Python: While Loop

Introduction

In this article we will discuss about while loop in Python. A while loop is a control flow statement which checks condition, and condition is true then executes block of code again and again.

 

Syntax:

while(condition):
        Body of the loop

Flow Chart: The flow chart of while loop is given as follows:

 

In the above flow chart you can see whenever control of the program enters into the loop then, first of all on the entry time it will check some condition. If the condition will become true then only it will go inside the loop and execute the body of the the loop and again it will check after first execution and if condition is again true then it will again execute body of the loop. This iteration goes continuously while condition is true. Whenever condition will be false, the control of the program come out.

 

Example: Printing 1 to 20.

i=1
while(i<=20):
    print(i)
    i=i+1

Output: 

 

 

Example 2: Printing table of given number.

num=int(input("Enter a Number : "))
i=1
while(i<=10):
    print(num*i)
    i=i+1

Output: 

 

 

Example 3:  Printing reverse of a number.

num = int(input("Enter a Number : "))
rev = 0
while(num != 0):
   rem = num % 10
   rev = rev * 10 + rem
   num = int(num / 10)
print(rev)

Output:

 

 

While loop with else

In python we can also execute while loop with else part. While loop works same like a normal while loop, but whenever while loop condition will be false its else part will execute.

 

Note: else part will only execute when while loop condition will be false. If while loop will be terminated by break statement then while loop's else part never executes.

 

Example 4: 

#program to check
#weather the given number is
#prime number or not
num = int(input("Enter a Number : "))
i=2;
while(i<num):
    if(num%i==0):
        print("%d is not a prime number..."%num)
        break
    i=i+1
else:
    print("%d is a prime number..."%num)

Output: