Question

How to find the position of the first set bit (1-bit) of a number in C++

Answer 1

#include <iostream>

// Function to find the position of the first set bit
int findFirstSetBitPosition(int num) {
    // Edge case: If the number is 0, there are no set bits
    if (num == 0) {
        return 0; // Return 0 to indicate no set bit
    }

    int position = 0; // Position starts from 0 (LSB)
    while ((num & 1) == 0) { // Check if the least significant bit is 0
        num >>= 1;           // Right shift the number by 1
        position++;          // Increment the position
    }
    
    return position;
}

int main() {
    int num = 4224; // 0001 0000 1000 0000

    // Find and display the position of the first set bit
    int position = findFirstSetBitPosition(num);
    if (position == 0) {
        std::cout << "The number has no set bits (it's 0)." << std::endl;
    } else {
        std::cout << "The position of the first set bit is: " << position << std::endl;
    }
}

  
  
  
/*
run:
  
The position of the first set bit is: 7
7

*/

 

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Answer 2

#include <iostream>
#include <cmath>

// Function to find the position of the first set bit
int findFirstSetBitPosition(int num) {
      // Edge case: If the number is 0, there are no set bits
    if (num == 0) {
        return 0; // Return 0 to indicate no set bit
    }
    
    /*
    num & -num isolates the lowest set bit.
    log2(...) gives the zero-based index of that bit.
    */
        
    return static_cast<int>(log2(num & -num));
}

int main() {
    int num = 4224; // 0001 0000 1000 0000

    // Find and display the position of the first set bit
    int position = findFirstSetBitPosition(num);
    if (position == 0) {
        std::cout << "The number has no set bits (it's 0)." << std::endl;
    } else {
        std::cout << "The position of the first set bit is: " << position << std::endl;
    }
}


  

/*
run:
  
The position of the first set bit is: 7

*/

 

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