Python 5
While loops in Python
The basic way that most while loops are implemented work like this:
#set a loop control variable that will be used to get in or stay in the while loop
while (some condition than involved that loop control variable previously set):
#some executable statements that are done while in the loop
#change that loop control variable in some way
The while loop is one of the looping constructs available in Python. The while loop continues until the expression becomes false. The expression has to be a logical expression and must return either a true or a false value.
The syntax of the while loop is:
while expression:
statement(s)
Here expression is evaluated first. If expression is true that is, then the statement(s) block is executed repeatedly until expression becomes false. Otherwise, the next statement following the statement(s) block is executed.
Note: In Python, all the statements indented by the same number of character spaces after a programming construct are considered to be part of a single block of code. Python uses indentation as its method of grouping statements.
Example:
count = 0
while (count < 9):
print "The count is: ", count
count = count +1
print “Good bye!”
This will produce following result:
The count is: 0
The count is: 1
The count is: 2
The count is: 3
The count is: 4
The count is: 5
The count is: 6
The count is: 7
The count is: 8
Good bye!
The block here, consisting of the print and increment statements, is executed repeatedly until count is no longer less than 9. With each iteration, the current value of the index count is displayed and then increased by 1.
Example:
ans = raw_input("More input? ").strip()while ans=="yes" or ans=="Yes":
print "I Love Python"
ans = raw_input("More input? ").strip()
print "Good bye!"
This will produce following result:
More input? YesI Love Python
More input? yes
I Love Python
More input? yes
I Love Python
More input? yes
I Love Python
More input? no
Good bye!
The Infinite Loops:
You must use caution when using while loops because of the possibility that this condition never resolves to a false value. This results in a loop that never ends. Such a loop is called an infinite loop.
An infinite loop might be useful in client/server programming where the server needs to run continuously so that client programs can communicate with it as and when required.
Example:
Following loop will continue until you enter 1 at command prompt:
var = 1while var == 1 : # This constructs an infinite loop since var doesn't change inside the loop
num = raw_input("Enter a number :")
print "You entered: ", num
print "Good bye!"
This will produce the following result:
Enter a number :20You entered: 20
Enter a number :29
You entered: 29
Enter a number :3
You entered: 3
Enter a number between :Traceback (most recent call last):
File "test.py", line 5, in <module>
num = raw_input("Enter a number :")
KeyboardInterrupt
Above example will go in an infinite loop and you would need to use CNTL+C to come out of the program.
------
Single Statement Suites:
Similar to the if statement syntax, if your while clause consists only of a single statement, it may be placed on the same line as the while header.
Here is an example of a one-line while clause:
while expression : statement------
The continue Statement:
The continue statement in Python returns the control to the beginning of the while loop. The continue statement rejects all the remaining statements in the current iteration of the loop and moves the control back to the top of the loop.
The continue statement can be used in both while and for loops.
Example:
for letter in "Python": # First Exampleif letter == "h":
continue
print "Current Letter :", letter
var = 10 # Second Example
while var > 0:
print "Current variable value :", var
var = var -1
if var == 5:
continue
#notice this continue at the end of the loop has no effect
print "Good bye!"
This will produce following result:
Current Letter : PCurrent Letter : y
Current Letter : t
Current Letter : o
Current Letter : n
Current variable value : 10
Current variable value : 9
Current variable value : 8
Current variable value : 7
Current variable value : 6
Current variable value : 5
Current variable value : 4
Current variable value : 3
Current variable value : 2
Current variable value : 1
Good bye!
While Example:
while x<3:
print x
x = x + 1
print x
Output?
Problem: Get a list of the email addresses that a person uses. The person can have any number of email addresses. Print out each email address after it is input.
Output should be:
What is your name?
George
Welcome, George
Are you done entering email addresses?
no
Ok, what is your next email address?
Are you done entering email addresses?
no
Ok, what is your next email address?
Are you done entering email addresses?
Yes
Ok, fine.