2.4 Python While Loop

The while loop in Python is a fundamental control flow statement used to repeatedly execute a block of code as long as a specified condition remains True. This makes it particularly useful when the number of iterations is not known in advance and depends on dynamic conditions.

Syntax of a while Loop

The basic structure of a while loop is as follows:

while condition:
    # Code block to be executed
    # This block runs as long as 'condition' is True

• condition: A boolean expression that is evaluated before each iteration. If the condition evaluates to True, the code block inside the loop is executed. If it evaluates to False, the loop terminates, and program execution continues with the statement immediately following the while loop.

How the while Loop Works – Step-by-Step

1. Evaluate the condition: Python checks if the condition is True or False.
2. Execute Code Block: If the condition is True, the code block within the loop is executed.
3. Repeat: After executing the code block, Python returns to Step 1 to re-evaluate the condition.
4. Terminate: If the condition is False, the loop stops, and execution proceeds to the next statement after the loop.

Example 1: Simple Counter Loop

This example demonstrates a while loop that increments a counter until a specific value is reached.

counter = 1
while counter <= 3:
    print(f"Count: {counter}")
    counter += 1
print("Loop completed.")

Output:

Count: 1
Count: 2
Count: 3
Loop completed.

In this example, the loop continues as long as counter is less than or equal to 3. Inside the loop, the current counter value is printed, and then counter is incremented by 1.

Example 2: Wait for Non-Numeric Input

This loop continues to prompt the user for input as long as the input consists only of numeric characters. It stops when the user enters something that is not a number.

user_input = "0"  # Initialize with a numeric string to enter the loop
while user_input.isnumeric():
    user_input = input("Enter a number (or a word to stop): ")
    if user_input.isnumeric():
        print(f"You entered: {user_input}")
print("Loop stopped. Non-numeric input detected.")

Sample Output:

Enter a number (or a word to stop): 45
You entered: 45
Enter a number (or a word to stop): 100
You entered: 100
Enter a number (or a word to stop): hello
Loop stopped. Non-numeric input detected.

The loop utilizes the isnumeric() string method to check if the entered input contains only digits.

Infinite while Loop in Python

An infinite loop occurs if the condition in a while loop never evaluates to False. These loops will run indefinitely unless explicitly terminated (e.g., by the user interrupting the program, typically with Ctrl+C). While often an error, infinite loops can be useful in specific scenarios like server processes or waiting for events.

while True:
    command = input("Type 'exit' to stop: ")
    if command.lower() == "exit":
        break  # Exit the loop when the command is 'exit'
    print(f"You said: {command}")

Sample Output:

Type 'exit' to stop: hey
You said: hey
Type 'exit' to stop: hello there
You said: hello there
Type 'exit' to stop: exit

In this case, while True: creates a condition that is always True. The break statement is crucial for providing an exit mechanism based on user input.

while Loop with else Block

Python supports an optional else block that can be used with while loops. The code within the else block is executed only when the loop terminates naturally (i.e., the condition becomes False), and not if the loop is exited prematurely using a break statement.

attempts = 0
while attempts < 3:
    print(f"Attempt {attempts + 1}")
    attempts += 1
else:
    print("All attempts done.")

Output:

Attempt 1
Attempt 2
Attempt 3
All attempts done.

Here, the else block executes because the loop condition attempts < 3 eventually becomes False. If a break statement were encountered within the loop, the else block would be skipped.

One-Liner while Statement

For very simple conditions, a while loop can be written on a single line. However, this is generally discouraged for readability unless the operation is trivial.

flag = False
while flag: print("This won't be printed")
print("Exited loop")

Output:

Exited loop

In this example, since flag is initialized to False, the condition is immediately false, and the loop body is never executed. If flag were set to True, this would result in an infinite loop unless an external mechanism (like Ctrl+C) interrupted it.

Key Points to Remember

• Prevent Infinite Loops: Always ensure that the condition controlling the while loop will eventually become False. Failure to do so will result in an infinite loop, which can freeze your program.
• break Statement: Use the break statement to exit the loop immediately, regardless of the loop's condition. This is useful for stopping the loop based on a specific event or condition occurring within the loop's body.
• continue Statement: Use the continue statement to skip the rest of the current iteration and proceed directly to the next evaluation of the loop's condition. This is helpful for bypassing certain steps under specific circumstances.

The while loop is a versatile tool in Python for managing repetitive tasks where the number of iterations is not predetermined. It offers flexibility and control over program flow based on dynamic conditions.