Learn While Loops

while loops are the key tool for handling indefinite iteration, which is useful in scenarios where the number of iterations isn't known in advance, like monitoring inventory levels until they meet a specific threshold.

Watch as Alex demonstrates how to use while loops to handle dynamic situations:

A while loop in Python continuously runs a block of code as long as a specified condition remains True.

Syntax

To start a while loop, you define a counter variable and follow it with the while keyword and a boolean condition. The condition is followed by a colon :, which indicates the start of the loop's code block.

The loop will execute repeatedly until the condition becomes False. Typically, the counter variable is updated inside the loop to eventually make the condition False and stop the loop.

Take this while loop for example:


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# Handling a queue at a grocery store checkout
queue_length = 5  # Initial number of people in the queue
while queue_length > 0:  # Start the `while` loop as long as the queue isn't empty
    print(f"Current queue size: {queue_length}")
    
    # Simulate serving a customer
    print("Serving the next customer...")
    
    # Decrease the queue length by 1 as a customer leaves
    # The `-=` operator is a shortcut for `queue_length = queue_length - 1`
    queue_length -= 1

In this example, queue_length acts as our counter variable, starting at 5. With each loop iteration, the value of queue_length decreases by 1 until it reaches 0. At that point, the loop ends because the condition 0 > 0 evaluates to False.

Note

It is crucial to update the counter variable inside the loop to eventually meet the condition for stopping the loop. Without this, the loop will run infinitely, preventing any further code from executing.

Example Application

Imagine you're managing the milk stock in a grocery store. You need to ensure the stock doesn't drop below a certain level, and when it does, you should restock to maintain the stock level.

A while loop is useful in this situation because it allows you to restock milk in increments over several iterations until the stock reaches the desired level. Without a while loop, you'd need to calculate the exact amount of milk to restock all at once.

For instance, consider that a store worker can only restock a fixed amount of milk in one trip (e.g., 20 packs per restock). If the stock isn't fully replenished, the worker returns to the warehouse to bring another batch of milk.


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# Initial amount of milk in stock
milk_stock = 15
# Minimum stock level before restocking is necessary
min_stock = 50
# Quantity a worker can restock at one time
restock_quantity = 20

# Start the loop to restock milk until the stock exceeds the minimum required level
while milk_stock < min_stock:
    # If the loop is running, the condition is `True`, indicating we need more milk
    print(f"Milk stock is low: {milk_stock} units remaining.")
    # Simulate the process of restocking milk
    print("Restocking milk...")
    # Increase the stock by the quantity the worker can bring in one trip
    milk_stock += restock_quantity
    
# Output the final stock level after restocking is complete
print(f"Milk stock updated: {milk_stock} units, which is now sufficient.")

Note

milk_stock += restock_quantity is equivalent to milk_stock = milk_stock + restock_quantity. It's simply a more concise and readable way to express the same operation.

The flowchart above illustrates the logic behind the while loop used for managing the milk inventory. Notice how the counter variable milk_stock, starting at 15, is incremented by the restock_quantity with each loop iteration.

The loop keeps running until milk_stock surpasses the min_stock threshold of 50, at which point it stops.

Task

Swipe to start coding

Create a discount countdown timer that collects all countdown values in a list using a while loop.

Use a while loop to count down from start_number to 1 (inclusive), decrementing by 1 each iteration.
During each iteration, append the current countdown value to the countdown_values list.
After the loop completes, print Discount countdown complete! and then print the countdown_values list.

Solution

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Section 5. Chapter 2

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