The next loop we will explore is the while loop. This type of loop will continue to execute as long as the specified condition remains True. Once the condition evaluates to False, the loop terminates. Here is how we can construct it:

while condition: # run code # close indent to finish while loop

Similar to the for loop, the while loop commences with the reserved keyword while. However, instead of iterating over a defined range of values, it operates based on a specified condition. The loop continues to execute its block of code as long as the condition evaluates to True.

It is akin to an if statement, but with repeated evaluations as long as the condition remains true. Therefore, it is essential to update a value within the while loop to ensure it eventually exits; failing to do so can result in an infinite loop! This makes the while loop potentially more hazardous compared to the for loop and as such should be used with caution.

Let's now see this type of loop in action, we will replicate the for (i in 1:5) loop from the previous lesson using a while loop.

In this structure, we first define a starting value with i = 1, which will be used to meet the condition set in the while loop. As the loop runs, it will print the current value of i. However, one crucial distinction from the for loop is that i does not increment automatically; we must explicitly increment it within the loop using i = i + 1.

You might be wondering about the practical utility of while loops when for loops are available. After all, from this demonstration they are longer to write and more prone to errors. The while loops shine in situations where we wish to perform iterations but the exact number of iterations required is unknown upfront. They offer flexibility, allowing the loop to run as long as a specific condition holds true.

Let's see such a scenario in the following example. We set a variable called coffee_beans initially to 100 and then randomly select 1, 2, or 3 and add this to coffee_beans until it reaches a threshold value of 115 while printing out the total sum at each iteration.

Feel free to run the code editor a couple of times to see that the while loop will end at different times due to the randomness in the sampling.

Skipping and exiting while loops

Just like with for loops, we can use the statements continue and break to skip anything past the former and exit early with the latter. We update the previous code to wrap the functionality that adds a random number of beans into a new function named add_beans. Additionally, we've introduced an if statement paired with a continue statement to skip the printing of the bean count when it falls between 101 and 105. To enhance our control over the program's flow further, we've also implemented a break statement to terminate the loop prematurely if the number of beans hits or exceeds 110.

Practice Question

Satella is set to read "Wuthering Heights" by Emily Brontë, a novel that spans across 464 pages. On a regular day, she reads about thirty pages. However, she takes a break every sixth and seventh day to unwind, choosing not to read on these days. Starting from the first day, can you calculate how many days it will take for Satella to finish the novel?

To solve this, you're invited to complete the function time_to_finish_novel. For this task, make use of a while loop, and feel free to utilize the helper function reading_day, which returns a boolean indicating whether Satella is up for reading on a given day. Don't worry about scoring—this exercise is designed to help you get comfortable with coding. Further note that this uses the modulo operator as well as not. For a review on those see the operators section.

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