#include <stdio.h>
 #define BASE  2
 #define VALUE 13
 #define SHIFT 2

 int changeBase(int in, int oldBase, int newBase) {
     int digit, out, factor;

     out = 0;
     factor = 1;
     while (in != 0) {
         digit  = in % oldBase;
         out    = factor * digit + out;
         factor = newBase * factor;
         printf("%d %6d %d\n", digit, in, out);
         in     = in / oldBase;
     }
    return out;
 }

 int main(void) {
     int c, i;

     c = changeBase(VALUE, BASE, 10);//函数里的renturn out是好几个数值，那么c的值是什么？不明白。
     printf("%03d in base %d is %d\n", VALUE, BASE, c);

     for (i = 0; i < SHIFT; i++)
         c = c * 10;

     printf("%03d * %d^%d in base %d is %06d\n", VALUE, BASE,  
      SHIFT, BASE, c);

     c = changeBase(c, 10, BASE);
     printf("%03d * %d^%d is %d\n", VALUE, BASE, SHIFT, c);

     return 0;
 }

明白了，谢谢。