Set Clear Toggle a Specific Bit in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to set a specific bit in a number?

Setting a bit means changing that bit to 1, regardless of its current value.

beginner

How do you clear a specific bit in a number?

Clearing a bit means changing that bit to 0, regardless of its current value.

beginner

What does it mean to toggle a specific bit in a number?

Toggling a bit means changing it from 0 to 1 or from 1 to 0 (flipping its value).

intermediate

In C#, how do you set the 3rd bit (0-based) of an integer variable num?

Use num = num | (1 << 3); which shifts 1 left by 3 and ORs it with num to set the bit.

intermediate

How do you clear the 2nd bit of an integer num in C#?

Use num = num & ~(1 << 2); which shifts 1 left by 2, negates it, and ANDs with num to clear the bit.

Which operator is used to set a specific bit in C#?

A Bitwise OR (|)

B Bitwise AND (&)

C Bitwise XOR (^)

D Bitwise NOT (~)

Setting a bit uses OR (|) to ensure the bit is 1 without changing others.

What does the expression 1 << 4 do?

A Sets the 4th bit to 1

B Shifts 1 left by 4 bits, resulting in 16

C Clears the 4th bit

D Toggles the 4th bit

It shifts the number 1 left by 4 bits, which equals 16 in decimal.

How do you toggle the 1st bit of a number num in C#?

A num = ~num;

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

C num = num | (1 << 1);

D num = num ^ (1 << 1);

XOR (^) flips the bit at position 1, toggling it.

What is the result of clearing a bit that is already 0?

A The bit becomes 1

B The entire number becomes 0

C The bit remains 0

D An error occurs

Clearing a bit that is already 0 keeps it 0 without changing other bits.

Which operator is used to clear a bit in C#?

A Bitwise AND (&) with negation

B Bitwise OR (|)

C Bitwise XOR (^)

D Bitwise NOT (~)

Clearing a bit uses AND (&) with the negated mask to set that bit to 0.

Explain how to set, clear, and toggle a specific bit in an integer using bitwise operators.

Think about how each operator affects bits at the target position.

Describe what happens when you toggle a bit twice in a row.

Toggling twice cancels out the change.

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