1. Output: Create a basic 4 function ( add, subtract, multiply, & divide ) calculator
2. What you learn from the example:
   1. Creates a menu
   2. Use while loop, conditionals, and functions
   3. Get input from user

# This function adds two numbers
def add(x, y):
    return x + y

# This function subtracts two numbers
def subtract(x, y):
    return x - y

# This function multiplies two numbers
def multiply(x, y):
    return x * y

# This function divides two numbers
def divide(x, y):
    return x / y


print('Select operation.')
print('1.Add')
print('2.Subtract')
print('3.Multiply')
print('4.Divide')

while True:
    # take input from the user
    choice = input('Enter choice(1/2/3/4): ')

    # check if choice is one of the four options
    if choice in ('1', '2', '3', '4'):
        try:
            num1 = float(input('Enter first number: '))
            num2 = float(input('Enter second number: '))
        except ValueError:
            print('Invalid input. Please enter a number.')
            continue

        if choice == '1':
            print(num1, '+', num2, '=', add(num1, num2))

        elif choice == '2':
            print(num1, '-', num2, '=', subtract(num1, num2))

        elif choice == '3':
            print(num1, '*', num2, '=', multiply(num1, num2))

        elif choice == '4':
            print(num1, '/', num2, '=', divide(num1, num2))
        
        # check if user wants another calculation
        # break the while loop if answer is no
        next_calculation = input('Next calculation? (yes/no): ')
        if next_calculation == 'no':
          break
    else:
        print('Invalid Input')