Tutorial on while loop

Okay, let’s dive deeper into the while loop! 😊 It’s a really fundamental concept in programming for making your computer do repetitive tasks.

Think of the while loop like setting a condition for Python and telling it: “Keep doing this set of actions ✨ as long as this condition remains true”.

This is different from an if statement, which checks a condition just once. A while loop keeps checking the condition and repeating its work. The code inside the loop will execute over and over again as long as the condition is True.

A key reason to choose a while loop is often when you don’t know beforehand exactly how many times you need to repeat something 🤔. For example, if you want to keep asking a user for input until they type a specific word like “stop”, a while loop is perfect because you don’t know how many tries it will take. In contrast, a for loop is often used when you do know the number of repetitions, like repeating something exactly 10 times. The sources describe while as a more general way of executing repetitive tasks compared to for.

Here is the basic structure, or syntax, of a while loop in Python:

while condition_is_true:
    # This block of code is the 'loop body'
    # It gets repeated while the condition is True
    # Remember, these lines must be indented! 👇
    # Something here MUST change so the condition eventually becomes False
    # ... other statements ...

Let’s break down the parts based on the sources:

• while keyword: This keyword signals the start of a while loop. It is one of the reserved words in Python.
• Condition (or expression): This is a test that Python checks before each repetition of the loop. It needs to evaluate to either True or False. The loop continues only if this condition is True. Any non-zero value is considered True.
• Colon :: Just like with other compound statements such as if, a colon is needed at the end of the while line. Forgetting the colon is noted as a common coding mistake for beginners.
• Indented block: The lines of code that are indented directly below the while header form the loop body. These are the statements that are repeatedly executed. Python uses indentation to define these blocks of code.

How the while loop works (Flow of Execution):

1. Python checks the condition_is_true.
2. If the condition is False, the loop body is skipped entirely, and execution continues at the first statement after the indented block. If the condition is initially false, the loop body is never run.
3. If the condition is True, Python executes all the statements in the indented block (the loop body).
4. After executing the last statement in the loop body, Python jumps back up to the while line and checks the condition_is_true again. This repeats from step 2. This process is often called iteration or looping because it repeats.

Crucially: Avoiding Infinite Loops ⚠️🔄

It is absolutely essential that something inside the while loop’s body changes a variable used in the condition, so that the condition will eventually become False. If the condition never becomes False, the loop will run forever, which is called an infinite loop. This can make your program unresponsive. You must be cautious when using while loops because of this possibility.

Useful Statements Inside Loops:

You can also use special statements within loops to control their execution:

• break: This statement immediately exits the current loop entirely. The program continues execution at the first statement after the loop. break can sometimes help avoid nesting. Using while True with a break inside is a common pattern for loops where the exit condition is checked somewhere in the middle.
• continue: This statement skips the rest of the current iteration of the loop and immediately jumps back to the top to check the condition again (for a while loop). It can also help avoid nesting in some cases.
• pass: This is an empty statement that does nothing. It’s sometimes used as a placeholder where a statement is syntactically required but you don’t want to do anything yet. It’s not tied only to loops but is often discussed with break and continue. while True: pass would be an infinite loop that does nothing.
• else clause: while loops (and for loops) in Python can have an optional else clause. The code in the else block is executed only if the loop finishes normally, without hitting a break statement.

Learning programming, including understanding loops, takes patience and practice. It might seem confusing at first. Looking back at examples and trying them yourself multiple times can help. Using tools like a pen and paper to draw diagrams of how your variables change step-by-step inside the loop can be very helpful.

We recently looked at calculating the factorial of a number using a while loop in our conversation. That was a great example of using the loop to repeat the multiplication and decrementing a counter (i = i - 1) until the condition (i > 0) became false, ensuring the loop terminated correctly.

Keep practising, and you’ll master while loops! 😊👍