However, Python also has an alternative to the for-in loop and it is called the while loop and even though you may not use a while loop as often as you will use your for in loop it still can be helpful in certain circumstances and we're going to go through a couple of different examples.

Before we do that I want to talk about the key difference between using a for-in loop vs a while loop because that's also going to help clarify when you'd want to choose one over the other. With a for-in loop you have a very clearly defined start and finish to your loop. So if you're looping over a list of strings when you're done iterating through the entire list the for-in loop just completely stops and that is the type of behavior that you want for it.

Now a while loop is very different a while loop will not stop when it reaches the end of a list if that's what you're iterating over a while loop has to explicitly be told when to stop and it's part of the reason why you would not want to use a while loop on a regular basis because there are many times where you might forget to tell the while loop to stop at a certain point and then you reach what is called an infinite loop which is something that can crash your program because the while loop will not stop until it has been told to stop and so you have to be able to define what that point is and just if you're interested in the formal term it's called a sentinel value. You have to set a sentinel value in order to tell your while loop when to stop.

We're going to create a couple of examples here. The first one is going to be very basic so I'm going to create a list here and I'm gonna say nums = list and then I'm going to set of range of 1 to 100

nums = list(range(1, 100))

which really means as you probably know by now that is going to be a range from 1 to 99 so if we want to see what this looks like you can say print nums run this and you'll see we have a list that goes from 1 to 99.

Now if we want to iterate over this list if we had a for-in loop we could just say for num in nums: And then print out each one of these items and this would work perfectly fine just like you'd expect

We've walked through this example a few times now but with a while loop, it is not quite this easy. So if we're going to use a while loop we're going to use this type of syntax so I'm going to say while len. So this is the function for finding the length of the list we're working with let's say while len(nums) so in other words well the length of nums so the count of the nums list is greater than zero. Then I want you to print nums.pop())

nums = list(range(1, 100))

while len(nums) > 0:
  print(nums.pop())

And what this is going to do is it's going to iterate over and list and it's going to pop an element off it's going to print out that value and if you remember the behavior of the pop function this will also remove the item from the names list. So this is going to create our sentinel value it's going to continually decrease the length of the nums list until it reaches 0 once it reaches that value. This condition. So where it says while the length of nums is greater than zero. This is finally going to be false and when it's false the while loop is going to stop.

So if I run this you're going to see that we get the behavior we're looking for

and notice here. Also a slightly different behavior. It starts at ninety-nine and then goes all the way down to 1 because technically we're printing out the nums and we're doing it from the last value so that is something that is good to keep in mind this is a basic example of how to use the while loop and this may also give you some clarification on why the for-in loop is so popular because this is not anywhere near as intuitive as what we saw with the for-in loop.

Now I'm going to delete this but it will be in your show notes if you want to reference it. And now we're going to do something fun. We are going to build a little guessing game. So are guessing game is going to be something different than we've done before. Because we going to build a function and do not worry if this looks weird because we are going to have an entire section dedicated to functions. I simply want to give you a little preview of it right here.

I'm going to say def guessing_game(): so this is going to create a function and we're going to declare a code block inside of it and this code block is going to have a while loop and so I'm to say while and this might look a little bit weird. I'm going to say while True: So what in the world is going on here? Well, this is actually a very common use of the while loop which is where you do not have an idea of when you want the while loop to stop.

So with our first example, we didn't know that we wanted to simply iterate through the collection of numbers. However, in a real-world python program, you wouldn't need to use a while loop for that because you have a for-in loop. However one of the most common uses for a while loop is when you don't know how many times you want your program to iterate. So in this case what we're going to do is we're going to create a guessing game where the user is going to be asked to guess the number and until they get the right number you want the program to continue running. So, in this case, we can't use a for-in loop because we don't know when the right answer has been guessed until they've actually done it. So we need to have a while loop here that will just continue to loop through the program over and over again until the right number has been guessed and then It'll break out of it.

So I'm going to say while True. And then I'm going to say if I can type correctly print what is your guess. And so this is going to print this out to the console. And then I'm going to store the guess in a variable and I'm going to call a function. We haven't talked about before because I haven't really gone into how to get values from the console and it's mainly because when you get into real Python development such as building web and mobile applications and machine learning applications. You rarely are going to be getting console input but in this case, for a little console guessing game, we'll need it.

And so what you can do is call input and it's a function. So you're going to call it with parens just like this. So this is going to do is it's going to set up a prompt here in the terminal and it's going to store whatever we type into it in this guess variable so I'm gonna say guess equals input and then say if guess is equal to and we can just put any number here so I'm going to say if the user guesses the number 42 and I need to add a call in here then I'm going to say print You correctly guessed it and then after that I am going to return false. And the reason I'm going to do that is because this while loop is going to continue as long as true is true. And that may sound very convoluted and doesn't make any sense but essentially true is always true unless we tell it that it is false.

def guessing_game():
  while True:
    print('What is your guess?')
    guess = input()

    if guess == '42':
      print('You correctly guessed it!')
      return False

And so when we say return false this is going to stop our while loop.

So, in this case, this is our sentinel value. So if you remember back to when we went through our number example our sentinel value was when the length of our number list wasn't greater than 0 here are sentinel values going to be whenever the correct guess is presented by the user. Then we return false which is going to stop this entire loop which is the behavior that we're looking for. And then you say else and this needs to be indented just like this. So I'm gonna say else I want to print out and let's format this. So I'm gonna say

print(f'No, {guess} is not the answer, please try again/n')

And then I'm also going to give a /n what that is going to do is that is going to just give us a new line so that they're guessing on a new line and that is it.

The way we can call a function in python is just by saying guessing game which we have talked about that before but here every time we've used it before we've called another function inside of the language itself kind of like we're doing input. This is in the core Python language here. We're actually calling our own function so we hit clear and let's see if this works if it works it should ask us what our guess is. And then it should present a prompt and then it should keep on allowing us to guess until we have gone through and finally guess the number 42.

So if I hit return here so far so good. It says, What is your guess?

And I'm gonna say 10 and it says no 10 is not the answer, please try again. It gives us a new line because we gave that newline character.

I'll say 55. No, 55 is not the answer. Please try again.

So notice the behavior here because we have a while loop. I could go on for this for hours and I could guess a thousand different numbers and it's going to keep on going through because we have a while loop here. It will keep the program running until it reaches that sentinel value until it returns false.

Now if I type in the number 42. It says you correctly guessed it and then notice the while loop ended.

So when I hit return False it is now breaking outside of the while loop and it returns false here. And so it ends the program and says that I correctly guessed it.

So that is one of the most common ways that you're going to be using a while loop in Python. It's when you don't really have a clear end of value. So with our for-in loops we know when we want them to start and we know when we want them to finish a while loop is going to be one of the best tools you can have for looping whenever you want a program to go on and on and only breaks out of it when it's given that sentinel value.

Code

nums = list(range(1, 100))

while len(nums) > 0:
  print(nums.pop())

def guessing_game():
  while True:
    print('What is your guess?')
    guess = input()

    if guess == '42':
      print('You correctly guessed it!')
      return False
    else:
      print(f"No, {guess} isn't the answer, please try again\n")

guessing_game()