Python While Loop | Docs With Examples

The Python while loop is a fundamental control flow tool that allows you to execute a block of code repeatedly as long as a condition is true.

Mastering the while loop will help you write dynamic and flexible programs in Python.

Basic Syntax

The syntax of a while loop is incredibly straightforward:

while condition:
    # Code to execute repeatedly

The key part here is the condition, which is a boolean expression that determines whether the loop runs. If it evaluates to True, the loop continues; if False, the loop ends.

Common Examples

1. Basic while Loop

Print numbers from 1 to 5:

count = 1
while count <= 5:
    print(count)
    count += 1

Output:

1
2
3
4
5

2. Infinite Loop

A loop that runs forever unless stopped manually (or with a break):

while True:
    print("This loop will run forever!")

Use with caution: Infinite loops can crash your program if not handled properly.

3. Using break to Exit a Loop

Terminate a loop when a specific condition is met:

count = 1
while True:
    print(count)
    count += 1
    if count > 5:
        break

Output:

1
2
3
4
5

4. Using continue to Skip Iterations

Skip the current iteration and move to the next one:

count = 0
while count < 5:
    count += 1
    if count == 3:
        continue
    print(count)

Output:

1
2
4
5

Common Use Cases

1. Waiting for User Input

Prompt the user until they provide valid input:

while True:
    password = input("Enter your password: ")
    if password == "1234":
        print("Access granted!")
        break
    else:
        print("Incorrect password. Try again.")

2. Reading Data Until a Condition is Met

Read numbers until the user enters 0:

sum = 0
while True:
    num = int(input("Enter a number (0 to quit): "))
    if num == 0:
        break
    sum += num
print(f"Total sum: {sum}")

Avoiding Common Pitfalls

1. Infinite Loops: Ensure your loop condition eventually becomes False to avoid infinite loops.
2. Off-by-One Errors: Double-check your loop condition and updates to avoid missing or extra iterations.

Key Takeaways

- The while loop runs as long as the condition is True.
- Use break to exit a loop prematurely and continue to skip iterations.
- Be cautious of infinite loops and ensure your loop has an exit condition in your Python projects.

Practice Exercise

Here's a simple challenge you can attempt, try writing a program that prints the Fibonacci sequence up to a given number:

limit = int(input("Enter the maximum number: "))
a, b = 0, 1
while a <= limit:
    print(a, end=" ")
    a, b = b, a + b

Wrapping Up

The while loop is a versatile tool for controlling program flow, especially in cases where the number of iterations isn’t predetermined. By mastering its syntax and understanding common use cases, you’ll unlock a powerful tool for dynamic programming. Happy coding!