函式指標 計算應用 (C)

//===============================================================
// 20090523/24/26 知識 +
// http://knowledge.yahoo.com.tw/question/question?qid=1609052303386
// http://knowledge.yahoo.com.tw/question/question?qid=1609052411327
// http://knowledge.yahoo.com.tw/question/question?qid=1609052606389
// 發問者 :(無 ) http://tw.knowledge.yahoo.com/my/my?show=AB04901004
//===============================================================

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

typedef double T (double *Op1, double *Op2);
T add, sub, mul, div, fac, por;
T *pM[] = { add, sub, mul, div, fac, por };
char* msg[] = { "相加", "相減", "相乘", "相除", "階乘", "次方" };

//=========== 主程式 ...
int main(void)
{
  double ans, Op1, Op2;
  int i, num, opN = sizeof(pM)/sizeof(*pM);
  while(1)
  {
    printf("\n要做哪種運算？ ");
    for(i=0; i<opN; i++) printf("%d)%s ", i, msg[i]);
    printf(",其它)離開 \n");
    if (scanf("%d",&num)!=1 || num<0 || num >= opN) break;
    ans = pM[num](&Op1, &Op2);
    printf("%s的結果 = %.2lf \n", msg[num], ans);
  }
  system("pause");
  return 0;
}

//---------
double Input(int no, int mode=-1)
{
   double d;
   while(!fflush(stdin))  //淨空輸入
   {
     printf("請輸入第%d個數值 : ", no );
     if (scanf("%lf", &d)!=1) printf("輸入錯誤!! 重新");
     else if (mode==0 && d==0) printf("數值不得為0!! 重新");
     else break;
   }
   return d;
}

int InputInt()
{
   int n;
   while (!fflush(stdin)) //淨空輸入
   {
     printf( "請輸入一個整數 : " );
     if (scanf("%d", &n)==1) break;
     printf("輸入錯誤!! 重新");
   }
   return n;
}
//---------
double add(double *Op1, double *Op2)
{
  *Op1 = Input(1); *Op2 = Input(2);
  return (*Op1) + (*Op2);
}

double sub(double *Op1, double *Op2)
{
  *Op1 = Input(1); *Op2 = Input(2);
  return (*Op1) - (*Op2);
}

double mul(double *Op1, double *Op2)
{
  *Op1 = Input(1); *Op2 = Input(2);
  return (*Op1) * (*Op2);
}

double div( double *Op1, double *Op2)
{
  *Op1 = Input(1); *Op2 = Input(2, 0);
  return (*Op1) / (*Op2);
}

double fac(double *Op1, double *Op2)
{
  double ans = 1; *Op2 = 0;
  int num = InputInt();
  for( *Op1 = num; num>=2; num--) ans *= num;
  return ans;
}

double por(double *Op1, double *Op2)
{
  *Op1 = Input(1); *Op2 = Input(2);
  return pow(*Op1, *Op2);
}
//---------------------------------