Question

How to round a number to the previous power of 2 in C

Answer 1

#include <stdio.h>
#include <math.h>

/**
 * Rounds an unsigned integer down to the previous power of 2.
 *
 * return the previous power of 2 less than or equal to n
 */
unsigned int roundToPreviousPowerOf2(unsigned int n) {
    if (n == 0) return 0;
    return (unsigned int)pow(2, floor(log2(n)));
}

int main() {
    unsigned int num = 31;

    printf("Previous power of 2: %u\n", roundToPreviousPowerOf2(num));

    return 0;
}


 
/*
run:
 
Previous power of 2: 16
 
*/

 

 

 

 

 

70+ SQL courses for beginners and professionals

Answer 2

#include <stdio.h>

int roundToPreviousPowerOf2(int n) {
    if (n <= 0) return 0;

    int power = 1;
    while (power <= n / 2) {
        power <<= 1;
    }
    
    return power;
}

int main() {
    int num = 21;
    
    printf("Previous power of 2: %d\n", roundToPreviousPowerOf2(num));
    
    return 0;
}


 
/*
run:
 
Previous power of 2: 16
 
*/

 

70+ SQL courses for beginners and professionals