How to Use Python List Comprehensions

welcome back to L Master Tech and in today's video I'm going to show you how to use an awesome and Powerful programming tool in the Python programming language called list [Music] comprehensions so if you've never seen list comprehensions in python or maybe you've seen them slard of them but you've kind of avoided learning about them because it seemed new and unnecessary um you're missing out because this is an awesome feature that uh python has available to you and to show you a basic list comprehension um I'm going to use a pretty exam a pretty good example of some basic python all right so let's say that I have a list and it comes in as all lowercase kind of poorly formatted and it's a list of names in this case just three names well the conventional way to write python code to capitalize all of those names and then store them in another list or even the same list would be code that looks something like this so it would be make a new list in this case we'd say like names unor proper and then for I in range of however many of those names there are so if we added a fourth name this code would not break it would still be able to handle that um and then you might have some if conditional maybe not so in this case just to show the power of list comprehensions I added if names start with P so in that case I'm only going to grab names that start with P so it would be Pete and philli but not Dale in this example um you don't need that step obviously if you want to capitalize all of the names you could just comment that out and skip it but then you would do two things you would capitalize each name turn into uh a string transform called tile this is a built-in python function that um parses a string and makes the first letter of each new word after a space um a a capital and then store that in the list of proper names and so if I run this you'll see down in the bottom I get all three names now capitalized and nicely formatted um and then to add back in the if conditional let's say if we only want the ones that start with a p and I run it again you'll see now I get the same list still nicely formatt and capitalized but just Pete and Phillip not Dale this time and this code is perfect valid and uh completely functional in Python and probably something very familiar to people with a little bit of basic Python programming experience but the incredibly powerful uh abilities of list comprehensions come in when we take a look at a way to define this all in one line so the basic format of a list comprehension is up here in the comment new list equals a for B in C if d and if D is optional but so what does this mean let's take a look at this example so names proper is going to equal the actual value that we want which would be each name turned into a title four and then name because this is just a variable that we're defining think of like an indexer variable that gets used in a for Loop so name. tile for each name that is in the list names and then we could stop right there uh except that title needs those uh uh parentheses um we could stop right there and to show you what that's going to do it goes back to giving us all three of the uh original names and capitalized um but we can go even one step further so this is where I said that if condition D is uh capitalized um or is optional um then what we could say after this too is if name Dot and then another buil in Python string function which is do starts with and then starts with the letter P so now this if I run it is going to give us just Pete and Phillip capitalized nicely formatted and so this is just one specific example obviously there's not really a world where you would badly need to take a list of three names capitalize them and use a list comprehension but this starts becoming incredibly powerful in much larger data sets and examples and maybe the conventional python programmer in you thinks like well this is a lot easier to read you know if you're not familiar with list comprehensions this is a little bit of like syntax sugar and it looks a little bit diff more difficult to read here I know exactly what's happening like I'm iterating through however long my names list is I'm checking to see if it starts with a p and if it is I capitalize it using the title function and then I add it to my name's proper list but once you learn to read a python list comprehension this line of code is doing the exact same thing it is taking each item inside of this list it is storing it in a variable called name and then it is doing thetitle transform to that variable and then it's checking just like before to see if that variable if that value starts with the letter P so we have taken five lines of code here and turned them into one line of code um and strings and capitalizing things are kind of just the beginning of uh list comprehension the use cases go all over the place so that's kind of a good beginner um example to introduce you to list comprehensions now let's take a look at a few different ways to do list comprehensions in a few different use cases okay so I think a good second example to show you guys would be iterating through nested lists also known as an array more correctly or a matrix um and to show you how let's say I have a matrix uh filled with lists and I want to combine all of the components in those lists into one list now typically in Python if you do list concatenation it will just add those together um but that's not always going to be what you actually want and so it's still several lines of code and in if or for Loop something like that and there's a really easy list comprehension for this so let's say that I have this Matrix um with a bunch of lists in it and you know obviously I've done the alphabet here uh J K L forgot the alphabet that's embarrassing m j k l m n o you know we'll leave it there good enough um let's say that we want to create a list comprehension to store all of the values in those different lists in just one list going forward and to do that we'll say okay the ABCs are going to be equal to a character for each list in The Matrix but that's not really enough yet because we haven't defined where character is coming from we want each character inside of each list in The Matrix and so what we do is we have this main Loop and we need to define a for loop after this main Loop that's going to run in each iteration of the loops so what we have to say is four character in each list so I know that this is where list comprehensions start to look really gummy and really kind of uh goofy if you're not familiar with it compared to just some nice plain uh for loops and it's perfectly okay to go back to what you're comfortable with but I also think it's very useful to acquaint yourself with these examples what we are saying here and the way to read this is take every character for each list inside of the Matrix for each character in that list so I know the best way to kind of read this is sort of backwards where we say take every character in each list for each list in the matrix it's a little bit of getting your syntax figured out but if we run this you'll see and I left all the code that we were using before in place so at the end I can just dump this all in a GitHub for you but what you can see there is we get one nicely formatted uh list at the end and you could do things here like character do um upper uh if you wanted in all capitals uh dictionary list or anything like that so you can still do the stuff that we figured out in the previous example of like string transforms or numerical transforms but this is how you iterate through lists within a list how you iterate through matrices and arrays that have lists inside of lists you could feel theoretically have more for conditions after that or you could have if conditions after that so that's just another example showing the power of list comprehensions now I want to take a look at a few examples with numbers this section of the tutorial will be called fun with numbers um no it's not called anything I don't do things that way okay so let's say we want to make a quick list and this is actually a list comprehension but this isn't really the uh tutorial let's say we want a quick list of numbers and this is just going to give us 0 1 2 3 4 5 6 7 8 and nine this is basically saying uh 4 I in range 10 uh you know numbers. append I okay this is basically just getting us a list of numbers well let's say I want to make a list of all of the squares okay I want to take each number and square it this is the python Syntax for uh raising to a power for Num in Num okay so easy enough if I print squares then what you'll see uh I spelled it wrong initially squares then what you'll see when I run this is I get the numbers 0 through N squared though okay so 0 squared is 0 1 squar 2 3 4 all the way up to 9 squar pretty cool um obviously you can do the same thing for cubes if you wanted I know it's silly to column squares but now just by changing that two to a three we've got all these cubed values and things like that so in a theoretical world where maybe this isn't uh 10 numbers maybe this is 100 numbers and maybe I want to know every cubed number that is divisible by if num Cub 3 is uh divisible by uh it doesn't matter six okay so every cubed root that is um divisible by three but or or divisible by six with no remainder so to check for no remainder I want to say modulo 6 is equal to zero right all right so I know that was kind of uh just pulled out of a hat example here but this is going to go through and Cube every value 0 to 99 and store it in a list if the cubed value is divisible by six and I honest to goodness have no idea which ones those will be okay um so there you go there are apparently about 15 of those values um I I couldn't actually tell you what these values are uh I guess what we could do here this is now it's just fun you know now we're just having fun guys um okay so here I have added the number and what its cubed value is if it's perfectly divisible by six um I don't know why this is such a surprise to me but it is all the values that are a multiple of six which makes perfect sense right um because all cubing something is is multiplying it by six but that was supposed to show you an example of cubed roots and I can't stop laughing at myself for uh how silly an example that is but hopefully you're still seeing the format of a some value and some value that can be manipulated or checked or even turned into a tupal right like we took a number and we turned it into a tupal by saying we want to store both the number and its cubed value so a for B which is B is the one that's going to change the least because it's just the name of your indexing variable and then in C so whatever list we're checking through if D some condition uh to check against or check for and that is a list comprehension so I think the last thing I want to do in this tutorial is talk about kind of the natural extension of this which is dictionary comprehensions it's almost identical to list comprehensions but there's a couple nuances about uh using list comprehension style checks against dictionaries so let's take a look at just a few quick examples for those okay so I'm going to run it all the way back here to kind of our first list which was just three names only now I'm going to make it a different uh list and instead of a list I'm actually going to make it a dictionary so assuming you know a little bit about python a dictionary is obviously a bunch of keys and then their values so dictionaries are organized into key value pairs separated by a colon and then uh each pair is separated by a comma so key colon value and then a comma and then the next key colon the next value comma stored inside these curly brackets that's what makes it a dictionary if I want to transform this dictionary into a new dictionary let's say adjusted salary okay where I want to go through and I want to take all of the values that were in that first dictionary and I want to do something new to them just like the list comprehensions the format here is very similar it's still a for B in C if D only now you have to know about a couple little nuances okay so let's say I want to do two things I want to do a full uppercase of the name so I want to do name. uper but then I also want to take a uh integer value of the salary to the nearest thousand okay so let's say I want name. uper and then colon and then the salary uh I want it to be an integer and I want to divide it by a th000 okay so an integer salary divided by a th000 for each name comma salary in salaries do items okay so and then you need these parentheses here I'm trying to make sure my little camera Square doesn't get in the way of the code um so what this is saying is take all of the items for each entry in salaries split it into two variables called name and then salary and then uppercase the name and then take an integer divided by a thousand of the salary and then store that in a new dictionary so if I print adjusted salaries gosh I hope this works run okay what you'll see is you've got all three names fully uppercase so this is different than tile which we did in the earlier example up here um and then you have the integer value of their salary in thousands okay so it's just a way of showing you you can transform both the keys and the values inside of a dictionary using a dictionary comprehension at the same time you just have to know this not tricky necessarily but slightly new syntax until you get used to dictionary and list comprehensions um just like before you could say if salary greater than uh 100,000 if you wanted to or if int of salary divided by a th000 is greater than 100 it's basically the same check just written in a different way um so this should only give us people whose salaries are above 100K and it does um Pete drops off of there and then you could add another thing that we haven't talked about yet in list comprehensions and in dictionary comprehensions you can combine if statements uh and you can do this two different ways so you can say if the salary is over a th000 and if name dot starts with p okay and so if you've been paying attention to our list you know this will probably only return Phillip all right um and you can do that by saying if and then another if or you can do it this way you can say if and then you can uh just like any other time you can put two conditionals inside of uh inside of parentheses and use the keyword and okay so uh this is a dictionary comprehension this is also shown that you can combine if conditional statements this is true for dictionaries and lists um and I think all of the examples combined in this video so far I will drop this code if you want exactly these examples um in a GitHub and I'll leave a link to it in the description of this video but you can probably cook up these examples or similar examples very quickly and easily yourself um there are so many use cases for list comprehensions and dictionary comprehension I'm sure there are probably things that I glossed over things that possibly we missed um but I hope all in all this video was useful and taught you what you were looking for um if you have any questions be sure to let me know about in the comments below it was really nice to be able to just put together a Python tutorial which is kind of how this channel got start I've been diving into a lot of hardware and going all over the place lately so it's nice to get back to the roots a little and do hopefully a useful Python tutorial for everybody be sure let me know in the comments below what you'd like to see more of on the channel uh don't forget to leave a like on the video subscribe to the channel if you've been enjoying the content consider checking out my patreon there's a link below in the video as well and as always a massive thank you to my patreon supporters as well as everyone leaving likes and watching on L Master Tech YouTube good luck with your projects until next time bye