Set Clear Toggle a Specific Bit in DSA Javascript - Build from Scratch

Set Clear Toggle a Specific Bit

📖 Scenario: Imagine you are controlling a simple electronic device with 8 switches. Each switch can be ON (1) or OFF (0). You want to write code to turn ON, turn OFF, or flip a specific switch using bits.

🎯 Goal: You will create a number representing the switches, then write code to set (turn ON), clear (turn OFF), and toggle (flip) a specific bit (switch) using bitwise operations.

📋 What You'll Learn

Create a variable switches with initial value 0

Create a variable bitPosition with value 3

Write code to set the bit at bitPosition in switches

Write code to clear the bit at bitPosition in switches

Write code to toggle the bit at bitPosition in switches

Print the value of switches after each operation

💡 Why This Matters

🌍 Real World

Controlling hardware switches or flags in software often uses bits to save space and speed.

💼 Career

Understanding bitwise operations is useful for embedded systems, game development, and performance-critical applications.

1

Create the initial switches variable

Create a variable called switches and set it to 0 to represent all switches OFF.

DSA Javascript

// Your code here

Hint

Use let switches = 0; to start with all switches OFF.

2

Create the bit position variable

Create a variable called bitPosition and set it to 3 to represent the 4th switch (counting from 0).

DSA Javascript

let switches = 0;
// Your code here

Hint

Remember bits start counting at 0 from the right.

3

Set the bit at bitPosition

Use bitwise OR to set the bit at bitPosition in switches. Write switches = switches | (1 << bitPosition).

DSA Javascript

let switches = 0;
let bitPosition = 3;
// Your code here

Hint

Use 1 << bitPosition to create a mask with only that bit ON.

4

Clear, toggle, and print the switches

Clear the bit at bitPosition using AND with NOT mask, then toggle the bit using XOR. Print switches after each operation using console.log(switches).

DSA Javascript

let switches = 0;
let bitPosition = 3;
switches = switches | (1 << bitPosition);
// Your code here

Hint

Use switches & ~(1 << bitPosition) to clear and switches ^ (1 << bitPosition) to toggle.

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