简单的表达式求值

#include 
using namespace std;
#define M 100
char ops[7] = {'+', '-', '*', '/', '(', ')', '='};
char cmp[7][7]= {{'>', '>', '<', '<', '<', '>', '>'},
    {'>', '>', '<', '<', '<', '>', '>'},
    {'>', '>', '>', '>', '<', '>', '>'},
    {'>', '>', '>', '>', '<', '>', '>'},
    {'<', '<', '<', '<', '<', '=', ' '},
    {'>', '>', '>', '>', ' ', '>', '>'},
    {'<', '<', '<', '<', '<', ' ', '='}
};
bool check(char ch)
{
    for (int i = 0; i < 7; i++)
        if (ch == ops[i])
            return true;
    return false;
}
char C(char ch1, char ch2)
{
    int i, m, n;
    for (i = 0; i < 7; i++)
    {
        if (ch1 == ops[i])
            m = i;
        if (ch2 == ops[i])
            n = i;
    }
    return   cmp[m][n];

}
void cal(int x, char op, int y, int &z)
{
    switch (op)
    {
    case '+':
        z = x + y;
        break ;
    case '-':
        z = x - y;
        break ;
    case '*':
        z = x * y;
        break ;
    case '/':
        z = x / y;
        break ;

    }
}

void work(char *str, int &result)
{
    int a, b, v;
    char ch, op;
    int temp, i = 0;
    stack  oper;
    stack  num;
    oper.push('=');
    ch = str[i++];
    while (ch != '=' || oper.top() != '=')
    {
        if (check(ch))
        {
            switch (C(oper.top(), ch))
            {
            case '<':
                oper.push(ch);
                ch = str[i++];
                break;
            case '=':
                oper.pop();
                ch = str[i++];
                break;
            case '>':
                op = oper.top();
                oper.pop();
                b = num.top();
                num.pop();
                a = num.top();
                num.pop();
                cal(a, op, b, v);
                num.push(v);
                break;

            }
        }
        else
        {
            temp = ch - '0';
            num.push(temp);
            ch=str[i++];
        }

    }
    result = num.top();
    printf("%d\n",result);
}
int main()
{
    int i = 0;
    int result;
    char *str;
    str = new char[M];
    cin >> str[i];
    while (str[i] != '=')
    {
        cin >> str[++i];
    }
    work(str, result);
    return 0;
}