#include <stdio.h>int sum(int x, int y){  return x + y;}int reduce(int x, int y){  return x - y;}int multiply(int x, int y){  return x * y;}float divide(int x, int y){  if(y ==0)    return 0;  else    return x *1.0f/ y;}struct Student{  int age;  float weight;  char *name;  void(*studentP());}int main(int argc, const char * argv[]){  // 1.定義函數指針  int (*p)(int x, int y);  // 2.初始化  p = sum;  // 3.賦值  int s = p(3,2);  // 4.打印  printf("%d/n",s);//=5;  //--------------  printf("請輸入數字1,2,3,4/n");  int num;  scanf("%d",&num);  switch (num) {    case 1:      p = sum;      break;    case 2:      p = reduce;      break;    case 3:      p = multiply;      break;    case 4:      p = divide;      break;  }  int result = p(22,2);  printf("%d/n",result);  //結構體：  struct Student stu = (22,88,"tom",studp);  printf("結構體長度=%d",sizeof(stu));  //1.  stu.studp;  //2.  struct stu* stp = &stu;  (*stp).studp;  //3.  stp -> studp;  return 0;}