Python While loop

Python While loop

The Python while loop allows a part of the code to be executed until the given condition returns false. It is also known as a pre-tested loop.

It can be viewed as a repeating if statement. When we don't know the number of iterations then the while loop is the most effective to use.

The syntax is given below.

1. while expression:    
2.     statements    

Here, the statements can be a single statement or a group of statements. The expression should be any valid Python expression resulting in true or false. The true is any non-zero value and the false is 0.

While loop Flowchart

Loop Control Statements

We can change the normal sequence of the while loop's execution using the loop control statement. When the while loop's execution is completed, all automatic objects defined in that scope are demolished. Python offers the following control statement to use within the while loop.

1. Continue Statement - When the continue statement is encountered, the control transfer to the beginning of the loop. Let's understand the following example.

Example:

1. # prints all letters except 'a' and 't'   
2. i = 0  
3. str1 = 'javatpoint'  
4.   
5. while i < len(str1):   
6.     if str1[i] == 'a' or str1[i] == 't':   
7.         i += 1  
8.         continue  
9.     print('Current Letter :', a[i])   
10.     i += 1  

Output:

Current Letter: j
Current Letter: v
Current Letter : p
Current Letter: o
Current Letter: i
Current Letter: n

2. Break Statement - When the break statement is encountered, it brings control out of the loop.

Example:

1. # The control transfer is transfered  
2. # when break statement soon it sees t  
3. i = 0  
4. str1 = 'javatpoint'  
5.   
6. while i < len(str1):   
7.     if str1[i] == 't':   
8.         i += 1  
9.         break  
10.     print('Current Letter :', str1[i])   
11.     i += 1  

Output:

Current Letter: j
Current Letter : a
Current Letter : v
Current Letter : a

3. Pass Statement - The pass statement is used to declare the empty loop. It is also used to define empty class, function, and control statement. Let's understand the following example.

Example -

1. # An empty loop   
2. str1 = 'javatpoint'  
3. i = 0  
4.   
5. while i < len(str1):   
6.     i += 1  
7.     pass  
8. print('Value of i :', i)  

Output:

Value of i : 10

Example-1: Program to print 1 to 10 using while loop

1. i=1  
2. #The while loop will iterate until condition becomes false.  
3. While(i<=10):    
4.     print(i)   
5.     i=i+1   

Output:

1
2
3
4
5
6
7
8
9
10

Example -2: Program to print table of given numbers.

1. i=1    
2. number=0    
3. b=9    
4. number = int(input("Enter the number:"))    
5. while i<=10:    
6.     print("%d X %d = %d \n"%(number,i,number*i))    
7.     i = i+1    

Output:

Enter the number:10
10 X 1 = 10 

10 X 2 = 20 

10 X 3 = 30 

10 X 4 = 40 

10 X 5 = 50 

10 X 6 = 60 

10 X 7 = 70 

10 X 8 = 80 

10 X 9 = 90 

10 X 10 = 100 

Infinite while loop

If the condition is given in the while loop never becomes false, then the while loop will never terminate, and it turns into the infinite while loop.

Any non-zero value in the while loop indicates an always-true condition, whereas zero indicates the always-false condition. This type of approach is useful if we want our program to run continuously in the loop without any disturbance.

Example 1

1. while (1):    
2.     print("Hi! we are inside the infinite while loop")  

Output:

Hi! we are inside the infinite while loop
Hi! we are inside the infinite while loop

Example 2

1. var = 1    
2. while(var != 2):    
3.     i = int(input("Enter the number:"))    
4.     print("Entered value is %d"%(i))    

Output:

Enter the number:10
Entered value is 10
Enter the number:10
Entered value is 10
Enter the number:10
Entered value is 10
Infinite time

Using else with while loop

Python allows us to use the else statement with the while loop also. The else block is executed when the condition given in the while statement becomes false. Like for loop, if the while loop is broken using break statement, then the else block will not be executed, and the statement present after else block will be executed. The else statement is optional to use with the while loop. Consider the following example.

Example 1

1. i=1   
2. while(i<=5):    
3.     print(i)    
4.     i=i+1    
5. else:  
6.     print("The while loop exhausted")    

Example 2

1. i=1    
2. while(i<=5):    
3.     print(i)    
4.     i=i+1    
5.     if(i==3):    
6.         break   
7. else:  
8.     print("The while loop exhausted")  

Output:

1
2

In the above code, when the break statement encountered, then while loop stopped its execution and skipped the else statement.

Example-3 Program to print Fibonacci numbers to given limit

1. terms = int(input("Enter the terms "))  
2. # first two intial terms  
3. a = 0  
4. b = 1  
5. count = 0  
6.   
7. # check if the number of terms is Zero or negative  
8. if (terms <= 0):  
9.    print("Please enter a valid integer")  
10. elif (terms == 1):  
11.    print("Fibonacci sequence upto",limit,":")  
12.    print(a)  
13. else:  
14.    print("Fibonacci sequence:")  
15.    while (count < terms) :  
16.        print(a, end = ' ')  
17.        c = a + b  
18.        # updateing values  
19.        a = b  
20.        b = c  
21.      
22.     count += 1  

Output:

Enter the terms 10
Fibonacci sequence:
0 1 1 2 3 5 8 13 21 34