Lesson 03-08: The while Loop¶

while Loop Syntax¶

Here is a for loop and a while loop that does the exact same thing:

for n in range(5):
    print(n)
print("Done!")

n = 0
while n < 5:
    print(n)
    n = n + 1
print("Done!")

Let’s take the second one and look at the step-by-step process. Click on “Show Codelens” below to step through the code:

Let’s go over how the while loop works:

A while loop works very similarly to an if statement, with one big difference: as long as the statemet following while is True, it will loop the code inside!

WARNING: If you get caught in an infinite loop in the terminal, you can use CTRL+C to stop code execution! But it’s best to avoid infinite loops - which will be explored in the next lesson.

Here’s an example that isn’t based on numbers.

Here, we write a loop that waits for a user to type “quit” and repeatedly asks the user to type something in until that happens. You might notice on line 1 that we actually ask the user for an input before we even start looping - this is because on line 5, our condition is word != "quit", so word needs a value (needs to be initialized). Then, on line 7, we have to re-ask the user to type in something else, so that the user can type in “quit” to quit.

Write a while loop on line 4 below so that the loop will continue as long as x is less than 200. If done correctly, it should print: 2 4 8 16 32 64 128 done

for VS. while¶

Now that we know how to use both for loops and while loops, it’s important to understand when to use each one. While they are both similar in the sense that they both repeat code, they are used in different situations.

In for loops, you know exactly how many times you are looping. This is defined either within the range() function or in another iterable (which we will learn about later). In while loops, you continue to loop until a certain condition is met - it is not always clear exactly how many times the code will loop.

Therefore, when choosing between for and while loops, you should follow the following guidelines:

• If you know exactly how many times you need to loop, use a for loop.
• If not, use a while loop.

u0308-1: Let’s say you need to create a program to add up all prime numbers up to 1000. What kind of loop should you use?
• (A) for
• (B) while
• Do you know exactly how many times you'll need to loop?

u0308-2: Let’s say you need to create a program to add up all powers of two as long as the sum is less than 5000. What kind of loop should you use?
• (A) for
• Do you know exactly how many times you'll need to loop? Be especially careful of the wording "as long as"
• (B) while
• Nice job! The key phrase is "as long as"