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Like other programming languages, Python language also provides us a feature of while loop, which allows us to execute a block of statements in a loop until a condition declared in while loop is being fulfilled.

Working of while loop

The condition of while loop is tested for the boolean value - true or false.

• If the condition is true then the block of statements in while loop is executed.
• If the condition is false then the block of statements associated with the while loop is skipped and not executed, but the optional else statement is executed(if declared).

Important points about conditional statements in Python

• As you can see in the diagram above, In Python, a colon: is required after declaring the condition of a conditional statement, be it an if, else or loop i.e. while, etc, which helps us in declaring the start of an associated indented block, and it also improves readability.


• In Python, the condition of an if, elif statement or while loop can even be declared without a pair of parentheses () around it(contrary to many programming languages like C, C++ which require us to wrap a condition within a pair of parentheses).


• Unlike many programming languages like C, C++, Java which require a pair of braces { } to hold multiple statements associated with a conditional statement like if, else etc or loop like while, the Python language does not require you to use a pair of braces { } to contain one or more statements with any conditional statement.


• Using a pair of braces { } to hold one or more statements with any conditional statement like if or else statement or with a while loop, gives an error.


• Then how do we associate one or multiple statements with a conditional statement like if, elif or else?
  In Python, indentation is meaningful and using it we can associate one or more statements with any conditional statement or looping structure.

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while loop example

#Python - while loop example

i=0;

while(i<5):
	print("i =", i)
	i=i+1

Output

i = 0
i = 1
i = 2
i = 3
i = 4

Program Analysis

• The condition of while loop i.e. i<5 is evaluated, which turns out to be true, hence its associated two intended statements are executed, out of which one statement is printing the value of i on screen and the other statement is incrementing the value of i.


• The two statements associated with the while loop are executed until the condition of while loop evaluated to true.

While loop must have an iteration part in its body or it ends an endless loop

#Python - indefinite while loop example

i=0;

while(i<5):
	print("i =", i)

Output

This program on execution runs indefinitely in a loop because of the absence of incremental part to increment the value of i.

While loop with optional else statement

Unlike other programming languages like C, C++, and Java, we can optionally declare else statement with while loop. The statement associated with this optional else statement is only executed when the condition of while loop evaluates to false.

#Python - while loop with optional else statement example


i=0;

while(i>5):
	print("i =", i)
else:
	print("i is not greater than 5")

Output

i is not greater than 5

In the above program, the condition of while loop is evaluated to false, therefore the statement associated with the optional else statement is executed.

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