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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the 3rd bit of number n.

DSA Javascript

let n = 8; // 1000 in binary
let bitPosition = 3;
n = n | (1 << [1]);
console.log(n.toString(2));

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

AbitPosition

B2

C3

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 instead of 2 for bit position

Forgetting zero-based indexing

Bits are zero-indexed from the right, so the 3rd bit means position 2.

2fill in blank

medium

Complete the code to clear the 2nd bit of number n.

DSA Javascript

let n = 14; // 1110 in binary
let bitPosition = 2;
n = n & ~ (1 << [1]);
console.log(n.toString(2));

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

A1

BbitPosition

C2

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using 2 instead of 1 for bit position

Forgetting to invert the mask

To clear the 2nd bit, shift 1 left by 1 (zero-based), then invert and AND.

3fill in blank

hard

Fix the error in toggling the 1st bit of number n.

DSA Javascript

let n = 5; // 0101 in binary
let bitPosition = 0;
n = n [1] (1 << bitPosition);
console.log(n.toString(2));

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

A&

B|

C~

D^

Attempts:

3 left

💡 Hint

Common Mistakes

Using OR instead of XOR

Using AND which clears bits

Toggling a bit uses XOR (^), which flips the bit at the position.

4fill in blank

hard

Fill both blanks to set and then clear the 4th bit of number n.

DSA Javascript

let n = 10; // 1010 in binary
let bitPosition = 3;
n = n [1] (1 << bitPosition);
n = n [2] ~ (1 << bitPosition);
console.log(n.toString(2));

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

A|

B&

C^

D~

Attempts:

3 left

💡 Hint

Common Mistakes

Using XOR to set or clear bits

Forgetting to invert mask when clearing

Use OR to set a bit and AND with NOT to clear it.

5fill in blank

hard

Fill all three blanks to toggle the 0th bit, set the 2nd bit, and clear the 3rd bit of number n.

DSA Javascript

let n = 13; // 1101 in binary
n = n [1] (1 << 0);
n = n [2] (1 << 1);
n = n [3] ~ (1 << 2);
console.log(n.toString(2));

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

A^

B|

C&

D~

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up operators for set/clear/toggle

Forgetting to invert mask when clearing

Toggling uses XOR (^), setting uses OR (|), clearing uses AND (&) with NOT.

Practice

1. What does the operation num | (1 << k) do to the number num in terms of bits?

easy

A. Toggles the k-th bit of num

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

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

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

Solution

1. Step 1: Understand bitwise OR with shifted 1

   The expression 1 << k creates a number with only the k-th bit set to 1.

2. Step 2: Apply OR operation to set 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 B

4. Quick Check:

   Bitwise OR with 1 at position k sets that bit [OK]

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

Common Mistakes:

• Confusing set with clear operation
• Thinking it toggles the bit
• Assuming it checks the bit instead

2. Which of the following JavaScript expressions correctly clears the k-th bit of a number num?

easy

A. num ^ (1 << k)

B. num | ~(1 << k)

C. num & (1 << k)

D. num & ~(1 << k)

Solution

1. Step 1: Understand clearing a bit

   To clear the k-th bit, we want to set it to 0 while keeping others unchanged.

2. Step 2: Use AND with NOT mask

   The mask ~(1 << k) has all bits 1 except the k-th bit 0. ANDing with this clears the k-th bit.

3. Final Answer:

   num & ~(1 << k) -> Option D

4. Quick Check:

   AND with inverted mask clears bit [OK]

Hint: AND with NOT of shifted 1 clears a bit [OK]

Common Mistakes:

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

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

let num = 10; // binary 1010
let k = 1;
num = num ^ (1 << k);
console.log(num);

medium

A. 8

B. 10

C. 12

D. 14

Solution

1. Step 1: Understand initial values and bit positions

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

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

   1 << 1 = 2 (binary 0010). XOR toggles bit 1: 1010 ^ 0010 = 1000 (binary), which is 8 decimal.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   XOR toggles bit 1: 10 -> 8 [OK]

Hint: XOR toggles the bit at position k [OK]

Common Mistakes:

• Confusing XOR with OR or AND
• Miscounting bit positions (0-based)
• Not converting binary to decimal correctly

4. The following code is intended to toggle the k-th bit of num. What is the error?

function toggleBit(num, k) {
  return num & (1 << k);
}
console.log(toggleBit(5, 0));

medium

A. Function does not return a number

B. Incorrect bit shift direction

C. Using AND instead of XOR to toggle the bit

D. Missing parentheses around bit shift

Solution

1. Step 1: Identify intended toggle operation

   Toggling a bit requires flipping it: 0 to 1 or 1 to 0.

2. Step 2: Check the operator used

   The code uses AND (&) which masks bits, not toggles. XOR (^) is needed to toggle bits.

3. Final Answer:

   Using AND instead of XOR to toggle the bit -> Option C

4. Quick Check:

   Toggle needs XOR, not AND [OK]

Hint: Toggle bits with XOR, not AND [OK]

Common Mistakes:

• Using AND or OR instead of XOR for toggle
• Confusing toggle with set or clear
• Ignoring operator precedence

5. You want to write a function updateBit(num, k, bitValue) that sets the k-th bit of num to bitValue (0 or 1). Which of the following implementations is correct?

hard

A. return (num & ~(1 << k)) | (bitValue << k);

B. return (num | (bitValue << k)) & ~(bitValue << k);

C. return num ^ (bitValue << k);

D. return num & (bitValue << k);

Solution

1. Step 1: Clear the k-th bit first

   Use num & ~(1 << k) to clear the k-th bit regardless of its current value.

2. Step 2: Set the k-th bit to bitValue

   Shift bitValue to k-th position and OR it with cleared number to set the bit correctly.

3. Final Answer:

   return (num & ~(1 << k)) | (bitValue << k); -> Option A

4. Quick Check:

   Clear then set bit with OR [OK]

Hint: Clear bit then OR with shifted bitValue to update [OK]

Common Mistakes:

• Using XOR which toggles instead of sets
• Not clearing bit before setting
• Using AND which masks bits incorrectly