List Comprehensions - Python Tips and Tricks #1

Key Takeaways

[Music] what is going on guys welcome to this new tutorial series on neural nine it is about python tricks and tips which is essentially a tutorial series that is not for beginners not for intermediates necessarily it's just showing you some tricks that you can use in the python language uh i would not consider them to be very advanced skills or something that is very complicated but i would also say it's not necessarily a beginner topic which is why i made this extra playlist here where we're just going to talk about interesting things that you can do with python that speed up your coding make your code more fancy more effective more efficient whatever and in the first episode today we're going to start with list comprehension so let us get right into it so what list comprehensions essentially are just a concise way to create lists in python now let's say you have some sort of list here for example numbers equals and then you have a bunch of values here 18 16 22 99 23 11 54 and so on just some values and what you want to do is you want to filter out all the even values and you want to create a new list only containing the even values of this list so what you would do is you could say new list or even numbers whatever uh create an empty list and then you would go ahead and say 4 number and numbers what i'm going to do is i'm going to say okay if the number is even so if the division by the number two results in a remainder of zero um then i'm just going to say new list dot append the number that we're talking about so this is a way that definitely works so we can go ahead and say print new list and you will see that we'll end up with a list only containing the even values down here you can see 18 16 22 and 54. we don't have any other even values in the list um and this is not a wrong way it's not an inefficient way but it takes more lines of code than if we use list comprehensions now list comprehensions are very very simple to use instead of writing a full loop and then an if statement across multiple lies lines what we do is we use the square brackets and inside of those square brackets we specify what the list shall contain so i can say for example it shall contain the number so just the number that we're talking about i haven't defined what the number is yet i'm just saying the number for all the number uh elements in numbers so in this case what i'm doing is i'm saying the element that ends up in the list is the first thing that i say here so number but what is number so i can also call this x for example what is x x is just every element in numbers and now what i can do is i can add a condition to it so i can say x for x in numbers but only if x modulus 2 equals 0. so this is a one-liner here as you can see new lease new list equals x for x in numbers if x modulus 2 equals 0. that is the only condition and if i go ahead and print this here you will see that we'll end up with the exact same result in the terminal down here 18 16 22 and 54 because the code is essentially the same it's just written in a different way in a more concise way using list comprehensions now another thing that we can do inside of those list comprehensions is not only manipulate the condition and the loop we can also manipulate the actual object that we're adding into the list so let's say we have numbers here again but this time we have one two three four five six seven and what i want to do is i don't want to add those values to the list i want to add uh powers of two to the list so i'm going to say powers of two and what i want to do is i want to take 2 to the power of 1 2 to the power of 2 2 to the power 3 and so on and get it into the list so what i can do here is i can not only say x for x in numbers i can also say 2 to the power of x for x in numbers and the important thing is here i'm not only able to manipulate what i'm iterating over or what the condition is so i can of course also specify an if statement here but i can also manipulate the element itself i don't have to work with x uh directly i can use x just as in as an exponent for two so i can say 2 to the power of x here and what i'm ending up with at the end is just the powers of 2 which we can see when we run the script you can see i end up with 2 4 8 16 32 64 128 because those are the powers of two and of course i can also get rid of this list here and just replace it with range uh whatever for example 30 and i'll get all the powers of two up until 30. so up until the exponent of 30 1 2 4 and so on until we get to this large number here 2 to the power 30 or 2 to the power of 29 not sure where it stops but you can specify any kind of list at the end here and then you can just go ahead iterate through it and then apply an operation to the result you don't have to store x directly based on a certain condition you can also go ahead and say 2 to the power of x or do anything you want with x you can also apply a function that returns another value and of course i can add an if condition here as well so i can say if x is divisible by five for example if i want to do that and i'll only get two to the power of numbers that are divisible by five as you can see so last but not least we're going to look at an example that will show you that you can also use else branches not only if statements and conditions that have to be met but also what happens if they're not met and for this i prepared a list of words a list of string strings and what we have here is the word automobile we have the word car we have the word anger we have the word fox and we have the word anchor and the idea is or the trivial task here is to just get all the words that start with a and to make them uppercase while all the other words remain lowercase so of course we can do this in many different ways we can do this with a map function which is not a trick that we're going to learn about we can do this with an ordinary loop but we can also do this with a list comprehension so what we can do here is we can just say words equals and then we can say word dot upper so we're saying the word shall be upper if the word dot starts with which is a string function with the letter a and else we can say word so in this case we're using a so-called ternary operator i'm going to make in a separate video on this one but essentially we're using this concise if statement here we say okay if this condition in the middle is met we're going to do this here and if it's not met we're going to just use the word uh and we're going to do this thing for all the words in words so as you can see we're saying word dot upper if word starts with a else just the ordinary word for every word in words and i think we could also add a condition to the end here uh but it's not necessary right now so what we can do here is we can just go ahead and print words and you're going to see that automobile anger and anchor are now uppercase because they satisfy the condition where it starts with a and car and fox remain the same now of course if i don't use this else thing here if i just say uh word.upper if word starts with a at the end for example here um i will only get the words that start with a of course they're going to be uppercase but i'm losing carr and fox because i don't have an alternative of what to do if the word does not start with a so this is also one thing we can do with list comprehensions so that's it for today's video hope you enjoyed and hope you learned something if so let me know by hitting the like button and leaving a comment in the comment section down below let me know if you like this tutorial series if you like this kind of video um if so i'm going to make much more of it because it's a very uh simple tutorial playlist you know you just have to find something in python that's exciting and you can make a video about it it's not too complicated you don't have to understand uh machine learning mathematics or any complex things you can just go ahead and find a nice feature of the language and then use it so if you want to see more of those videos let me know in the comment section based on your response i'm going to decide if i'm going to make just a couple more videos or a lot more videos so let me know in the comment section and by hitting the like button also make sure to subscribe to this channel if you want to see more future videos for free other than that thank you very much for watching sooner next video and bye [Music]