C语言实现四则运算

#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include <stack> 

//判断运算符的优先级函数 
int Precede(char a, char b) 
{ 
   switch (a) 
   { 
     case '*': 
     case '/': 
        if ((b == '+') || (b == '-')) 
        { 
           return 1; //a的优先级高于b 
        } 
        else 
        { 
           return 0; //a的优先级等于b 
        } 

 case '+': 
     case '-': 
    if ((b == '*') || (b == '/')) 
    { 
       return -1; //a的优先级低于b 
        }
    else 
{ 
   return 0; 
} 

default: 
     printf("运算符错误！\n"); 
     break; 
} 
} 


int Operate(int j, int k, char operater) 
{ 
switch (operater) 
{ 
case '+': 
return k + j; 
//break;  
case '-': 
return k - j; 
//break; 
case '*': 
return k * j; 
//break;  
case '/':  
return k / j; 
//break;  
default: printf("运算错误！\n"); 
         break; 
} 
} 

int calculate(int len, char *expStr) 
{ 
//表达式异常处理，注意len <= 0要加括号 
if ((len <= 0 ) || (expStr == NULL)) 
{ 
printf("表达式为空！\n"); 
} 
int i = 0, j = 0, k = 0; 
std::stack<int> date; 
std::stack<char> operate; 
while (i < len) 
{ 
if ((expStr[i] >= '0') && (expStr[i] <= '9')) 
{ 
date.push(expStr[i] - '0'); 
i++; 
} 
else if ((expStr[i] == '+') || (expStr[i] == '-') || expStr[i] == '*' || expStr[i] == '/') 
{ 
if (operate.empty()) 
{ 
operate.push(expStr[i]);
i++; 
} 
else 
{ 
//与栈顶运算符判断优先级 
switch (Precede(expStr[i], operate.top())) 
{ 
  case 0: 
  case -1: //栈顶运算符优先级高 
   j = date.top(); 
   date.pop(); 
   k = date.top(); 
   date.pop(); 
   date.push(Operate(j, k , operate.top()));
   operate.pop(); 
   //i++;  
   break; 

  case 1: //栈运算符顶优先级低 
   operate.push(expStr[i]); 
   i++; 
   break; 
  default: 
   printf("优先级判断错误！\n");
   break; 
 } 
} 
} 
else 
{ 
printf("表达式无法识别！\n");
break; 
}
} 

while (!operate.empty()) 
{ 
j = date.top(); 
date.pop(); 
k = date.top(); 
date.pop(); 
date.push(Operate(j, k , operate.top())); 
operate.pop(); 
} 

return date.top(); 
} 

int main() 
{ 
   int len = 0; int i= 0, input_flag = 0; 

   while (!input_flag) 
   { 
  printf("请输入表达式的长度："); 
  input_flag = scanf("%d", &len); 
  if (!input_flag) 
  { 
printf("输入有误，仅可输入数字！\n"); 
  } 
  flushall(); 
    } 
char *expStr = (char *)malloc(len * sizeof(char)); 
input_flag = 0;

while (!input_flag) 
{ 
printf("请输入表达式："); 
for (i = 0; i < len; i++) 
{ 
input_flag = scanf("%c", &expStr[i]); 
//scanf("%c", &expStr[i]);  
/*
if ((i < len) && (expStr[i] == '\n')) 
{
printf("长度不够，请重新输入："); 
i = 0; 
}
*/ 
if (!input_flag) 
{ 
printf("表达式输入有误！\n"); 
flushall(); 
break; 
} 
 } 
 } 

 printf("表达式的计算结果为：%d\n", calculate(len, expStr)); 

  return 0; 
}