прост калкулатор

#include<iostream>
#pragma warning (disable:4996) //no errors with strcat

const int STR_MAX = 100;

bool isOperator(const char& c) {
    return c == '+' || c == '-' || c == '*' || c == '/' || c == '=';
}

int calculate(const char* str) {
    int firstNumber = 0;
    int secondNumber = 0;

    bool firstNumInput = true;

    int strSize = strlen(str);

    char prevSign;

    for (int i = 0; i < strSize; i++) {
        if (!isOperator(str[i])) {
            if (firstNumInput) {
                firstNumber *= 10;
                firstNumber += str[i] - '0';
            }
            else {
                secondNumber *= 10;
                secondNumber += str[i] - '0';
            }
        }
        else {
            if (firstNumInput) {
                firstNumInput = false;
            }
            else {
                switch (prevSign){
                case '+':
                    firstNumber = firstNumber + secondNumber;
                    secondNumber = 0;
                    break;
                case '-':
                    firstNumber = firstNumber - secondNumber;
                    secondNumber = 0;
                    break;
                case '*':
                    firstNumber = firstNumber * secondNumber;
                    secondNumber = 0;
                    break;
                case '/':
                    firstNumber = firstNumber / secondNumber;
                    secondNumber = 0;
                    break;
                default:
                    std::cout << "Invalid operator!";
                    return -1;
                    break;
                }
            }
            prevSign = str[i];
        }
    }

    return firstNumber;
}

int main() {
    char str[STR_MAX];

    std::cin.getline(str, STR_MAX, '=');

    //dobavq ravno nakraq
    strcat(str, "=");

    std::cout << calculate(str);
}