Python While Loops: Tutorial & Examples

What is the Point?

The while loop in Python is a type of loop that executes a block of code repeatedly as long as a given condition is True. Unlike the for loop, which iterates over a fixed sequence of values, the while loop can run indefinitely until the condition is False or the loop is interrupted by a break or return statement.

General syntax:

while condition:
    # code to be executed
    # while the condition is true

Explanation:

• while keyword: Used to start a while loop.
• condition: A Boolean expression that determines whether the loop should continue or exit.
• : colon: Indicates the start of the loop block.
• Indented code block: The block of code that will be executed repeatedly as long as the condition is True.

In practice, using the while loop wisely will make your code much more efficient and concise. Let’s examine some examples for a deeper and better understanding.

Examples

Basic

This example uses a while loop to print the numbers from 1 to 10:

# initialize a counter variable
n = 1

# loop until n is greater than 10
while n <= 10:
    # print the current value of n
    print(n)
    # increment n by 1
    n = n + 1

Output:

1
2
3
4
5
6
7
8
9
10

In this example, the condition is n <= 10, which means the loop will run as long as n is less than or equal to 10. The body of the loop prints the current value of n and then increases it by 1. When n becomes 11, the condition becomes False, and the loop ends.

Infinite Loop with Break

In this example, the while loop runs indefinitely until the user enters the command quit. As soon as the input is quit, the loop will be exited immediately using the break statement.

while True:
    user_input = input("Enter a command: ")
    if user_input == "quit":
        break
    else:
        print(f"Executing command: {user_input}")

Output (may vary depending on what you type):

Enter a command: start
Executing command: start
Enter a command: stop
Executing command: stop
Enter a command: quit

While loop with continue statement

This example uses a while loop to print all the odd numbers from 1 to 20, skipping the even numbers:

# initialize a counter variable
n = 1

# loop until n is greater than 20
while n <= 20:
    # check if n is even
    if n % 2 == 0:
        # increment n by 1 and skip to the next iteration
        n = n + 1
        continue
    
    # print the current value of n (odd number)
    print(n)
    # increment n by 1
    n = n + 1

Output:

1
3
5
7
9
11
13
15
17
19

Let me explain the code snippet above. The condition is n <= 20, which means the loop will run as long as n is less than or equal to 20. The body of the loop checks if n is even using the modulo operator (%). If n is even, it increments n by 1 and uses the continue statement to skip to the next iteration of the loop without executing the rest of the body. If n is odd, it prints n and then increments it by 1.

While loop with the else clause

This example uses a while loop with an else clause to find a prime number between two given numbers:

# define a function to check if a number is prime or not
def is_prime(n):
    # check if n is less than or equal to one
    if n <= 1:
        # return False
        return False

    # check if n is divisible by any number from two to its square root
    for i in range(2, int(n**0.5) + 1):
        # if n is divisible by i
        if n % i == 0:
            # return False
            return False

    # return True
    return True


# define the lower and upper bounds
lower = 10
upper = 20

# loop from lower to upper
while lower <= upper:
    # check if lower is prime
    if is_prime(lower):
        # print lower and break the loop
        print("Found a prime number:", lower)
        break
    # increment lower by one
    lower = lower + 1
else:
    # print a message if no prime number is found
    print("No prime number found between", lower, "and", upper)

Output:

Found a prime number: 11

In this code example, the condition is lower <= upper, which means the loop will run as long as lower is less than or equal to upper. The body of the loop calls the is_prime() function to check if lower is a prime number or not. If lower is prime, it prints it and breaks the loop. If lower is not prime, it increments by 1 and continues the loop. The else clause of the loop executes only if the loop ends normally without a break statement. In this case, it prints a message that no prime number was found between the given bounds.