用栈的方法实现表达式求值运算，但是为什么得不出结果？求大家解释一下

#include <stdio.h>
#include <stdlib.h>
#define STACK_INIT_SIZE 100
typedef int Status;
typedef char SElemType;
typedef struct {
    SElemType* base;
    SElemType* top;
    int stacksize;
}SqStack;
Status InitStack(SqStack& S) {    //初始化
    S.base = (SElemType *)malloc(STACK_INIT_SIZE * sizeof(SElemType));
    if (!S.base)    return NULL;
    S.top = S.base;
    S.stacksize = STACK_INIT_SIZE;
    return 99;
}
Status Push(SqStack& S, SElemType e) {    //入栈
    if (S.top - S.base == S.stacksize)
        return NULL;    //栈满
    *S.top++ = e;    //元素e压入栈顶，栈顶指针加1
    return 99;
}
Status Pop(SqStack& S, SElemType& e) {    //出栈
    if (S.top == S.base)    //栈空
        return NULL;
    e = *--S.top;    //栈顶指针减1，将栈顶元素赋给e
    return 99;
}
Status GetTop(SqStack S) {    //取栈顶元素
    if (S.top != S.base)    //栈非空
        return *(S.top-1);
}
int In(SElemType e) {    //判断读入字符是否为运算符
    if (e == '+' || e == '-' || e == '*' || e == '/' || e == '(' || e == ')' || e == '#')
        return 99;
    else
        return NULL;
}
SElemType Precede(SElemType a, SElemType b,SElemType &f) {    //比较运算符的优先级
    if (a == '+' || a == '-') {
        if (b == '+' || b == '-' || b == ')' || b == '#')
            f = '>';
        else if (b == '*' || b == '/' || b == '(')
            f = '<';
    }
    else if (a == '*' || a == '/') {
        if (b == '+' || b == '-' || b == '*' || b == '/' || b == ')' || b == '#')
            f = '>';
        else if (b == '(')
            f = '<';
    }
    else if (a == '(') {
        if (b == '+' || b == '-' || b == '*' || b == '/' || b == '(')
            f = '<';
        else if (b == ')')
            f = '=';
    }
    else if (a == ')') {
        if (b == '+' || b == '-' || b == '*' || b == '/' || b == ')' || b == '#')
            f = '>';
    }
    else if (a == '#') {
        if (b == '+' || b == '-' || b == '*' || b == '/' || b == '(')
            f = '<';
        else if (b == '#')
            f = '=';
    }
    return f;
}
SElemType Operate(SElemType a, SElemType theta, SElemType b,SElemType &e) {    //运算
    a = a - '\0';
    b = b - '\0';
    if (theta == '+')
        e = a + b + '\0';
    else if (theta == '-')
        e = a - b + '\0';
    else if (theta == '*')
        e = a * b + '\0';
    else if (theta == '/')
        e = a / b + '\0';
    return e;
}
int EvaluateExpression() {
    SqStack OPND, OPTR;
    char c, a, b, theta, x,f,e;
    InitStack(OPND);    //运算数栈
    InitStack(OPTR);    //运算符栈
    Push(OPTR, '#');
    c = getchar();
    while (c != '#' || GetTop(OPTR) != '#') {
        if (!In(c)) {
            Push(OPTR, c);
            c = getchar();
        }
        else
            switch (Precede(GetTop(OPTR), c,f)) {
            case '<':    //栈顶元素优先权低
                Push(OPTR, c); c = getchar();
                break;
            case '=':    //脱括号并接收下一字符
                Pop(OPTR, x); c = getchar();
                break;
            case '>':    //退栈并将运算结果入栈
                Pop(OPTR, theta);
                Pop(OPND, b); Pop(OPND, a);
                Push(OPND, Operate(a, theta, b,e));
                break;
            }
    }
    return GetTop(OPND)-'\0';
}
int main() {
    printf("输入表达式:\n");
    printf("结果是:%d\n", EvaluateExpression());
}