#include  <iostream>

using namespace std;

int getLength(char first[]);
int fromCharToInt(char arr[]);
int Pow(int num, int pow);
int Factorial(int n);

int main()
{
	char* expression = new char[100];
	int result = 0;
	int number = 0;

	cout << "Enter an expression to calculate or write 'exit' to close the program:" << endl;

	cin.getline(expression, 100);
	char operation = ' ';

	if (expression[0] == 'e' && expression[1] == 'x' && expression[2] == 'i' && expression[3] == 't' && expression[4] == '\0')
		return 0;

	int sizeOfArray = getLength(expression);

	for (int i = 0; i < sizeOfArray; i++)
	{
		if (expression[i] != '+' && expression[i] != '-' && expression[i] != '*' && expression[i] != '!' && expression[i] != '/'
			&& expression[i] != ' ' && expression[i] != '=' && (expression[i] < '0' || expression[i]>'9'))
		{
			cout << "This expression is invalid!" << endl;
			return 0;
		}
		if (expression[i] == '!' && expression[i + 1] == '!')
		{
			cout << "This expression is invalid!" << endl;
			return 0;
		}
		if (expression[i] == ' ' && expression[i + 1] == '!')
		{
			cout << "This expression is invalid!" << endl;
			return 0;
		}
	}
	if (expression[0] == ' ')
	{
		cout << "This expression is invalid!" << endl;
		return 0;
	}
	if (expression[sizeOfArray - 1] != '=')
	{
		cout << "This expression is invalid!" << endl;
		return 0;
	}

	char* firstNum = new char[35];
	int elementsFirstNum = 0;
	int i = 0;
	while (expression[i] != ' ')
	{
		if (expression[0] == '-')
		{
			i++;
		}
		firstNum[elementsFirstNum] = expression[i];
		elementsFirstNum++;
		i++;
	}
	firstNum[elementsFirstNum] = '\0';
	result = fromCharToInt(firstNum);


	for (int i = getLength(firstNum); i < sizeOfArray - 1; i++)
	{
		if (expression[i] != ' ')
		{
			cout << "This expression is invalid!" << endl;
			return 0;
		}
		else
		{
			i++;
		}

		if (expression[i] == '+' || expression[i] == '-' || expression[i] == '*' || expression[i] == '/' || expression[i] == '!')
		{
			operation = expression[i];
			i += 2;
		}

		char* secondNum = new char[35];
		int elementsSecondNum = 0;
		while (expression[i] != ' ')
		{
			if (expression[i] == '!')
			{
				i++;
				break;
			}
			secondNum[elementsSecondNum] = expression[i];
			elementsSecondNum++;
			i++;
		}
		secondNum[elementsSecondNum] = '\0';
		number = fromCharToInt(secondNum);

		if (expression[i - 1] == '!')
		{
			number = Factorial(number);
		}
		i--;

		switch (operation)
		{
		case '+':
			result = result + number;
			break;
		case '-':
			result = result - number;
		break;
		case '*':
			result = result * number;
			break;
		case '/':
			if (number == 0)
			{
				cout << "This expression is invalid!" << endl;
				return 0;
			}
			result = result / number;
			break;
		default:
			break;
		}

		delete[]secondNum;

		if (expression[i + 2] == '=')
			break;
	}

	cout << result;

	delete[]expression;
	delete[]firstNum;
	return 0;
}

int getLength(char first[])
{
	int counter = 0;
	while (first[counter] != '\0')
	{
		counter++;
	}
	return counter;
}

int fromCharToInt(char arr[])
{
	int size = getLength(arr);
	int number = 0;
	int pow = size - 1;

	for (int i = 0; i < size; i++)
	{
		number += (arr[i] - '0') * Pow(10, pow--);
	}
	return number;
}

int Pow(int num, int pow)
{
	int result = 1;

	for (int i = 0; i < pow; i++) {
		result *= num;
	}
	return result;
}

int Factorial(int n)
{
	if (n == 0)
	{
		return 1;
	}
	else
	{
		return n * Factorial(n - 1);
	}
}