簡易型計算機

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<math.h>
void calculation_add(double *a)
{
   double b;
   scanf("%lf",&b);
   system("cls");
   printf("%.2lf + %.2lf\n",*a,b);
   *a=*a+b;
   printf("result so far is %.2lf\n",*a);
}
void calculation_subtract(double *a)
{
   double b;
   scanf("%lf",&b);
   system("cls");
   printf("%.2lf - %.2lf\n",*a,b);
   *a=*a-b;
   printf("result so far is %.2lf\n",*a);
}
void calculation_multiply(double *a)
{
   double b;
   scanf("%lf",&b);
   system("cls");
   printf("%.2lf * %.2lf\n",*a,b);
   *a=(*a)*b;
   printf("result so far is %.2lf\n",*a);
}
void calculation_divide(double *a)
{
    double b,loop=1;
   while(loop==1)
   {
      scanf("%lf",&b);
      if(b==0)
      {
       printf("null\n");
       printf("pause");
       system("cls");
       printf("%.2lf / ",*a);
      }
      else loop=0;
   }
   system("cls");
   printf("%.2lf / %.2lf\n",*a,b);
   *a=(*a)/b;
   printf("result so far is %.2lf\n",*a);
}
void calculation_power(double *a)
{
   system("cls");
   printf("%.2lf * %.2lf\n",*a,*a);
   *a=(*a)*(*a);
   printf("result so far is %.2lf\n",*a);
}
void calculation_end(double *a)
{
   system("cls");
   printf("\t = %.2lf\n",*a,*a);
   printf("final so far is %.2lf\n",*a);

}
int main()
{
    double a=0,b,input_test_loop=0;
    char operation;
    printf("please input your operation(+,-,*,/,^,=) "
           "and input a number :\n");
    while(input_test_loop==0)
    {
       scanf(" %c ",&operation);
       switch(operation)
       {
           case '+':calculation_add(&a);
                    break;
           case '-':calculation_subtract(&a);
                    break;
           case '*':calculation_multiply(&a);
                    break;
           case '/':calculation_divide(&a);
                    break;
           case '^':calculation_power(&a);
                    break;
           case '=':calculation_end(&a);
                    input_test_loop=1;
                    break;
           default :printf("\nerror operation\n");
                    system("pause");
                    system("cls");
                    break;
       }
    }
    return 0;
}