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The big topic that we're going to explore today is a concept known as a list

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comprehension. And as I mentioned before,

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this is something that's really unique to the Python language.

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In many other programming languages,

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you don't actually have access to something like this.

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There's nothing that really compares to it.

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And it's something that Python developers really,

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really love because it cuts down on the amount of typing and it just makes the

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code a lot shorter, and in most cases, a lot easier to read.

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So what exactly is a list comprehension? Well, it's a case where you 

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create

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a new list from a previous list. So far we've been doing that using for loops.

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So for example, if I have a list of numbers, 1, 2, 3,

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and I want to create a new list where each number is increased by one,

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well

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then I would use a for loop creating a new empty list and then looping through

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each of the numbers in that list in order to add one to

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each of those numbers.

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And then I would append each of those increased numbers to my new list.

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And I would end up with a new list that consists of three items,

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and it will be 2, 3, and 4. Now using list comprehension

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we can take these four lines of code and turn it into one.

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And the way that we create a list comprehension looks something like this.

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I like to use the keyword method where you type out the list comprehension

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keywords,

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and then replace each of the words with the actual item in your code.

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The way that we would create any list comprehension is we will create the name

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of the new list and instead of creating a empty list and then having to append

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to it or populate it, we actually going to create it straight in the same line.

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So we open up a set of square brackets to denote that we're creating a new list,

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and then we have this new _item for item in list

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keyword pattern. Here's how it would work.

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Let's take that same thing that we did before

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where we took you a list of numbers

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and we created a new list where each number is increased by one,

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and let's see how we would do it using list comprehension.

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So the first keyword I'm going to replace is the list.

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So this is the list that we're going to iterate through. And in our case,

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it's this list of numbers. So let's replace that list

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keyword with the actual list, which is numbers.

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The next thing that is easy to understand is each item in the list.

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So we can call this anything we want. In the for loop here,

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we called it n, so why don't we go ahead and call it n again?

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This n is going to represent each of the numbers in that list of numbers

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just as it does in this for-loop.

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And the final keyword that we haven't yet replaced is the new_item.

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So what do we want each of the new items to be in this new list?

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Well,

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each of the new items is basically calculated by taking n and then adding one

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to it.

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So this line is the expression or the code that we need to execute in order to

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get a new_item. So this becomes our final list

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comprehension. We have our lists of numbers

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and then we create a new list where we loop through each of the numbers, n,

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inside this list of numbers

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and then for each of the n, we add one to it in order to create the final list.

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So when you look at a list comprehension just like this as in the real code,

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it can be a little bit confusing to see how would you read it,

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which order does it happen.

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But when you're using this keyword method that I often use

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where you just type out what you need to have inside a list comprehension,

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you need a name for the new list, you need a set of square brackets

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and then inside the square brackets, you have new_item for item in list.

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Then you go through it replacing each of the keywords, list item

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and new_item. So I want you to have a go at actually writing out this code.

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Go ahead and create a new PyCharm project,

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and I want you to go into the Python console.

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We're not actually going to be writing code inside our main.py.

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And the reason is because the console allows us to write code line by line

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and then we can view our variables really easily on the right-hand side pane

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here. For example, if I have my list of numbers, 1, 2, 3,

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and then I have to write out my list comprehension

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where we create our new list which I'll call new_numbers,

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and then we have to create our list comprehension.

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So the way to remember it is new_item for item in list.

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So now go ahead and write this out and see if you can remember how we actually

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structure our list comprehension and see if you can get it to work within your

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PyCharm console. So pause the video, give that a go.

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All right. So our list is of course, this list of numbers.

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So let's replace that first.

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And then for each of the items in that list of numbers, well,

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we can call it anything we want. So I'm just going to call it n or num,

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it doesn't matter. But as long as you're consistent in using it

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when you create the new item

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because each new item is going to be the n,

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which is each number in the list, +1.

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So now when I hit enter,

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you can see I've got this new list called new_numbers created from the previous

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list, numbers, and this now has the same number of items

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but each item is now increased by one.

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Now it's important to remember that when we say list comprehension,

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it doesn't strictly mean that you can only work with list.

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You can work with other sequences like strings, for example.

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So here I've got a variable called name, which is just a string. It's the word

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Angela.

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And I'm creating a new list by using a list comprehension that has this kind of

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code. Now,

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I want you to pause for a second and think about what would be in this new list,

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what would it look like? And then I want you to try it out inside PyCharm.

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So pause the video now. All right.

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So name equals Angela, this is the variable that we're working with.

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And then I'm going to create a new list

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which I'll call letters_list.

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This new list is going to be created using my list comprehension.

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So it's going to be letter for letter in name. And what this does is it takes

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this sequence, this string,

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it goes through each of the letters in that string

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and then it adds each of the letters into this new list.

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So when I hit enter, you can see, this is what letters_list

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looks like. It's just split off my string into individual letters

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and it's added it into a brand new list.

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As I mentioned, these things in Python

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like a list or a string or a range or tuple, they're called sequences because

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they have a specific order. And when you perform a list comprehension,

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it's going to take that sequence and it's going to go through it in order either

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be it the letters in the string or the items in a list.

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And then it's going to take each of those items in that correct order

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and then do something with it, either add one to it, or in this case,

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do nothing and just add it to a new list. Here's a challenge for you.

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Can you take the range,

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which is also a sequence that we can iterate through, and then create a range

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between 1 and 5?

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And remember that the way that the range works is it's going to take 1 and

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then 2, and then 3 and then 4, but it's not going to go up to 5.

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I want you to loop through this range and then create a list where each of the

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numbers in the range is doubled.

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And the final list should look like this. It will be 2, 4, 6,

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and 8. Pause the video and see if you can complete this challenge.

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All right. So let's create our new lists

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which I'm just going to call range_list.

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And we're going to use that keyword pattern to remind ourselves.

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So it was going to be new_item for item in list.

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And this list, in this case, is actually not going to be a list,

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it is going to be a range between 1 and 5.

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And then each of the items, we get to give it a name, which I'll just call num.

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And then in this new list, what is each new item going to be? Well,

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it's going to be num multiplied by 2. So now when I hit enter,

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I get my range_list, which is 2, 4, 6, 8.

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So its loop through each of the numbers in the range

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and its multiplied each of those numbers by 2,

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and then its created a new list using each of those numbers.

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Now the final thing I want to show you regarding list comprehensions is that

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there's also such a thing as a conditional list comprehension.

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This takes our keywords a little bit further.

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Whereas previously we stopped right here,

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new_item for item in list. We can also tag on two more keywords;

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if and a test.

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What this is going to allow us to do is to only add this new item and only to

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perform this code if the test actually passes.

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Here's an example. I've got a bunch of names here; Alex, Beth, Caroline,

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Dave, Eleanor, and Freddy. Now,

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one of the neat things that you can actually do with the Python console is not

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only can you view what all of the variables are equal to,

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but you can also edit them if you have an issue.

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The sharp-eyed amongst you will know that I've actually made a typo in this

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name, it's not Elanor, its actually spelled a little bit differently.

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So what I can do on the right-hand pane here is I can right-click on that piece

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of data that I want to change and then click set value,

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and then actually change the data here.

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Now that I've got that spelled out correctly, if I type names and hit enter,

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you can see it's now been corrected and this is my new list of names.

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So using this list of names,

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I'm going to try and create a new list of names,

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but I only want the short names.

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I only want the names which is made up of four letters or less, like Alex or

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Beth.

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The way I would do this is I would create a new list,

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which is called short_names. And then I would use my list comprehension.

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And remember, in this case, the keywords are new_item

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for item in list. And then if and a test.

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So let's fill in the easy parts first.

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The list is going to be our list of names, the item I can call it

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anything I want. I'll just call it name in terms of the singular form of names.

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And then once I loop through each of the names inside my list of names,

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what is the new item going to be? Well, it's actually going to be unmodified.

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It's just going to be that same name. But in this case,

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I'm only going to add that name to the list if it passes this test.

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And this test is going to look at the length of the name

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which I created here,

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so if I named this n then I would be testing it against n and it would be adding

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n as well. But because I've called it name

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so I'm going to use that name right here in the condition.

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So if the length of the name is less than five,

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so it has four or less characters,

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then I'm going to add that name to my new list. Now, when I hit enter,

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you can see that I've got this new list of short names and it only contains the

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names, Alex, Beth and Dave.

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So this list comprehension is now a little bit more complex because the first

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thing it does is it goes through each of the names inside this list of names,

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it checks each of those names for its length. And if the length is five,

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then it adds the name to this new list.

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Now here's a challenge for you.

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I want you to take all of the names from this list of names

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which are made up of five letters or more, so basically Caroline,

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Eleanor and Freddie,

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and I want you to turn each of these names to the uppercase version.

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What you should end up with is a list where you have CAROLINE in all caps,

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ELEANOR in all caps and FREDDIE in all caps.

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But of course, you're not going to just type this out,

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you're going to use list comprehension.

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Pause the video and see if you can complete this challenge.

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All right. I'm going to call this new list long_names,

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cause I haven't eaten for a while and I lack imagination. Now,

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in order to create this list of long names,

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I'm going to use that new keyword pattern that we learned,

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which is new item for item in list

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if test. The list is, again, going to be our list of names.

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Each of the items we can call it anything we want,

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but I'm going to call it name. And if you find that this is too similar to this,

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you can call anything you want.

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Nom, you can use French or you can just write n or you can do anything you want.

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This name that we're looping through is going to be tested in order to see if

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we're actually going to create a new item.

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So the test, in this case, is we're checking to see if the length of the name is

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now greater than five.

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And if this name that we're currently looping through passes this test,

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then we get to specify what the new item should be.

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So the new item is going to be that particular name that passed the test,

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but then it's going to be uppercased,

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so we're going to call name.upper in order to create the new item

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that's going to be added to the list. So now once I hit enter,

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you can see the new list that I've created, long_names, has Caroline,

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Eleanor and Freddy, all in uppercase.

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Essentially, we looped through each of the names in the list,

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we took each of those names and checked that they'ree longer than five.

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If they were, in fact, longer than five,

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then we looked at this part of the code to see what we should do with each of

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those names. And in our case, we've turned it into uppercase.

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So there's quite a lot of theory covered in today's lesson.

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And if you want to see the code that I've been writing so far,

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then you can always head over to the day-26-end Repl.it in order to either

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download it or just read through the code that I've been typing. Now,

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in order to be able to fully understand what's going on,

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you can't just watch me talk about it. You have to practice.

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So in the coming lessons,

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I've got a whole bunch of exercises for you to practice using and creating list

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comprehensions for yourself. And that way, once you've done the drills,

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you've done the code press-ups, then you'll be able to show off your list

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comprehension muscles. For all of that and more, I'll see you on the next lesson.