learn-co-curriculum/ds-python-functions-lab2

Basic calculator program using Python

SWABT

• Combine and experiment with methods taught upto this point including: 1. Data types 2. Iteration 3. Conditionals 4. Functions 5. USER I/O.

EXERCISE: Create a simple calculator which can perform basic arithmetic operations like addition, subtraction, multiplication or division depending upon the 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.

Example 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

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):
    return None # Return addition

Function to subtract two numbers

def subtract(num1, num2):
    return None # Return subtraction

Function to multiply two numbers

def multiply(num1, num2):
    return None # Return Product

Function to divide two numbers

def divide(num1, num2):
    return None # Return Division

The main program body to take user input and call the relevent function

Print user menu

print("Please select operation -\n" \
        "1. Add\n" \
        "2. Subtract\n" \
        "3. Multiply\n" \
        "4. Divide\n")

select = None
 
number_1 = None
number_2 = None

#Use a series of if-elif-else commands to pass the number to desired function defined above

  
DESIRED OUTPUT: 

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

Select operations form 1, 2, 3, 4 :3
Enter first number: 3
Enter second number: 3
3 * 3 = 9

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**

Write the code below to achieve this functionality

#initialize the code with cont (continue) flag set to yes (y)
cont = 'y'

#Check for user input after each iteration of the code
#Repeat the code above as long as cont = y 
#Break out of code if cont = n (Hint: use a while loop)

#Expected output 

#Select operations form 1, 2, 3, 4 :3
#Enter first number: 2
#Enter second number: 4
#2 * 4 = 8
#Continue? y/n:y
#Select operations form 1, 2, 3, 4 :2
#Enter first number: 4
#Enter second number: 5
#4 - 5 = -1
#Continue? y/n:n

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).

Exercise: 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.

#Redefine the division function including modulo and integer division

def divide(num1, num2):

    # Add a condition to existing function which prompts user for selecting division vs. modulo.
    
#Expected Output: 

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

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).