 In this program, we ask the user to choose the desired operation. Options 1,2,3 and 4 are valid. Two numbers are taken and an `if...elif...else` branching is used to execute a particular section. User-defined functions `add()`, `subtract()`, `multiply()` and `divide()` evaluate respective operations.

## Source Code

``` Program make a simple calculator that can add, subtract, multiply and divide using functions

# define functions
"""This function adds two numbers"""

return x + y

def subtract(x, y):
"""This function subtracts two numbers"""

return x - y

def multiply(x, y):
"""This function multiplies two numbers"""

return x * y

def divide(x, y):
"""This function divides two numbers"""

return x / y

# take input from the user
print("Select operation.")
print("2.Subtract")
print("3.Multiply")
print("4.Divide")

choice = input("Enter choice(1/2/3/4):")

num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))

if choice == '1':

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))
else:
print("Invalid input")```

Output

```Select operation.