count_bits_set.c

#include <stdio.h>

#define BYTETOBINARYPATTERN "%d%d%d%d%d%d%d%d"
#define BYTETOBINARY(byte)  \
  (byte & 0x80 ? 1 : 0), \
  (byte & 0x40 ? 1 : 0), \
  (byte & 0x20 ? 1 : 0), \
  (byte & 0x10 ? 1 : 0), \
  (byte & 0x08 ? 1 : 0), \
  (byte & 0x04 ? 1 : 0), \
  (byte & 0x02 ? 1 : 0), \
  (byte & 0x01 ? 1 : 0)

void print_16_bits(unsigned int n) {
  printf("%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d%d",
      (n & 0x08000 ? 1 : 0), \
      (n & 0x04000 ? 1 : 0), \
      (n & 0x02000 ? 1 : 0), \
      (n & 0x01000 ? 1 : 0), \
      (n &  0x0800 ? 1 : 0), \
      (n &  0x0400 ? 1 : 0), \
      (n &  0x0200 ? 1 : 0), \
      (n &  0x0100 ? 1 : 0), \
      (n &   0x080 ? 1 : 0), \
      (n &   0x040 ? 1 : 0), \
      (n &   0x020 ? 1 : 0), \
      (n &   0x010 ? 1 : 0), \
      (n &    0x08 ? 1 : 0), \
      (n &    0x04 ? 1 : 0), \
      (n &    0x02 ? 1 : 0), \
      (n &    0x01 ? 1 : 0));
}

int main(int argc, const char *argv[])
{
  unsigned int n = 92;
  unsigned int v = n; // count the number of bits set in v
  unsigned int c;     // c accumulates the total bits set in v

  for (c = 0; v; c++)
  {
    printf("c:       %d\n", c);

    /*
    printf("v:       "BYTETOBINARYPATTERN""BYTETOBINARYPATTERN"\n",
        BYTETOBINARY(v>>8), BYTETOBINARY(v));
    printf("v-1:     "BYTETOBINARYPATTERN""BYTETOBINARYPATTERN"\n",
        BYTETOBINARY((v-1)>>8), BYTETOBINARY(v-1));
    printf("v&(v-1): "BYTETOBINARYPATTERN""BYTETOBINARYPATTERN"\n\n",
        BYTETOBINARY((v&(v-1))>>8), BYTETOBINARY(v&(v-1)));
    */

    printf("v:       ") ; print_16_bits(v)       ; printf("\n") ;
    printf("v-1:     ") ; print_16_bits(v-1)     ; printf("\n") ;
    printf("v&(v-1): ") ; print_16_bits(v&(v-1)) ; printf("\n") ;

    v &= v - 1; // clear the least significant bit set
  }
  printf("There are %d ones and in %d.\n", c, n);

  return 0;
}