#include <stdio.h>
#include <stdlib.h>
#define MAX 80

void inToPostfix(char*, char*); // 中序轉後序
int priority(char); // 運算子優先
double eval(char*);
double cal(char, double, double);
int main(void) {
char infix[MAX] = {'\0'};
printf("運算式：");
scanf("%s", infix);
printf("%f", eval(infix));
return 0;
}
void inToPostfix(char* infix, char* postfix) {
char stack[MAX] = {'\0'};
int i, j, top;
for(i = 0, j = 0, top = 0; infix[i] != '\0'; i++) switch(infix[i]) {
case '(':              // 運算子堆疊
            stack[++top] = infix[i];
break;
case '+': case '-': case '*': case '/':
while(priority(stack[top]) >= priority(infix[i])) {
postfix[j++] = stack[top--];
}
stack[++top] = infix[i]; // 存入堆疊
            break;
case ')':
while(stack[top] != '(') { // 遇 ) 輸出至 (
                postfix[j++] = stack[top--];
}
top--;  // 不輸出 (
            break;
default:  // 運算元直接輸出
            postfix[j++] = infix[i];
}
while(top > 0) {
postfix[j++] = stack[top--];
}
}
int priority(char op) {
switch(op) {
case '+': case '-': return 1;
case '*': case '/': return 2;
default:            return 0;
}
}
double eval(char* infix) {
char postfix[MAX]= {'\0'};
char opnd[2] = {'\0'};
double stack[MAX] = {0.0};
inToPostfix(infix, postfix);
int top, i;
for(top = 0, i = 0; postfix[i] != '\0'; i++) switch(postfix[i]) {
case '+': case '-': case '*': case '/':
stack[top - 1] = cal(postfix[i], stack[top - 1], stack[top]);
top--;
break;
default:
opnd[0] = postfix[i];
stack[++top] = atof(opnd);
}
return stack[top];
}
double cal(char op, double p1, double p2) {
switch(op) {
case '+': return p1 + p2;
case '-': return p1 - p2;
case '*': return p1 * p2;
case '/': return p1 / p2;
}
}