Behind every mouse click and touch-screen tap, there is a computer program that makes things happen. This course introduces the fundamental building blocks of programming and teaches you how to write fun and useful programs using the Python language.
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From the course by University of Toronto
Learn to Program: The Fundamentals
1147 ratings
University of Toronto
Learn to Program: The Fundamentals
1147 ratings
From the lesson
While Loops, Lists, and Mutability
This module introduces another way to repeat code (using a while loop), how to properly document your code to help other programmers understand it, Python's list data type, and the concept of mutation.
Meet the Instructors
Jennifer CampbellAssociate Professor, Teaching Stream
Department of Computer Science
Paul GriesAssociate Professor, Teaching Stream
Department of Computer Science
As a reminder, a method is a function that is inside an object. You've heard of the methods in type list. Ignoring the methods with underscores, we see append, count, extend, index, insert, pop, remove, reverse, and sort. Some of these methods modify the list, and some just return information. We will now give examples of using each of them. We will keep track of someone's favorite colours. Here's a prompt that we will use to gather the information.
We are going to use a loop in order to gather all the person's favorite colours, but we need to start by asking them for their first. So far, then, variable colours is assigned an empty list, and variable prompt is assigned. Enter another one of your favorite colours, Type return to end. We ask the user to enter their first favorite color by calling function input and passing in variable prompt. Let's say, the user types blue. So, color now refers to blue. Variable color still refers to an empty list. We are accumulating the information that the user types. So, what we're going to do is use a while loop and say that, while color is not the empty string, we will append the color to the colours variable, Colour.apprend color. Now, we've appended the color the user just typed, and so we need to get ready for the next iteration of the loop by asking the user for another color. So, color is assigned input with argument prompt. We will now enter the colours yellow, and brown, and, let's say, that that's our whole list of favorite colours so we're going to follow the instructions and type return to end this while loop.
When we examine the colours variable, we see that it contains strings blue, yellow, and brown. But we forgot some, so we are going to use method extend to extend the list of colours by sending in the list of colours that we forgot. We will call colours.extend, and pass in the list with the strings hot pink and neon green. Now, when we examine colours, we see blue, yellow, brown, hot pink and neon green.
to remove it. When we examine colours, we see the list blue, yellow, brown and hot pink, Neon green is no longer there. One other point to make is that when you call colours.pop, not only does it remove the last item, but it also returns it. So, you can use it in an assignment statement. A function that both returns information and does something to data, such as colours.pop is a function that is said to have side-effects. We're not enamored of brown either, so we will use pop with its optional argument in order to remove it.
Brown is at index two, so if I call colours.pop two, it will remove brown from the list, as well as return it. Examining colours, we see we're down to blue, yellow and hot pink. Colours.pop will remove the last item in the list. And, colours.pop with an index will remove the item at the given index. Method remove picks an object to remove, not an index. And it will remove the first occurrence of that object from the list. Let's try to use it to remove black, Which isn't in the list. And we see we get a value error. It won't work if we call remove with an item that is not in the list. So, before calling a method or function that might raise an error, we want to check that, that won't actually happen. We can count the number of a occurrences of a particular value in, in a list. And, if it ends up being greater than zero, In other words, if yellow exists in the list, then we can call remove. Examining colours, we end up with just blue and hot pink. Rather than counting, we could just say, if yellow in colours, Then colours.removeyellow. This is more standard than calling colours.count and counting the number of items. Just say, if it's in there, then call colours.remove on it. Of course, yellow wasn't there. And, so the if statement condition evaluated to false, And we didn't call colours.remove cuz we had previously removed it.
We will now call colours.extend with arguments auburn, taupe, and magenta. But we get a type error. Extend takes exactly one argument, three given. The problem is that, we need to put auburn, taupe, and magenta into a list because extend is expecting a list. Now, colours refers to the list with strings blue, hot pink, auburn, taupe, and magenta. We can sort it.
Now, colours refers to the alphabetical list, auburn, blue, hot pink, magenta, and taupe. We can reverse this. We call colours.reverse, and now colours refers to taupe, magenta, hot pink, blue, and auburn. We are feeling a little guilty about brown, we will call colours.insert in order to put it back in the list where it belongs. We used arguments minus two and brown to say insert brown at index minus two into our colours. We have not replaced the item that was at index minus two, instead blue and auburn have been shifted over to make room for brown. So, insert does not get rid of anything that is in the list, it moves things over to make room. Last, let's get rid of hot pink because that's just a little bit bright for our aged selves. In order to remove it, we need to know it's index. We can call the list index method in order to find this out. But again, index might crash if the item is not in the list. For example, colours.index remember we don't have neon green anymore. It gives us a value error neon green is not in list. We want to prevent errors from happening. So here, we will use and if statement if hot pink in colours to check to see whether we can call index without crashing. So, if hot pink is in colours, we will then assign the variable, where, the result of calling colours.hotpink, And then, we can use that index where to call colours.pop to remove hot pink and now we are left with taupe, magenta, brown, blue, and auburn.
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