The while Loop in Python

 

The while Loop in Python

The while loop in Python is used to repeatedly execute a block of code as long as a specified condition is True. Unlike the for loop, which iterates over a sequence, the while loop continues indefinitely until the condition evaluates to False.

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1. Basic Syntax of the while Loop

The basic syntax of a while loop is as follows:

while condition:
    # Code to execute as long as the condition is True

• condition: This is an expression that is evaluated before each iteration. If it evaluates to True, the loop runs; if it evaluates to False, the loop stops.
• The indented block of code is executed repeatedly until the condition becomes False.

Example:

counter = 0

while counter < 5:
    print(counter)
    counter += 1  # Increment the counter by 1

Output:

0
1
2
3
4

In this example, the loop runs as long as the counter is less than 5. Each time the loop executes, the counter is incremented by 1, and once the counter reaches 5, the condition becomes False, and the loop stops.

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2. Infinite Loops

A while loop can run indefinitely if the condition always evaluates to True. These are called infinite loops and can be dangerous if not handled properly.

Example of an Infinite Loop:

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

To stop an infinite loop, you can interrupt the program (in many environments, this is done by pressing Ctrl+C).

However, you can use a condition to break out of an infinite loop safely, such as with a break statement.

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3. The break Statement

The break statement is used to exit a while loop prematurely, regardless of the condition. This is useful when you need to stop the loop based on some other condition inside the loop.

Example:

counter = 0

while counter < 10:
    if counter == 5:
        break  # Exit the loop when counter reaches 5
    print(counter)
    counter += 1

Output:

0
1
2
3
4

Here, the loop will stop when counter reaches 5, even though the condition (counter < 10) is still True.

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4. The continue Statement

The continue statement is used to skip the rest of the code inside the while loop for the current iteration and jump to the next iteration of the loop.

Example:

counter = 0

while counter < 5:
    counter += 1
    if counter == 3:
        continue  # Skip the iteration when counter equals 3
    print(counter)

Output:

1
2
4
5

In this example, when counter equals 3, the continue statement is executed, and the loop moves to the next iteration without printing 3.

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5. Using else with the while Loop

The else block can also be used with a while loop. It will execute if the loop terminates normally (i.e., without being interrupted by a break statement).

Example:

counter = 0

while counter < 5:
    print(counter)
    counter += 1
else:
    print("The loop has finished.")

Output:

0
1
2
3
4
The loop has finished.

In this case, the else block runs once the loop completes normally (i.e., when counter reaches 5).

Example with break:

counter = 0

while counter < 5:
    print(counter)
    counter += 1
    if counter == 3:
        break  # Break out of the loop when counter reaches 3
else:
    print("The loop has finished.")

Output:

0
1
2

In this case, the else block is not executed because the loop was terminated early by the break statement.

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6. Nested while Loops

You can nest while loops within other while loops. This is useful when you need to perform more complex repeated tasks.

Example:

i = 0

while i < 3:
    j = 0
    while j < 3:
        print(f"i = {i}, j = {j}")
        j += 1
    i += 1

Output:

i = 0, j = 0
i = 0, j = 1
i = 0, j = 2
i = 1, j = 0
i = 1, j = 1
i = 1, j = 2
i = 2, j = 0
i = 2, j = 1
i = 2, j = 2

In this example, the outer loop (i) runs 3 times, and for each iteration of i, the inner loop (j) also runs 3 times. This is an example of a nested while loop.

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7. User Input and while Loop

A common use case for the while loop is to repeatedly ask for user input until a valid input is given. The loop will continue until a specified condition is met.

Example:

while True:
    user_input = input("Enter a number between 1 and 10: ")
    if user_input.isdigit() and 1 <= int(user_input) <= 10:
        print(f"Thank you! You entered: {user_input}")
        break
    else:
        print("Invalid input. Please try again.")

In this example, the program will continue asking for a valid input until the user enters a number between 1 and 10.

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Summary

• Basic syntax: The while loop executes a block of code as long as a condition is True.
• Infinite loops: A while loop can run forever if the condition never becomes False. It can be controlled using break or continue.
• break: Used to exit the loop prematurely.
• continue: Skips the current iteration and proceeds to the next iteration.
• else: Executes if the loop finishes normally (without break).
• Nested loops: You can nest while loops within other while loops.
• User input: while loops are commonly used to validate or repeat tasks based on user input.

Mastering the while loop allows you to create more flexible and dynamic programs that can handle different types of repetitive tasks.