C primer plus (第六版）第十二章 编程练习第1题

题目：

1. 不使⽤全局变量，重写程序清单12.4。

2. 程序清单12.4：

   #include <stdio.h>
   int units = 0;         /* 外部变量  */
   void critic(void);
   int main(void)
   {
        extern int units;  /* 可选的重复声明 */
        printf("How many pounds to a firkin of butter?\n");
        scanf("%d", &units);
        while (units != 56)
             critic();
        printf("You must have looked it up!\n");
        return 0;
   }
   void critic(void)
   {
        /* 删除了可选的重复声明 */
        printf("No luck, my friend. Try again.\n");
        scanf("%d", &units);
   }

思路：

1. 用自动变量units代替全局变量units，此时需要critic()函数带返回值并将返回值赋值给unit用来和56进行比较；

   #include <stdio.h>
   int critic(void);
   int main()
   {
       int units;
       
       printf("How many pounds to a firkin of butter?\n");
       scanf("%d",&units);
       while (units != 56)
           units = critic();
       printf("You must have looked it up!\n");
       return 0;
   }
   
   int critic(void)
   {
       int temp;
       printf("No luck, my friend. Try again.\n");
       scanf("%d", &temp);
       return temp;
   }

2. 还是用自动变量units代替全局变量units，不同的是critic()函数只传入units的指针地址，输入的数据是通过直接修改相同指针下的数据实现的。

   #include <stdio.h>
   void critic(int * pt);
   int main()
   {
       int units;
           
       printf("How many pounds to a firkin of butter?\n");
       scanf("%d", &units);
       while (units != 56)
       {
           critic(&units);
       }
       printf("You must have looked it up!\n");
       return 0;
   }
   void critic(int * pt )
   {
       printf("No luck, my friend. Try again.\n");
       scanf("%d", pt);
   }