Project - 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 upon the user input.

Objectives

You will be able to:

• Perform operations on various data types
• Use loops to perform iteration
• Use conditionals to make programming decisions
• Define and use functions
• Ingest and parse user input

Approach

• User choose the desired operation. Options 1, 2, 3, 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% exactly 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 form 1, 2, 3, 4 : 1
Enter first number : 20
Enter second number : 13
20 + 13 = 33

Creating Arithmatic Functions

We shall create four functions, one for each arithmatic operation which will perform the required operation and resturn 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 shall now write the main program body to take user input and call the relevent 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 form 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 form 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:

Lets 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 calculationa 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 form 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 loops.

# 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 form 1, 2, 3, 4 :4
# Enter first number: 5
# Enter second number: 4
# Press d for division and m for modulo operatorm
# 5 / 4 = 1
# Continue? y/n:y
# Select operations form 1, 2, 3, 4 :4
# Enter first number: 5
# Enter second number: 4
# Press d for division and m for modulo operatord
# 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 he/she wants a normal division / (int) or // (float) , or a module % operator 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 incrementaly added more functionality to it by adding loops for iteration and further conditions allowing different type of calculations. We also practiced User I/O by taking choices from the users and dealing with exceptions (unexpected input).