//表达式求值
#include <stdio.h>
#include <malloc.h>
#include <string.h>
/*
*功能:根据运算符计算
*参数:a, b参与运算的数, ch运算符
*返回值:计算结果,操作符错误则返回0
*/
int cal(int a, char ch, int b)
{
    switch(ch)
    {
    case '+':
        return a+b;
        break;
    case '-':
        return a-b;
        break;
    case '*':
        return a*b;
        break;
    case '/':
        return a/b;
        break;
    }
    return 0;
}
/*
*功能:计算表达式的值(用数组模拟栈)
*参数:表达式字符串
*返回值:计算结果
*/
int evaluateExpression(char *str)
{
    int i = 0, result, numSub = 0, operSub = 0;
    int tmp, len = strlen(str);
    int *operand = (int*)malloc(sizeof(int)*len);
    char *operat = (char*)malloc(sizeof(char)*len);
    while(str[i] != '\0')
    {
        switch(str[i])
        {
        case '+':
            while(operSub > 0 && operat[operSub-1] != '(')
            {
                printf("%d %c %d = ", operand[numSub-2], operat[operSub-1], operand[numSub-1]);
                operand[numSub-2] = cal(operand[numSub-2], operat[operSub-1], operand[numSub-1]);
                printf("%d\n", operand[numSub-2]);
                --numSub;
                --operSub;
            }
            operat[operSub++] = '+';
            break;
        case '-':
            while(operSub > 0 && operat[operSub-1] != '(')
            {
                printf("%d %c %d = ", operand[numSub-2], operat[operSub-1], operand[numSub-1]);
                operand[numSub-2] = cal(operand[numSub-2], operat[operSub-1], operand[numSub-1]);
                printf("%d\n", operand[numSub-2]);
                --numSub;
                --operSub;
            }
            operat[operSub++] = '-';
            break;
        case '*':
            if(str[i+1] >= '0' && str[i+1] <= '9')
            {
                tmp = 0;
                while(str[i+1] >= '0' && str[i+1] <= '9')
                {
                    tmp = tmp * 10 + str[i+1] - '0';
                    ++i;
                }
                --i;
                printf("%d * %d = ", operand[numSub-1], tmp);
                operand[numSub-1] = cal(operand[numSub-1], '*', tmp);
                printf("%d\n", operand[numSub-1]);
                ++i;
            }
            else
                operat[operSub++] = '*';
            break;
        case '/':
            if(str[i+1] >= '0' && str[i+1] <= '9')
            {
                tmp = 0;
                while(str[i+1] >= '0' && str[i+1] <= '9')
                {
                    tmp = tmp * 10 + str[i+1] - '0';
                    ++i;
                }
                --i;
                printf("%d / %d = ", operand[numSub-1], tmp);
                operand[numSub-1] = cal(operand[numSub-1], '/', tmp);
                printf("%d\n", operand[numSub-1]);
                ++i;
            }
            else
                operat[operSub++] = '/';
            break;
        case '(':
            operat[operSub++] = '(';
            break;
        case ')':
            while(operat[operSub-1] != '(')
            {
                printf("%d %c %d = ", operand[numSub-2], operat[operSub-1], operand[numSub-1]);
                operand[numSub-2] = cal(operand[numSub-2], operat[operSub-1], operand[numSub-1]);
                printf("%d\n", operand[numSub-2]);
                --numSub;
                --operSub;
            }
            --operSub;
            break;
        default:
            tmp = 0;
            while(str[i] >= '0' && str[i] <= '9')
            {
                tmp = tmp * 10 + str[i] - '0';
                ++i;
            }
            --i;
            operand[numSub++] = tmp;
            break;
        }
        ++i;
    }
    while(numSub > 1 && operSub >= 1)
    {
        printf("%d %c %d = ", operand[numSub-2], operat[operSub-1], operand[numSub-1]);
        operand[numSub-2] = cal(operand[numSub-2], operat[operSub-1], operand[numSub-1]);
        printf("%d\n", operand[numSub-2]);
        --numSub;
        --operSub;
    }
    result = operand[numSub-1];
    free(operand);
    free(operat);
    return result;
}
int main()
{
    char *str = "225/15-20+(4-3)*2";
    int result;
    printf("计算过程:\n");
    result = evaluateExpression(str);
    printf("计算结果:result = %d\n", result);
    return 0;
}

计算过程:
225 / 15 = 15
15 - 20 = -5
4 - 3 = 1
1 * 2 = 2
-5 + 2 = -3
计算结果:result = -3