getting masked value
• Confusing bitwise operators

5. You have an integer num and want to toggle the bit at position pos only if it is currently set (1). Which code snippet correctly does this?

hard

A. if ((num ^ (1 << pos)) != 0) num ^= (1 << pos);

B. if ((num & (1 << pos)) != 0) num ^= (1 << pos);

C. if ((num & ~(1 << pos)) != 0) num ^= (1 << pos);

D. if ((num | (1 << pos)) != 0) num ^= (1 << pos);

Solution

1. Step 1: Check if bit at pos is set

   Using num & (1 << pos) checks if bit at pos is 1. If result is not zero, bit is set.

2. Step 2: Toggle bit only if set

   If bit is set, toggle it using XOR: num ^= (1 << pos);. This flips the bit from 1 to 0.

3. Final Answer:

   if ((num & (1 << pos)) != 0) num ^= (1 << pos); -> Option B

4. Quick Check:

   Check bit with AND, toggle with XOR [OK]

Hint: Check bit with AND before toggling with XOR [OK]

Common Mistakes:

• Using OR instead of AND to check bit
• Toggling without checking bit first
• Using incorrect mask complement