CS256 - Principles of Structured Design Fall 2021

Displaying ./code/cs256su21code/jul19/powerOf2.c

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/*
this program finds the biggest power of two smaller than
the given number
*/
#include <stdio.h>
#include <stdlib.h>
int isPowerOf2(int x);
int biggestPowerOf2(int x);
int setBit(int n, int bit);
int main(int argc, char *argv[]){
        
        if(argc < 2){
                fprintf(stderr, "Usage %s <number>\n", argv[0]);
                exit(1);
        }
        int a, pwr2;
        a = atoi(argv[1]);
        if(a < 0){
                printf("Enter a positive number\n");
                exit(1);
        }
        if(isPowerOf2(a) == 1){
                printf("%d is a power of 2\n", a);
        } else {
        pwr2 = biggestPowerOf2(a);
        printf("Largest power of 2 smaller than %d is %d\n", a, pwr2);  
        }
        return 0;
}
int isPowerOf2(int x){
        int bit, count = 0;
        for(int i = 31; i >= 0; i--){
                bit = (x >> i) & 1;
                if(bit == 1)
                        count++;
                if(count > 1)
                        return 0;
        }
        return 1;
}
int biggestPowerOf2(int x){
        int bit, p = 0;
        //ints are 32 bits, 
        //starting with leftmost bity (bit 31) and going all the way to bit 0
        for (int i = 31; i >= 0; i--){
                bit = (x >> i) & 1; //right-shifting given number i times and ANDing it with 1
                if(bit == 1){ //if the bit is 1
                        p = setBit(p, i); //set the i'th bit of p and exit the loop
                        break;
                }       
        } 
        return p;               
}
int setBit(int n, int bit){
        return n | 1 << bit;
}
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