Count Set Bits Brian Kernighan Algorithm in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the count variable to zero.

DSA C

int countSetBits(int n) {
    int count = [1];
    while (n) {
        n = n & (n - 1);
        count++;
    }
    return count;
}

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

A0

B1

C-1

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing count to 1 instead of 0.

Using the input variable n as initial count.

The count variable must start at zero to correctly count the number of set bits.

2fill in blank

medium

Complete the code to update n by removing the rightmost set bit.

DSA C

int countSetBits(int n) {
    int count = 0;
    while (n) {
        n = n [1] (n - 1);
        count++;
    }
    return count;
}

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

A+

B|

C^

D&

Attempts:

3 left

💡 Hint

Common Mistakes

Using | (bitwise OR) instead of &.

Using + instead of bitwise operator.

Using bitwise AND (&) between n and (n-1) removes the rightmost set bit.

3fill in blank

hard

Fix the error in the loop condition to correctly count set bits.

DSA C

int countSetBits(int n) {
    int count = 0;
    while ([1]) {
        n = n & (n - 1);
        count++;
    }
    return count;
}

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

An != 0

Bcount < n

Cn > 0

Dcount != 0

Attempts:

3 left

💡 Hint

Common Mistakes

Using count in the loop condition instead of n.

Using n > 0 which works but less explicit than n != 0.

The loop should continue while n is not zero to count all set bits.

4fill in blank

hard

Fill both blanks to create a function that counts set bits using Brian Kernighan's algorithm.

DSA C

int countSetBits(int [1]) {
    int count = 0;
    while ([2]) {
        [1] = [1] & ([1] - 1);
        count++;
    }
    return count;
}

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

An

Bcount

Cn != 0

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using count as loop variable.

Using incorrect loop condition.

The function parameter and loop variable is n; the loop continues while n is not zero.

5fill in blank

hard

Fill all three blanks to complete the main function that reads an integer and prints its set bit count.

DSA C

#include <stdio.h>

int countSetBits(int n) {
    int count = 0;
    while (n != 0) {
        n = n & (n - 1);
        count++;
    }
    return count;
}

int main() {
    int [1];
    scanf("%d", &[1]);
    int [2] = countSetBits([3]);
    printf("%d\n", [2]);
    return 0;
}

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

Anum

Bresult

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Passing wrong variable to countSetBits.

Variable num stores input; result stores count; num is passed to countSetBits.

Practice

1. What does the Brian Kernighan algorithm do when counting set bits in a number?

easy

A. It repeatedly clears the least significant set bit until the number becomes zero.

B. It counts the total number of digits in the binary representation.

C. It shifts the number right and counts zeros.

D. It converts the number to decimal and counts digits.

Solution

1. Step 1: Understand the algorithm's main operation

   The algorithm uses n = n & (n-1) to clear the least significant set bit in each iteration.

2. Step 2: Effect of clearing bits

   Each operation reduces the number of set bits by one until the number becomes zero.

3. Final Answer:

   It repeatedly clears the least significant set bit until the number becomes zero. -> Option A

4. Quick Check:

   Clearing bits one by one = It repeatedly clears the least significant set bit until the number becomes zero. [OK]

Hint: Clear least set bit repeatedly to count set bits [OK]

Common Mistakes:

• Counting zeros instead of ones
• Shifting without clearing bits
• Confusing decimal digits with bits

2. Which of the following is the correct syntax to clear the least significant set bit in C using Brian Kernighan's method?

easy

A. n = n & (n - 1);

B. n = n | (n - 1);

C. n = n & (n + 1);

D. n = n ^ (n - 1);

Solution

1. Step 1: Recall the bit clearing operation

   The Brian Kernighan algorithm clears the least significant set bit by n = n & (n - 1).

2. Step 2: Verify other options

   Options A, B, and D do not clear the least significant set bit correctly.

3. Final Answer:

   n = n & (n - 1); -> Option A

4. Quick Check:

   Use n & (n-1) to clear bits [OK]

Hint: Use n & (n-1) to clear least set bit [OK]

Common Mistakes:

• Using +1 instead of -1
• Using OR or XOR instead of AND
• Confusing bitwise operators

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

unsigned int n = 13; // binary 1101
int count = 0;
while (n) {
  n = n & (n - 1);
  count++;
}
printf("%d", count);

medium

A. 5

B. 2

C. 4

D. 3

Solution

1. Step 1: Analyze initial value and binary form

   13 in binary is 1101, which has three set bits (1s).

2. Step 2: Count iterations clearing bits

   Each loop clears one set bit: 1101 -> 1100 -> 1000 -> 0000, so loop runs 3 times.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Number of set bits in 13 = 3 [OK]

Hint: Count set bits by clearing one bit per loop [OK]

Common Mistakes:

• Counting total bits instead of set bits
• Off-by-one in loop count
• Misreading binary representation

4. Identify the error in this C code snippet that tries to count set bits using Brian Kernighan's algorithm:

int countSetBits(int n) {
  int count = 0;
  while (n != 0) {
    n = n & (n + 1);
    count++;
  }
  return count;
}

medium

A. Missing initialization of count variable.

B. Using n + 1 instead of n - 1 in bit clearing step.

C. Incorrect return type of the function.

D. Loop condition should be while (n > 0).

Solution

1. Step 1: Check the bit clearing operation

   The code uses n = n & (n + 1), which does not clear the least significant set bit correctly.

2. Step 2: Correct operation for clearing bits

   The correct operation is n = n & (n - 1) to clear the least significant set bit.

3. Final Answer:

   Using n + 1 instead of n - 1 in bit clearing step. -> Option B

4. Quick Check:

   Use n & (n-1) to clear bits, not n & (n+1) [OK]

Hint: Use n & (n-1) to clear bits, not n & (n+1) [OK]

Common Mistakes:

• Using +1 instead of -1
• Ignoring loop condition correctness
• Forgetting to initialize count

5. Given an array of unsigned integers, which approach using Brian Kernighan's algorithm is most efficient to find the total count of set bits in all elements?

hard

A. Shift each number bit by bit and count set bits using bitwise AND with 1.

B. Convert each number to binary string and count '1's using string functions.

C. For each element, repeatedly clear least significant set bit and count until zero, then sum counts.

D. Use a lookup table for each byte and sum counts for all bytes in each number.

Solution

1. Step 1: Identify Brian Kernighan's approach

   For each element, repeatedly clear least significant set bit and count until zero, then sum counts.: For each element, repeatedly clear the least significant set bit until zero and count (using n = n & (n-1)), then sum the counts across elements. This is the direct application of Brian Kernighan's algorithm.

2. Step 2: Why it is most efficient using Brian Kernighan

   B (strings) is slow; C (shifts) is O(word size) always; D (lookup) is fast but a separate method, not Brian Kernighan's. Brian Kernighan excels for sparse bits (O(set bits) time).

3. Final Answer:

   For each element, repeatedly clear least significant set bit and count until zero, then sum counts. -> Option C

4. Quick Check:

   Brian Kernighan on array: apply n & (n-1) loop per element [OK]

Hint: Brian Kernighan per element: clear LSB until zero, sum counts [OK]

Common Mistakes:

• Using string conversion which is slow
• Bit shifting each bit without optimization
• Confusing lookup table with Brian Kernighan method