简单字符四则运算

#include <assert.h>
#include <stdio.h>

int cal(int c1, char op, int c2)
{
    if (op=='+') return c1+c2;
    if (op=='-') return c1-c2;
    if (op=='*') return c1*c2;
    if (op=='/') return c1/c2;
    return 0;
}

int calculate(int len, char *expStr)
{
    assert(expStr);
    if (len < 3) return -1;
    char *p = expStr;
    int c1 = p[0]-'0';
    char op = p[1];
    int c2 = p[2]-'0';
    p += 3;
    while(*p) {
        if (*p=='*' || *p=='/') {
            c2 = cal(c2, *p, p[1]-'0');
        } else {
            c1 = cal(c1, op, c2);
            op = *p;
            c2 = p[1]-'0';
        }
        p += 2;
    }
    return cal(c1, op, c2);
}

int main(void)  {
    int ret1 = calculate(3, "1+2");
    int ret2 = calculate(11, "1+2*3/5-6*7");
    int ret3 = calculate(11, "1+2-3/5-6*7");
    printf ("values: %d, %d, %d", ret1, ret2, ret3);

    return 0;
}

// output:
// values: 3, -40, -39