运算字符串解析（c语言）

运算字符的解析，如：43+表示：4+3， 25* 表示：2*5， 25*1+表示（2*5）+1，435/+表示：4 +（3/5）

#include <stdio.h>
#include <stdbool.h>
#include <string.h>

bool isNumber(const char *c);
int getNumber(const char *c);
bool isOperator(const char *c);
float calculate(char *string, int len);

int main(int argc, const char * argv[])
{

    // insert code here...
    printf("Begin>>
");
    printf("%d",'0');
    printf("%d
",'/');
    char string[100] = "435/+";
    size_t len = strlen(string);
    float ret = calculate(string, (int)len);
    printf("result = %f
", ret);
    return 0;
}

bool isNumber(const char *c)
{
    if ( (int)(*c) >= (int)('0') && (int)(*c) <= (int)('9')) {
        return true;
    }
    return false;
}

int getNumber(const char *c)
{
    bool isN = isNumber(c);
    if (isN) {
        int n = (int)((*c) - (int)'0');
        return n;
    }else {
        return 0;
    }
}

bool isOperator(const char *c)
{
    if ((int)(*c) == (int)'+' || (int)(*c) == (int)'-' || (int)(*c) == (int)('*') || (int)(*c) == (int)('/') ) {
        return true;
    } else {
        return false;
    }
    
}

float calculate(char *string, int len)
{
    if (len == 1) {
        return (float)getNumber(string);
    }
    
    if (len == 2) {
        printf("error!");
    }
    
    if (!isOperator(string + len - 1)) {
        return 0.0f;
    }
    
    float a = 0.0f;
    float b = 0.0f;
    if (isOperator(string + len - 1 -1)) {
        a = (float)getNumber(string);
        b = calculate(string + 1, len - 2);
    } else {
        a = calculate(string, len - 2);
        b = (float)getNumber(string + len - 1 - 1);
    }
    
    char operator = *(string + len - 1);
    if ((int)operator == (int)'+') {
        return (a + b);
    } else if ((int)operator == (int)'-') {
        return (a - b);
    } else if ((int)operator == (int)'*') {
        return (a * b);
    } else if ((int)operator == (int)'/') {
        return (a / b);
    } else {
        return 0.0f;
    }
    
}