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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after setting the 3rd bit?

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

DSA Java

int num = 8;
int bitPosition = 3;
int result = num | (1 << bitPosition);
System.out.println(result);

A 16

B 8

C 12

D 0

💡 Hint

Remember that setting a bit means making sure that bit is 1 using OR operation.

The number 8 in binary is 1000. The 3rd bit (0-based) is already set (1). Setting it again keeps the number unchanged, so the output is 8.

❓ Predict Output

intermediate

What is the output after clearing the 2nd bit?

Given the integer value 14 (binary 1110), what is the output after clearing the 2nd bit (0-based from right)?

DSA Java

int num = 14;
int bitPosition = 2;
int result = num & ~(1 << bitPosition);
System.out.println(result);

A 8

B 6

C 14

D 10

💡 Hint

Clearing a bit means making that bit 0 using AND with NOT mask.

14 in binary is 1110. Clearing the 2nd bit (counting from 0) changes the bit from 1 to 0, resulting in 1010 binary which is 10 decimal.

❓ Predict Output

advanced

What is the output after toggling the 1st bit?

Given the integer value 5 (binary 0101), what is the output after toggling the 1st bit (0-based from right)?

DSA Java

int num = 5;
int bitPosition = 1;
int result = num ^ (1 << bitPosition);
System.out.println(result);

A 5

B 3

C 7

D 4

💡 Hint

Toggling flips the bit: 0 becomes 1, 1 becomes 0 using XOR.

5 in binary is 0101. Toggling the 1st bit flips it from 0 to 1, resulting in 0111 binary which is 7 decimal.

🧠 Conceptual

advanced

Which operation clears a specific bit at position k?

Which bitwise operation correctly clears the bit at position k (0-based from right) in integer n?

A n & ~(1 << k)

B n | (1 << k)

C n & (1 << k)

D n ^ (1 << k)

💡 Hint

Clearing means setting the bit to 0 without affecting others.

To clear a bit, use AND with the complement of the bit mask. This sets the target bit to 0 and leaves others unchanged.

🔧 Debug

expert

Why does this toggle bit code fail for negative numbers?

Consider this code to toggle the 4th bit of an integer n: int result = n ^ (1 << 4); Why might this code produce unexpected results when n is negative?

DSA Java

int n = -16;
int result = n ^ (1 << 4);
System.out.println(result);

A Because negative numbers use two's complement and toggling bit 4 changes sign unexpectedly

B Because left shift on negative numbers causes sign bit corruption

C Because XOR cannot be used with negative numbers

D Because bit positions start at 1, not 0

💡 Hint

Think about how negative numbers are stored and how toggling bits affects sign.

Negative numbers use two's complement. Toggling bit 4 may flip the sign bit or change the number's sign unexpectedly.

Practice

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

easy

A. Toggles the k-th bit of num

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

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

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

Solution

1. Step 1: Understand the bit shift operation

   1 << k creates a number with only the k-th bit set to 1 and all others 0.

2. Step 2: Apply bitwise OR with num

   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 C

4. Quick Check:

   Set bit = OR with 1 shifted [OK]

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

Common Mistakes:

• Confusing OR with AND operation
• Thinking it clears or toggles the bit
• Misunderstanding bit positions starting at 0

2. Which of the following Java statements correctly clears (sets to 0) the 3rd bit (0-based) of an integer variable num?

easy

A. num = num & (1 << 3);

B. num = num & ~(1 << 3);

C. num = num ^ (1 << 3);

D. num = num | (1 << 3);

Solution

1. Step 1: Understand clearing a bit

   To clear a bit, we AND with a mask that has 0 at that bit and 1s elsewhere.

2. Step 2: Create mask and apply

   ~(1 << 3) creates a mask with all 1s except 0 at bit 3. ANDing num with this clears bit 3.

3. Final Answer:

   num = num & ~(1 << 3); -> Option B

4. Quick Check:

   Clear bit = AND with NOT mask [OK]

Hint: Clear bit by AND with NOT of shifted 1 [OK]

Common Mistakes:

• Using OR instead of AND to clear bits
• Using XOR which toggles bits
• Using AND with shifted 1 which keeps only that bit

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

int num = 10; // binary 1010
int k = 1;
num = num ^ (1 << k);
System.out.println(num);

medium

A. 10

B. 14

C. 12

D. 8

Solution

1. Step 1: Understand initial value and bit positions

   num = 10 in binary is 1010. Bit positions (0-based) from right: bit0=0, bit1=1, bit2=0, bit3=1.

2. Step 2: Apply XOR to toggle bit 1

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

3. Final Answer:

   8 -> Option D

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
• Misreading bit positions starting at 1 instead of 0
• Incorrect binary to decimal conversion

4. The following Java code is intended to toggle the 2nd bit of num. What is the error?

int num = 5; // binary 0101
int k = 2;
num = num & (1 << k);
System.out.println(num);

medium

A. It clears all bits except the 2nd bit instead of toggling

B. It toggles the wrong bit

C. It causes a syntax error

D. It sets the 2nd bit to 1 always

Solution

1. Step 1: Analyze the operation used

   The code uses AND with 1 << k, which keeps only the k-th bit and clears others.

2. Step 2: Understand toggle operation

   Toggle requires XOR, not AND. AND here does not toggle but masks all bits except bit 2.

3. Final Answer:

   It clears all bits except the 2nd bit instead of toggling -> Option A

4. Quick Check:

   Toggle = XOR, not AND mask [OK]

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

Common Mistakes:

• Using AND instead of XOR to toggle bits
• Confusing bit positions
• Expecting output without understanding bitwise operations

5. Given an integer num, write a Java expression to toggle the 0th bit only if it is currently set (1), otherwise leave num unchanged. Which expression achieves this?

hard

A. num = (num & (1 << 0)) != 0 ? num ^ (1 << 0) : num;

B. num = num & ~(1 << 0);

C. num = num ^ (1 << 0);

D. num = num | (1 << 0);

Solution

1. Step 1: Check if 0th bit is set

   Use (num & (1 << 0)) != 0 to test if bit 0 is 1.

2. Step 2: Toggle bit 0 only if set

   If bit 0 is set, toggle it with XOR; else keep num unchanged.

3. Final Answer:

   num = (num & (1 << 0)) != 0 ? num ^ (1 << 0) : num; -> Option A

4. Quick Check:

   Conditional toggle only if bit set [OK]

Hint: Use conditional check before toggling bit [OK]

Common Mistakes:

• Toggling bit unconditionally
• Using OR which always sets bit
• Clearing bit without condition