#include <stdio.h>  void main(){   int a = 3;   int b = 4;    a = a ^ b;//使用異或交換   b = b ^ a;   a = a ^ b;    printf("%d, %d/n", a, b);    a = a - b;//使用加減交換   b = a + b;   a = b - a;    printf("%d, %d/n", a, b);    a ^= b ^= a ^= b;    printf("%d, %d/n", a, b); } 

#include <stdio.h> #include <stdlib.h>  int sumof1(int x)//求一個數轉換成二進制以后1的個數 {   int countx = 0;   while(x)   {     countx ++;     x &= x-1; //每位與一次x - 1；就能消掉最后一個1   }   return countx; }  void main(){    char c[10];   int i = 999;    itoa(i, c, 10);//以10進制轉換成字符數組   puts(c);    itoa(i, c, 16);//以16進制轉換成字符數組   printf("0x%s/n", c);    itoa(i, c, 8);//以8進制轉換成字符數組   printf("0%s/n", c);    itoa(i, c, 2);//以2進制轉換成字符數組   puts(c);    i = atoi(c);//再將字符串轉成整形   printf("%d/n", i);    printf("%d/n", sumof1(i));//以2進制表示時1的個數 }