Python
Python While Loops
Python while Loop is used to execute number of statements or body till the condition passed in while is true. Once the condition is false, the control will come out of the loop.
We generally use this loop when we don't know beforehand, the number of times to iterate.
Syntax :
while <expression>:
    Body of while
Here, body will execute multiple times till the expression passed is true. The Body may be a single statement or multiple statement.
>>> a=10
>>> while a>0:
	print ("Value of a is",a)
	a=a-2

	
Value of a is 10
Value of a is 8
Value of a is 6
Value of a is 4
Value of a is 2
>>> 

Explanation :

Firstly, the value in the variable is initialized.

Secondly, the condition/expression in the while is evaluated. Consequently if condition is true, the control enters in the body and executes all the statements . If the condition/expression passed results in false then the control exists the body and straight away control goes to next instruction after body of while.

Thirdly, in case condition was true having completed all the statements, the variable is incremented or decremented. Having changed the value of variable step second is followed. This process continues till the expression/condition becomes false.

Finally Rest of code after body is executed.

>>> n = 10
>>> sum = 0
>>> i = 1
>>> while i <= n:
	sum = sum + i
	i = i+1    # update counter
	# print the sum
	print("The sum is", sum)

	
The sum is 1
The sum is 3
The sum is 6
The sum is 10
The sum is 15
The sum is 21
The sum is 28
The sum is 36
The sum is 45
The sum is 55
>>> 

While loop with else

Same as that of for loop, we can have an optional else block with while loop as well.

The else part is executed if the condition in the while loop evaluates to False. The while loop can be terminated with a break statement.

In such case, the else part is ignored. Hence, a while loop's else part runs if no break occurs and the condition is false.

>>> # Example to illustrate
>>> # the use of else statement
>>> # with the while loop
>>> counter = 0
>>> while counter < 3:
	print("Inside loop")
	counter = counter + 1
else:
	print("Inside else")

	
Inside loop
Inside loop
Inside loop
Inside else
>>>