# Erle Robotics Learning Python GitBook Free

## While loops

The while loop is similar to an if statement: it executes the code inside of it if some condition is true. The difference is that the while loop will continue to execute as long as the condition is true. In other words, instead of executing if something is true, it executes while that thing is true.

###### Practice 1

We are going to use whileand ifto see the difference:

>>> count = 0
>>>
>>> if count < 10:
...     print "Hello, I am an if statement and count is", count
...
Hello, I am an if statement and count is 0
>>>


When using the following code we print the sentence 9 times(< is not the same as <=).Remember always to actualize the count, if not you get an infinite loop.

... while count < 10:
...     print "Hello, I am a while and count is", count
...     count += 1
...
Hello, I am a while and count is 0
Hello, I am a while and count is 1
Hello, I am a while and count is 2
Hello, I am a while and count is 3
Hello, I am a while and count is 4
Hello, I am a while and count is 5
Hello, I am a while and count is 6
Hello, I am a while and count is 7
Hello, I am a while and count is 8
Hello, I am a while and count is 9
>>>


###### Practice 2

Create a while loop that prints out all the numbers from 1 to 10 squared (1, 4, 9, 16, ... , 100), each on their own line.

>>> num = 1
>>>
>>> while num<=10:  # Fill in the condition
...  print num**2    # Print num squared
...  num += 1 # Increment num (make sure to do this!)
...
1
4
9
16
25
36
49
64
81
100
>>>


While/else

Something completely different about Python is the while/else construction. while/else is similar to if/else, but there is a difference: the else block will execute anytime the loop condition is evaluated to False. This means that it will execute if the loop is never entered or if the loop exits normally. If the loop exits as the result of a break, the else will not be executed.

###### Practice 3

Copy this code in a file called game.pyand run it:

import random

print "Lucky Numbers! 3 numbers will be generated."
print "If one of them is a '5', you lose!"

count = 0
while count < 3:
num = random.randint(1, 6)
print num
if num == 5:
print "Sorry, you lose!"
break
count += 1
else:
print "You win!"


###### Practice 4

A common application of a while loop is to check user input to see if it is valid. For example, if you ask the user to enter y or n and they instead enter 7, then you should re-prompt them for input.Analyze the code bellow:

>>> choice = raw_input('Enjoying the course? (y/n)')
Enjoying the course? (y/n) p
>>> while choice!="y" and choice !="n":  # Fill in the condition (before the colon)
...     choice = raw_input("Sorry, I didn't catch that. Enter again: ")
...
Sorry, I didn't catch that. Enter again: e
Sorry, I didn't catch that. Enter again: y
>>>