Count Set Bits in an Integer

#include <stdio.h>

int countSetBits(unsigned int n) {
    unsigned char i,a;
    for(i=0;i<=31;i++){
      if((n>>i) & 1){
        a++;
      }
    }
    return a;
}

int main() {
    int n;
    scanf("%d", &n);
    printf("%d", countSetBits(n));
    return 0;
}