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Hi guys and welcome back.

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In this video, we're going to talk about

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list comprehensions in Python.

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A list comprehension is a great feature of Python

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and allows us to create new lists on the fly

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from existing lists.

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In addition, it's very succinct and very powerful.

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Let's get started.

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Here we've got a list of numbers,

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and I want to end up with these numbers,

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but multiplied by two,

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so I'll say something like doubled equal empty list.

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How do we put each of these numbers,

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multiplied by two, into this new list?

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Traditionally and in most languages,

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you would end up doing something like for num in numbers,

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and then you'd do doubled.append num times two.

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This is quite a long piece of code

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just for creating a new list of doubled numbers,

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but this is how you do it normally.

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You would iterate over the numbers list,

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create a new variable for each of the numbers in it,

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and then run through it three times,

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each time putting the appropriate number,

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multiplied by two, into your new list.

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Python has a much more succinct way of doing this

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which is with a list comprehension.

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In a list comprehension,

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the order of these operations, first the iteration,

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and then the appending, seems to be reversed,

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but it's not really.

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So the first thing you do is you would put in

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what you want to add to your new list,

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which is num times two.

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But of course, num doesn't exist in this context.

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It only existed here because we've got our for loop.

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So the next thing you do is you put your for loop,

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and what this does in Python is it puts num times two

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into a new list for every num in the numbers list.

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So essentially you're iterating over the numbers list,

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and you're putting the variable multiplied by two

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every time that the loop runs into your new list.

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You end up with the doubled numbers.

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Because list comprehension are usually in a single line,

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you do want to keep them short if you can,

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so instead of num times two for example in this case,

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you could do x times two, and for x numbers.

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That just helps you keep it a little bit shorter

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and a little bit more readable.

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After all, the most important thing here

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is that you're putting x times two into your new list

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while you iterate over the previous list.

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So again to recap, how you build a list comprehension

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is what you wanna put into your new list

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for the variable that you're using in numbers.

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Now let's say you've got a friend's list,

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and you wanna create a new list

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which contains the names of those friends

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that start with an s.

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So Sam, Samantha, and Saurabh.

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But it seems that a list comprehension

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may not be the tool for this job.

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After all, you do need to only get some of these out.

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Let's start with a for loop,

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and then I'll show you how the list comprehension

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can deal with it.

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So you would do for friend in friends.

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Then you would add an if statement to check

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that the friend name in question starts with an s.

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You could say if friend.startswith capital s,

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then you're gonna do starts_s.append friend,

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and then at the end, we're gonna print starts_s like that.

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Let me run this and show you what happens.

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Down here, you have Sam, Samantha, and Saurabh,

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and I think I'm not pronouncing that right,

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so my apologies if that's the case.

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So what we've got is a for loop,

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and then an if statement, and then an append.

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This structure here can also be reproduced

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with a list comprehension although it does start to get

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a little bit more confusing.

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Again, the process is more or less the same.

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First of all, you put what you wanna add to your new list,

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so that's your friend name.

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Then you put your iteration or your for loop,

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for friend in friends.

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And now if you wanna add an if statement,

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you do it at the end,

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so you're gonna add your friend

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for the friend in your list if the friend starts with s,

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and that does exactly the same thing,

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and you no longer need that loop there,

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so you can save that and run it again,

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and you see that the same thing comes out.

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So this is how you add a conditional, an if statement,

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into a list comprehension.

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Very important to nice that

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when you use a list comprehension,

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a new list is created,

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so even though these two lists may be the same

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if we take these names out,

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so now these two lists will be the same

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if we print them both out,

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you'll see that they are both identical.

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When you print friends is starts_s,

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you'll see that false comes out.

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That's because these two lists,

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although they have the same content,

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are not the same list.

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They are two different lists, two different places

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in your computer's memory with the same values.

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For context, friends zero is starts_s a zero,

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so Sam is Sam will give you true, see that?

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But the lists themselves are not the same,

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so the elements in them are the same,

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but the lists themselves are not the same,

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and by using the is keyword, then you can check that,

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but there's also something else you can do,

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which is check the IDs of these lists,

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so you can do something like friends,

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then print out the ID of friends,

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and then starts_s and print the ID of starts-s,

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and what I've done here by the way

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is I have passed multiple different things

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to the print function, and you can do that in print,

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and it just separates them with a space,

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so it's gonna separate these different parts with a space,

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the different arguments there.

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So by running this, you get that friends has an ID

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of this, ends in 2096, and starts_s has an ID

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that ends in 5280.

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So these two have different IDs, and in Cpython,

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which is what most of you will be using,

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certainly what I'm using,

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the ID is related to the memory address

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in which the list is stored,

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so you can see that these two things

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are in different places in memory.

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They're different objects entirely.

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They just happen to have the same contents.

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So this is something that will be very important

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as we proceed with the course,

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so this is why I wanted to show it to you at this point.

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Creating new lists gives you a different thing

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entirely in Python.

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It's not the same as the thing before

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even if it has the same contents,

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but as I mentioned earlier,

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if you do want them to be the same exact list,

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then you can do this.

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Starts_s equal friends, and now when you press play,

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you'll see that you get true,

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and these IDs are identical.

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So that is how you make two lists the same thing.

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Otherwise, you do end up with two different lists.

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All right, that's enough of that.

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Thank you for joining me in this video.

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I hope it's been useful,

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and I'll see you in the next one.