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Getting Started with Loops and Standard Inputs in Python

As a programmer, many programs that you'll write will solve an end user's problem. You will always want to get some data from the user. This article will explain how to handle user input and how to use while loops.

Prerequisites

Python installed on your computer.
Basic Python knowledge.

Objectives

In this article we will go through:

How the input() function works
Using int() to accept numerical input
Introduction to while loops
Using the while loop with the else statement
Exiting the while loop using break
Removing all instances of specific values from a list using a while loop
Filling a dictionary with user input using a while loop

How the input() function works

The input() function halts the execution of a program and waits for the user to key in some data. When Python receives the user's input, it stores the input in the variable that you choose to work with.

For example, let's create a program that accepts a user's name and prints back the name.

name = input("Please enter your name: ")
print(name)
#output
#Please enter your name: John
#John

The input() function takes in one argument, that is, the instruction you want the user to see. In this example, Python executes the first line and requests the user to input his/her name. The program execution will halt and wait for the user to input his/her name and continue after pressing ENTER key on the keyboard. The name of the user is then loaded in the variable name, then print(name) displays the user's name back.

Using int() to accept numerical input

Any text the user input using the input() function, is interpreted as a string. If you only need to print out the input then using the input() function is sufficient. Using the input directly as a numerical value throws an exception. Consider the example below:

amount = input("Please enter your amount? ")
rate=0.056
interest= amount*rate
print("Your interest is : ")
print(interest)

The example above throws an error because Python interprets the input amount as a string. Python can't multiply a string and float.

To solve the above problem we use the int() function which informs Python to use the input as a numeric value. The int() function converts the string to integer, shown below:

amount = input("Please enter your amount? ")
amount=int(amount)
rate=0.056
interest= amount*rate
print("Your interest is : ")
print( interest)

Output:

Please enter your amount? 9000
Your interest is : 
504.0

When we input 9000 in the above example, Python interprets it as a string. The amount is then converted to an integer using the int() function. Now Python calculates the value of the interest.

Introducing while loops

In python, a while loop executes a given block of code provided that certain condition remains true.

Syntax:

while expression:
 statement(s)

The following while loop counts from 10 to 15:

count = 10
while (count <=15):
 print ('The count is:', count)
 count = count + 1 #add 1 to count
#output
#The count is: 10
#The count is: 11
#The count is: 12
#The count is: 13
#The count is: 14
#The count is: 15

The while loop continues to execute if the value of count is equal to or less than 15.

Using else statement with while loop

When you use the else statement in a while loop, it will only execute when the condition becomes false.

The example below involves both the else statement and the while statement.

count = 10
while (count <=15):
 print ('The count is:', count)
 count = count + 1
else:
 print(count, " is not less than 5")

The above program prints a number less or equal to 15, otherwise the else block executes.

Results:

The count is: 11
The count is: 12
The count is: 13
The count is: 14
The count is: 15
16 is not less than 15

Using break to exit a loop

Use the break statement to exit a while loop without executing any remaining code in the loop. With the break statement you can control which lines of your program will execute or not.

Syntax:

while expression:
 #code for while loop
 if_expression:
 break
 #code for while loop
# code outside of while loop

Example:

i = 1
while i < 11:
 if i == 6:
 break
 print(i)
 i = i + 1
print('Bye')
#output
#1
#2
#3
#4
#5
#Bye

Removing all instances of specific values from a list using a while loop

The remove() method only removes a single value from a list. We remove all occasions of a value from a list using the while loop.

Say we have a list of employees and the name John appears more than once. We can remove all instances of the name John using the while loop. The while loop executes until John is no longer in the list as illustrated below:

employees = ['Mary', 'John', 'Paul', 'John', 'Yusuf', 'John'] #list containing many instances of 'John'
print(employees) # ['Mary', 'John', 'Paul', 'John', 'Yusuf', 'John']
while 'John' in employees:
 employees.remove('John')
print(employees) #['Mary', 'Paul', 'Yusuf']

Filling a dictionary with user input using a while loop

We use the while loop to prompt the users to enter as much input as we need. Let's create a program that accepts the username and the name of the mountain that each user likes to climb. Since we want to connect each response with a particular user, we will store data in a dictionary.

responses = {} # define an empty dictionary
# Set a flag to show that polling is active.
polling_active = True
while polling_active:
# Prompt for the person's name and response.
 name = input("\nEnter your name? ")
 response = input("Enter the name of the mountain you would like to climb? ")
 responses[name] = response # Store the response in the dictionary:
 # Find out if anyone else is going to take the poll.
 repeat = input("Please refer another person? (yes/ no) ")
 if repeat == 'no':
 polling_active = False
print("\n....Poll Results...")
for name, response in responses.items():
 print(name + " wishes to climb " + response + ".") # print results of the poll

When you execute this program and enter some responses, the output should look like this:

Enter your name? Peter
Enter the name of the mountain you would like to climb? Everest
Please refer another person? (yes/ no) yes
Enter your name? John
Enter the name of the mountain you would like to climb? Turin
Please refer another person? (yes/ no) no
....Poll Results...
Peter wishes to climb Everest.
John wishes to climb Turin.