计算中缀表达式

1.计算后缀表达式

 2.中缀表达式转后缀表达式

#include <cstdio> 
#include <cstring> 
#include <cctype> 
#include<iostream>
using namespace std;

//char stack
char stack[25]; 
int top = -1; 

void push(char item) {
   stack[++top] = item; 
} 

char pop() {
   return stack[top--]; 
} 

//returns precedence of operators
int precedence(char symbol) {

   switch(symbol) {
      case '+': 
      case '-':
         return 2; 
         break; 
      case '*': 
      case '/':
         return 3; 
         break; 
      case '^': 
         return 4; 
         break; 
      case '(': 
      case ')': 
      case '#':
         return 1; 
         break; 
   } 
} 

//check whether the symbol is operator?
int isOperator(char symbol) {

   switch(symbol) {
      case '+': 
      case '-': 
      case '*': 
      case '/': 
      case '^': 
      case '(': 
      case ')':
         return 1; 
      break; 
         default:
         return 0; 
   } 
} 

//converts infix expression to postfix
void convert(char infix[],char postfix[]) 
{
   int i,symbol,j = 0; 
   stack[++top] = '#'; 
    
   for(i = 0;i<strlen(infix);i++) 
   {
      symbol = infix[i]; 
        
      if(isOperator(symbol) == 0) 
      {
         postfix[j] = symbol; 
         j++; 
      } 
      else 
      {
         if(symbol == '(') 
         {
            push(symbol); 
         } 
         else 
         {
            if(symbol == ')') 
            {
               while(stack[top] != '(') 
               {
                  postfix[j] = pop(); 
                  j++; 
               } 
                    
               pop();   //pop out (. 
            } 
            else 
            {
               if(precedence(symbol)>precedence(stack[top])) 
               {
                  push(symbol); 
               }
               else 
               {
                    
                  while(precedence(symbol)<=precedence(stack[top])) 
                  {
                     postfix[j] = pop(); 
                     j++; 
                  } 
                  push(symbol); 
               }
            }
         }
      }
   }
    
   while(stack[top] != '#') 
   {
      postfix[j] = pop(); 
      j++; 
   } 
    
   postfix[j]='';  //null terminate string. 
} 

//int stack
int stack_int[25]; 
int top_int = -1; 

void push_int(int item) 
{
   stack_int[++top_int] = item; 
} 

char pop_int() 
{
   return stack_int[top_int--]; 
} 

//evaluates postfix expression
int evaluate(char *postfix)
{
   char ch;
   int i = 0,operand1,operand2;

   while( (ch = postfix[i++]) != '') 
   {
    
      if(isdigit(ch)) 
      {
         push_int(ch-'0');  // Push the operand 
      } 
      else 
      {
         //Operator,pop two  operands 
         operand2 = pop_int();
         operand1 = pop_int();
            
         switch(ch) 
         {
            case '+':
               push_int(operand1+operand2);
               break;
            case '-':
               push_int(operand1-operand2);
               break;
            case '*':
               push_int(operand1*operand2);
               break;
            case '/':
               push_int(operand1/operand2);
               break;
         }
      }
   }
    
   return stack_int[top_int];
}

int main() 
{ 
    char infix[100] ,postfix[25]; 
    cin>>infix;
    convert(infix,postfix); 
    
    printf("Infix expression is: %s
" , infix);
    printf("Postfix expression is: %s
" , postfix);
    printf("Evaluated expression is: %d
" , evaluate(postfix));
   
    return 0;
}