Functions Practice: Building a Calculator

Introduction

In this project, you'll create a simple calculator which can perform basic arithmetic operations like addition, subtraction, multiplication, or division depending on the user input.

Objectives

In this lab you will:

• Use a while loop
• Incorporate input/output functionality in code to allow for user interaction
• Declare and use a function with arguments
• Use break and continue to add control flow to a while loop

Approach

• User chooses the desired operation. Options 1, 2, 3, and 4 are valid options for operations
• Two numbers are taken and an if…elif…else branching is used to execute a particular section
• Using functions add(), subtract(), multiply(), and divide() evaluate respective operations
• The code should handle exceptions and must return "invalid input" when an unexpected character is given in the input (anything other than 1 - 4)

Example interface

Here is the interface you are expected to build. Don't worry if it is not 100% the same as what is shown. Focus more on the getting the logic correct at this stage.

Please select an operation:
1. Add
2. Subtract
3. Multiply
4. Divide

Select operations from 1, 2, 3, 4 : 1
Enter first number : 20
Enter second number : 13
20 + 13 = 33

Creating arithmetic functions

We'll create four functions, one for each arithmetic operation which will perform the required operation and return the resulting value as shown below:

# Function to add two numbers 
def add(num1, num2):
    # Perform the calculation
    return None

# Function to subtract two numbers 
def subtract(num1, num2):
    # Perform the calculation
    return None

# Function to multiply two numbers
def multiply(num1, num2):
    # Perform the calculation
    return None

# Function to divide two numbers
def divide(num1, num2):
    # Perform the calculation
    return None

Create a command-line user interface

We'll now write the main program body to take user input and call the relevant function:

# Print user menu 


# Take input from the user for operation, followed by numbers  


# Based on operation, pass the two numbers to respective function
# Print the output in a nice manner
# Print "Invalid input" if an unexpected character is seen in input


# Expected output    

# Please select operation -
# 1. Add
# 2. Subtract
# 3. Multiply
# 4. Divide

# Select operations from 1, 2, 3, 4 :1
# Enter first number: 2
# Enter second number: 3
# 2 + 3 = 5

Please select operation -

1. Add
2. Subtract
3. Multiply
4. Divide

Select operations from 1, 2, 3, 4 :1
Enter first number: 2
Enter second number: 3
2 + 3 = 5

Bring in the while loop

We can see how the logic set by using if-else statements, along with functions can be used to control the flow of the program in an easy way. Let's add more functionality to our calculator as below:

Let's try to make it a bit more interesting by introducing the behaviour of a real calculator so our users can choose to either continue with calculations OR exit the system. Users gets this functionality by pressing y for yes and n for no towards continuation.

Example interface

Notice continue: y/n at the bottom of interface.

Please select an operation:
1. Add
2. Subtract
3. Multiply
4. Divide

Select operations from 1, 2, 3, 4 : 1
Enter first number : 20
Enter second number : 13
20 + 13 = 33

Continue: y/n

Let's work towards implementing iteration into the equation and enclose above I/O interface inside a while loop.

# Initialize the code with cont (continue) flag set to yes (y)


# Check for user input after each iteration of the code in a while loop


        # Enclose the I/O  code block inside the while loop
        


# Expected output format

# Select operations from 1, 2, 3, 4 :4
# Enter first number: 5
# Enter second number: 4
# Press d for division and m for modulo operator :m
# 5 / 4 = 1
# Continue? y/n:y
# Select operations from 1, 2, 3, 4 :4
# Enter first number: 5
# Enter second number: 4
# Press d for division and m for modulo operator :d
# 5 / 4 = 1.25

Level up (Optional)

The while loop shown above allows the iteration through the code until a specific input from user i.e. n is noticed. Let's add some more functionality to this code by asking users about the type of division they are interested in, and this could be either normal division (as before) or a modulo operator (shows remainder).

Change the code in the division function so that if a user selects division operation, the code should ask the user if they want a normal division / or floor division //, or a modulo division % which only returns the remainder of a division. The program should return an exception for any other inputs.

def divide_v2(num1, num2):
    # Perform the calculation
    return None

Summary

In this lab, we saw how loops and conditions can be used to control the logic of a program execution based on user input. We started with building a simple calculator and incrementally added more functionality to it by adding loops for iteration and further conditions allowing different types of calculations. We also practiced user I/O by taking choices from the users and dealing with exceptions (unexpected input).