Python While Loop

25 Aug, 2021
Python While Loop is used to execute a block of statements repeatedly until a given condition is satisfied. And when the condition becomes false, the line immediately after the loop in the program is executed. While loop falls under the category of indefinite iteration. Indefinite iteration means that the number of times the loop is executed isn’t specified explicitly in advance. 
Syntax: 
while expression:
    statement(s)
Statements represent 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. When a while loop is executed, expr is first evaluated in a Boolean context and if it is true, the loop body is executed. Then the expr is checked again, if it is still true then the body is executed again and this continues until the expression becomes false.
Python3
# Python program to illustrate# while loopcount = 0while (count < 3):    count = count + 1    print("Hello Geek")
Hello Geek
Hello Geek
Hello Geek
In the above example, the condition for while will be True as long as the counter variable (count) is less than 3. 
Python3
# checks if list still# contains any elementa = [1, 2, 3, 4] while a:    print(a.pop())
4
3
2
1
In the above example, we have run a while loop over a list that will run until there is an element present in the list.
Just like the if block, if the while block consists of a single statement we can declare the entire loop in a single line. If there are multiple statements in the block that makes up the loop body, they can be separated by semicolons (;). 
Python3
# Python program to illustrate# Single statement while blockcount = 0while (count < 5): count += 1; print("Hello Geek")
Output: 
Hello Geek
Hello Geek
Hello Geek
Hello Geek
Hello Geek
Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. Python supports the following control statements.
Python Continue Statement returns the control to the beginning of the loop.
Python3
# Prints all letters except 'e' and 's'i = 0a = 'geeksforgeeks' while i < len(a):    if a[i] == 'e' or a[i] == 's':        i += 1        continue             print('Current Letter :', a[i])    i += 1
Output: 
Current Letter : g
Current Letter : k
Current Letter : f
Current Letter : o
Current Letter : r
Current Letter : g
Current Letter : k
Python Break Statement brings control out of the loop.
Python3
# break the loop as soon it sees 'e'# or 's'i = 0a = 'geeksforgeeks' while i < len(a):    if a[i] == 'e' or a[i] == 's':        i += 1        break             print('Current Letter :', a[i])    i += 1
Output: 
Current Letter : g
The Python pass statement to write empty loops. Pass is also used for empty control statements, functions, and classes.
Python3
# An empty loopa = 'geeksforgeeks'i = 0 while i < len(a):    i += 1    pass   print('Value of i :', i)
Output: 
Value of i : 13
As discussed above, while loop executes the block until a condition is satisfied. When the condition becomes false, the statement immediately after the loop is executed. The else clause is only executed when your while condition becomes false. If you break out of the loop, or if an exception is raised, it won’t be executed.
Note: The else block just after for/while is executed only when the loop is NOT terminated by a break statement. 
Python3
# Python program to demonstrate# while-else loop i = 0while i < 4:    i += 1    print(i)else:  # Executed because no break in for    print("No Break\n") i = 0while i < 4:    i += 1    print(i)    breakelse:  # Not executed as there is a break    print("No Break")
Output: 
1
2
3
4
No Break

1
In this, we don’t use any counter variable because we don’t know that how many times the loop will execute. Here user decides that how many times he wants to execute the loop. For this, we use a sentinel value. A sentinel value is a value that is used to terminate a loop whenever a user enters it, generally, the sentinel value is -1.
Python3
a = int(input('Enter a number (-1 to quit): ')) while a != -1:    a = int(input('Enter a number (-1 to quit): '))
Output:
Explanation:
First, it asks the user to input a number. if the user enters -1 then the loop will not execute
User enter 6 and the body of the loop executes and again ask for input
Here user can input many times until he enters -1 to stop the loop
User can decide how many times he wants to enter input
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