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Python Tutorial for Beginners 4: Lists, Tuples, and Sets — Transcript

by Corey Schafer · 5,473 words · 692 segments · language en · Watch on YouTube

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0:00hey there how's it going everybody in
0:01this video we'll be learning about lists
0:03tupal and sets in Python now lists and
0:06tupal allow us to work with sequential
0:08data and sets are unordered collections
0:10of values with no duplicates and we'll
0:13look at all of these to see exactly what
0:15that means so first let's look at list
0:17and we're going to spend a majority of
0:19the time on list just because it has a
0:21lot more functionality than the other
0:22data types so just like the name implies
0:25it allows us to work with a list of
0:27values so for example let's say that I
0:29wanted a list of courses so I could
0:31create a variable here called courses
0:34and to create a list we're going to use
0:36these square brackets and Within These
0:39brackets we put each value that we want
0:42separated by a comma so for example
0:44let's say that we wanted a list of
0:46courses so I'll say history and math and
0:53physics and also lastly we'll put in
0:57compy so now if we print out this list
1:02so I'll print out courses and run that
1:05then we can see that it prints out our
1:06entire list now if we wanted to see how
1:09many values are in our list then we can
1:11get this by using the Len function which
1:14stands for length so if instead we print
1:16out the length using Len of courses and
1:20run that then you can see that it says
1:22that we have four values in our list and
1:24we can access each of these values
1:26individually so first let's go back to
1:28just printing out our list and running
1:32that so now to access the values in our
1:34list we can use square brackets after
1:37our list and pass in the location of the
1:40value that we want so we can put in
1:42square brackets and this location is
1:44called an index and it starts at zero so
1:47to access the first value of our list we
1:50can access uh the location at index0 if
1:54I print that then we can see that we get
1:56history now since the length of our list
1:58is four that means with the first value
2:01starting at index zero then the last
2:03value will be at index three so it's our
2:06total length minus one so let's say I
2:09wanted to grab the last value for my
2:11list which is compy I could count these
2:14indexes out and see which one should be
2:16able to access it so 0 1 2 and three the
2:21third index so if I print out courses
2:24three and access that index of three and
2:27run that then we can see that we printed
2:28out compy now we can actually use
2:31negative indexes too and negative
2:33indexes will just start from the end of
2:35the list so since zero is the first item
2:38of our list I can also get the last item
2:40of our list using a negative one so if
2:43we print out courses uh index of -1 and
2:47run that then we can see that we got
2:49compi again which is the last item it's
2:52more convenient a lot of the time using
2:55a negative one to get the last item
2:57because we don't have to worry about
2:59what the link length of the list is so
3:01for example if my list grew by 10 items
3:04then the third index would no longer be
3:06the last item but the negative -1 will
3:09always be the last item and also if you
3:11accidentally try to access an index that
3:13doesn't exist then you'll get an index
3:15error so in this example if we tried to
3:18access an item at index 4 and run that
3:21since there is no index 4 you can see
3:23that we get this list index out of range
3:26now instead of only grabbing one value
3:28we can also access a range of values so
3:31if I wanted to grab the first two values
3:33from this list then I could say that I
3:36want to access starting at index zero
3:39and go to but not including index two so
3:43this first index is our starting point
3:45and the second index which is separated
3:47by this colon is the stopping point now
3:50one thing that's a little confusing is
3:51that the first index is inclusive but
3:54the second index is not and there's good
3:56reasons for that but it's easy to forget
3:58so what we're saying here is that we
4:00want all of the values between the
4:04beginning and up to but not including
4:06the second index so if we run this then
4:10we can see that it printed out history
4:11and math because it printed out zero and
4:15one all the way up to two but not
4:17including two which is physics now since
4:19our first index here is just the start
4:21of the string we can actually just leave
4:23that off and it'll assume that we want
4:24to start at the beginning so if we said
4:27print the courses with nothing there
4:30colon 2 and run this then you can see
4:33that we get the same result because it
4:35just assumed that we wanted to start at
4:37the beginning now if we just wanted to
4:39grab physics and compi from this list
4:42then we can say that we want to start at
4:44index 2 and then put in our colon and
4:47now just like our starting index if we
4:49don't put anything here then it's going
4:52to assume that we want to go all the way
4:53to the end of the list so if I run this
4:56then you can see that it printed out
4:57physics and compi because it started at
5:00our second index which is physics and
5:02just went all the way to the end now
5:03what we're doing here is called slicing
5:05and if you'd like to learn more about
5:07slicing in depth then you can watch my
5:09detailed video on that which shows you
5:11how you can skip values go in reverse
5:13and things like that and I'll leave a
5:15link to that video in the description
5:16section below okay so let's look at some
5:19list methods that we have available to
5:21us that allow us to modify our list so
5:23first let's say that we wanted to add an
5:25item to our list there are a couple of
5:27ways we can do this so first if we just
5:30wanted to add an item to the end of our
5:32list then we could use the append method
5:35so let's say that we want to add art to
5:37our courses so we can just say
5:41courses. append and we want to append
5:44art so now if I remove this slicing and
5:47just print out our course's list and run
5:50that then you can see the art was
5:52appended here to the end of the list now
5:54if we wanted to instead add art to a
5:56specific location in our list then we
5:59could instead use the insert method now
6:01insert takes two arguments first it
6:03takes the index where you want the to
6:05insert the value and then the value
6:07itself so if I wanted to insert art to
6:10the beginning of our list then we could
6:13say
6:14courses. insert and now the first
6:17argument is the location so let's just
6:19say location zero which is the beginning
6:21and then the value that we want to
6:23insert which is Art so if we run this
6:26now we can see that art was inserted at
6:28position zero
6:30now that only inserted the value it
6:31didn't overwrite anything so you can see
6:33that all the other courses are still
6:35here but they just got shifted now
6:37another way of adding values to our list
6:40is using the extend method now sometimes
6:42this confuses people so let's look at
6:44what this does so we want to use extend
6:47when we have multiple values that we
6:49want to add to our list so for example
6:52let's say that we have another list here
6:54called courses 2 and we'll set this
6:57equal to another list uh with art and
7:01education in this list and we want to
7:04add these values to our original courses
7:07list so first let's see what happens if
7:10we use this insert method so instead of
7:13inserting art to the beginning of our
7:15list let's instead insert these courses
7:18to the beginning of our list so let's go
7:20ahead and run this so we can see that at
7:22the beginning of our list that it
7:25actually added the entire list of
7:27courses to and not each each individual
7:30value so we can actually have a list
7:33within a list like we have here so if we
7:36were to print the first value of our
7:38courses list so I'll print out the index
7:41of zero and run that then we can see
7:43that now the first value is actually
7:46this list itself but this isn't really
7:48what we wanted we wanted to add all of
7:50those values from our second list to our
7:53original list now that is why we use the
7:55extend method so let's go ahead and set
7:58this back to the way was to just print
8:00the courses now instead of inserting
8:02here we'll instead use extend and it
8:05only takes one argument which is the
8:08iterable so we will extend courses with
8:12courses to so now if we run this then we
8:15can see that when we did courses. extend
8:18with courses 2 that it added the values
8:21from our second list uh here to our
8:24original so a lot of people get that
8:26mixed up with append and extend so again
8:30uh if you were to append it just like
8:31with insert then it's just going to
8:33append the list itself on there instead
8:37of the each individual item so if we use
8:40extend then now we can see that each
8:42individual item is extended onto that
8:45list okay so now let's look at how we
8:48can remove some items or remove some
8:51values from our list now one way to
8:53remove values is to just use the remove
8:56method so if we were to say courses do
9:00remove then let's say that we wanted to
9:02remove math so if we save that and run
9:05it then we can see that math was removed
9:08from the courses list now there's also a
9:10way of removing values with this pop
9:13method so if we say courses. pop Now by
9:17default this will remove the last value
9:19of our list now this is useful if we
9:22want to use our list like a stack or a
9:25CU so if we run this like it is then we
9:28can see that comp I was popped of off of
9:31our list and now we just have these
9:33three courses now one other thing about
9:35pop is that it Returns the value that it
9:38removed so we can actually set a
9:40variable uh and grab that returned value
9:43so if I set a variable here and say
9:45popped equals courses. poop and then I
9:49was to print this above our courses here
9:52and run that then we can see that it
9:55grabbed that compi value that was popped
9:57off of the list so if you had a or a
9:59queue then you could go through and just
10:02keep popping off values until your list
10:04is empty okay so now let's look at how
10:07we can sort our list in a couple of
10:09different ways here so first of all
10:11let's say that we just wanted to reverse
10:13our list as it currently is now this is
10:15pretty easy so we can just use the
10:17reverse method so I I can say courses.
10:20reverse and if we run this then we can
10:23see that now it prints out our uh
10:26courses but in Reverse so the last item
10:29all the way up until the first item now
10:31instead of reversing our list what if we
10:33wanted to sort our list now sorting is
10:36just as easy we can just use the sort
10:39method so I'll save that and run it and
10:42you can see that now our list is sorted
10:44in an alphabetical order so compi
10:47history math physics now if our list
10:49contained numbers then it would sort
10:51those in ascending order so let me call
10:54uh create another list here of numbers
10:57I'll just call that nums I'll set this
10:59equal to some random values so I'll say
11:011 5 uh 4 and three and we'll save that
11:05now below courses we will just say nums
11:08do sort and we will also print these out
11:11so I'll grab that variable print those
11:14out and run that so now we can see that
11:17our strings were sorted alphabetically
11:19and our numbers were sorted in ascending
11:21order now what if we wanted our values
11:24sorted in descending order now one way
11:26you might think to do this is just to
11:28use our reverse me method on the list
11:30after they're sorted and that would work
11:32but there's an easier way to do this uh
11:34instead we can just pass an argument to
11:37our sort method called reverse so if I
11:39come up here to the sort method and I
11:41pass reverse is equal to true and let me
11:45also grab this for our num sort and if I
11:49run this you can see that now these
11:51sorted are in descending order they're
11:54sorted in reverse order now one thing to
11:56notice here is that we don't need to
11:59reset our variables when we call most of
12:02these methods it's just altering the
12:05list in place but there's also a way
12:07that we can get a sorted version of our
12:09list without altering the original list
12:12so what if we wanted a sorted version of
12:15our courses list without uh altering the
12:19original so to do this we can use the
12:21sorted function so instead of calling
12:24this sort method on our list we'll
12:26instead use this sorted function and
12:29I'll pass courses in to sorted there now
12:32if we run this as it is then we can see
12:35that our list is not sorted it's exactly
12:37how uh we described it up here now
12:40that's because the sorted function
12:42doesn't sort the list in place it
12:45returns a sorted version of the list so
12:47to get that sorted list we have to make
12:50a new variable and set it to the return
12:52value of the sorted function so I could
12:55just call this sorted courses is equal
12:58to
12:59the sorted version of that course's list
13:03and now if we copy that and print that
13:05out so now we can see that this sorted
13:07courses is equal to the sorted version
13:09of that list so that's really useful
13:11because a lot of the times you won't
13:13want to alter your original list in any
13:16way um so using this sorted function is
13:18a nice way to get a sorted version of
13:20that list without altering the original
13:23and other than this sorted function
13:24there are a couple more useful built-ins
13:27uh that we can use with these SE quences
13:29so let's look at a few of these so we'll
13:31look at min max and sum so it's probably
13:35pretty obvious what these will do but if
13:37I wanted the minimum value of our
13:40numbers list here then let me comment
13:43out where we're printing out nums so a
13:45built-in function that we can use is
13:47just men and call men on our sequence of
13:50numbers and if I run that then you can
13:52see that it return returns one as the
13:54minimum number of that list and if we
13:56wanted the max value of that list then
13:58we could use Max and if I run that you
14:01can see it returns five um now if we
14:04wanted to print the sum of that entire
14:06sequence that I can just say sum of
14:09numbers if I print that then it gets 15
14:12because 1 + 5 + 2+ 4 + 3 is equal to 15
14:16okay so lastly let's see how we can find
14:20some values here within our list so let
14:22me go ahead and clean up here a little
14:25bit and uncomment out that so if we
14:28wanted to find the index of a certain
14:30value then we can use the index method
14:32for this so if I wanted to find the
14:34index of compi in our list then we could
14:38just print out
14:41courses. index and then search for compi
14:46now if we run this then we can see that
14:48we got three and three is the index
14:51where it found that compai value now if
14:53we try to find the index of a value that
14:56doesn't exist in the list then we'll get
14:58a value value errror so if we search
15:01that original list of courses for art
15:03and run that then we can see that we got
15:05a value error and it says art is not in
15:08the list now if we just wanted to Simply
15:11check if the value was in our list and
15:13simply get back a true or false result
15:16then for this we can use the in operator
15:19so if I wanted to check if art is in our
15:21list then I could say art in courses so
15:26if I run that and we can see that we got
15:28false but if instead we said math in
15:32courses and ran that then that returned
15:34true now this is going to be especially
15:37useful once we go over the topic of
15:39conditionals and if else statements and
15:41we're going to go over those in a couple
15:42of videos and we can also use this to
15:45Loop through values of our list by using
15:48a for Loop so if I was to say four item
15:53in courses and then scoot that over then
15:57we will just print out
15:59that item now this is the first time in
16:03our series that we've indented a block
16:05of code basically what we're saying here
16:07is that we want to create a loop We're
16:10looping through each value of our list
16:12and each Loop through this item variable
16:15will be equal to the next item in the
16:18list so that's why this line is indented
16:21because it tells us that this code is
16:23executed from within our for Loop so if
16:26we run this then we can see that it
16:28prints out each item of our list now the
16:31reason it prints them all out on a new
16:33line is because by default the print
16:35statement goes to a new line each time
16:37it's executed now this item variable is
16:40just a variable it's not a keyword or
16:41anything we can name this anything we
16:43want so instead if we wanted to you know
16:46call this course for course and courses
16:49and print that out then you can see that
16:50we get the same result so we can access
16:52each value as We're looping through but
16:55sometimes it might be useful to also
16:57have the index of what value were on Now
17:00to do this in Python we can access the
17:02index and the value by using the
17:05enumerate function so I will say
17:08enumerate and I will wrap courses within
17:11that enumerate function and this
17:13enumerate function returns two values it
17:15Returns the index that we're on and the
17:17value so instead of just getting the
17:19course here we're also going to need to
17:21get the index so I'll call that index so
17:24for index course in enumerate courses
17:28and now if we print out that index and
17:30the course and run that then you can see
17:33that we had access to each index and
17:36value as We're looping through our list
17:38and if we don't want to start at zero
17:41then we can pass in a start value to our
17:43numerate function so if instead we
17:45wanted to start at one then I can pass
17:50in the second argument say start is
17:52equal to one now if I run this then you
17:54can see that now our starting value is
17:57one okay so there is one one more thing
17:59that I want to go over before we move on
18:01to tupal and sets so it's pretty common
18:04that we'll want to turn our list into
18:06Strings separated by a certain value now
18:09to do this we're actually going to use a
18:11string method called join and we're
18:13going to pass in our list as the
18:15argument so for example let's say that
18:17we wanted to turn our list of courses
18:20into a string of comma separated values
18:22so we can say course string and that
18:25will hold the string version of our
18:27courses and now we'll type the string
18:29that we want to separate each item of
18:31our list so I want these to be comma
18:33separated so I'll put in a string of a
18:37comma and a space and then I'm going to
18:39use the join method to join the values
18:41of the list using this string so I'll
18:44say dojin and I want to join my courses
18:48on that string so now if I come down
18:50here and print that string version of
18:54our courses and run that then we can see
18:56that we get our values comma separated
18:58and if we want to change this then we
19:00can just change our string within uh
19:03that we're joining on so if we wanted to
19:05hyphenate these then I could just change
19:07this to space hyphen space and run that
19:10and you can see that when we ran that
19:12now we're separating these by The Hyphen
19:14so this is just one long string with all
19:17of our list values joined together now
19:19we can also do the reverse of this and
19:21turn a string back into a list so we can
19:24do this by splitting our string on a
19:26certain value so if I split the string
19:28that that we just created so let me
19:30create a new list here and I'll set this
19:33equal to the string version of our list
19:36and I'll do a DOT split and I'll split
19:39it on a space hyphen space so if we look
19:43at the string version that we have down
19:45here this is just one whole string what
19:47that's saying is it's saying hey split
19:49up all of these values on this space
19:51comma space and make a list out of all
19:53those values that you get so now if we
19:56print out our new list so I'll print
19:59that new list and run that and you can
20:02see that now we're back to the original
20:04list okay so I know that that was a lot
20:07to take in because there's so much that
20:08we can do with list but now let's move
20:11on to tupal and sets um now that we're
20:14familiar with list then these will
20:16probably go pretty quickly okay so tups
20:18are very similar to list but with one
20:21major difference so we can't modify
20:23tupal now in programming this is called
20:26mutable and immutable so list are
20:29mutable and tuples are not they are
20:31immutable so let's look at what this
20:33means so I'm going to grab a quick
20:35snippet of code here from my Snippets
20:38file uh just to compare these so I'm
20:40just going to grab from here all the way
20:43down to here and just go ahead and paste
20:47those in so first we have a regular list
20:50here that we've already looked at and
20:52we're going to look at one issue that
20:54you might run into with mutable objects
20:56so what I'm doing here is I'm creating
20:59our list of courses and I'm calling this
21:02list one and then we're creating another
21:04variable here called list two and we're
21:07setting that equal to list one and then
21:09we're printing out both of these so if I
21:12run this then we can see that both list
21:14one and list two have the same values so
21:17now let me uncomment out this code where
21:20we're changing a value in the first
21:23index of list one and then reprinting
21:26our values again so now if I run this
21:29then we can see here that all we did was
21:31change the value at the first index of
21:34list one but by changing list one's
21:37first value it also changed list two and
21:40the reason for that is because they're
21:42both the same mutable object now if you
21:45need to modify your list then this
21:47mutability is what you want but if you
21:50want a list of values that you no aren't
21:53going to change then we can use a tuple
21:55so let's look at a tupal example so I'm
21:58going to Common out our mutable example
22:00here and then uncommon out our Tuple
22:03example so a tupal looks almost exactly
22:06like a list but instead of the square
22:09brackets we're using instead using these
22:11parentheses and after we create our
22:13tupal we're doing the exact same thing
22:15we're calling a uh creating a variable
22:18Tuple one that is a tuple and then we're
22:21setting this Tuple two equal to Tuple
22:24one and for now I'll just comment out
22:26where we're changing that first value
22:28and I'll run that and you can see that
22:30both of these tupal are equal to the
22:33same list of values but now if we try to
22:36change that value at the first index of
22:39Tuple one just like we did with our list
22:42if we run that then we can see here that
22:44we get an error and it says type error
22:46Tuple does not support item assignment
22:49and that's because it's immutable um so
22:52now since a tupal is immutable it
22:55doesn't have nearly as many methods as a
22:58list because a lot of those list methods
23:00that we looked at involved mutating the
23:03values so we can't append we can't
23:05remove anything or anything like that
23:08but other than that they behave pretty
23:09much the same we can Loop through tupal
23:12we can access values and all the other
23:14things that we've already seen except
23:17for what mutates the list so that's
23:19basically the difference between list
23:21and tupal um if you need something that
23:23you can modify then use a list but if
23:26you just want something that you can
23:27Loop through and access then you might
23:30want to think about a tupal so that you
23:32don't run into the issues that we just
23:34saw okay and lastly let's look at sets
23:37now sets are values that are unordered
23:40and also have no duplicates so let me
23:43grab a sample from the Snippets file
23:45here um that we can see so I'm just
23:48going to go ahead and grab this small
23:50snippet and paste that in so we can see
23:52that this looks similar to lists and
23:54tupes but instead of the brackets or
23:57parentheses we're we're instead using
23:59these curly braces now if I run this
24:02code to print this set out then we can
24:04see that it prints out the values but if
24:06you notice here these aren't in the
24:07order that we added them so we had
24:09history first here and it has history
24:12last down here and if I run this a
24:14couple more times then we'll see that
24:16you know this order can change with each
24:19execution now the reason for this is
24:21because unlike our lists or tupal sets
24:24don't really care about order because
24:26some of the main uses for a set is
24:29either to test whether a value is part
24:31of a set and also it's used a lot to
24:33remove duplicate values because sets
24:35throw away duplicates so first let's
24:38look at how to get rid of duplicates so
24:40if I add another Math course here to the
24:43end of our set and now if I rerun this
24:47then we can see that we still only have
24:49four courses it got rid of that second
24:51Math course and just left us with the
24:54one now another thing I mentioned was
24:56that it's used to test whether a value
24:58is part of a set now this is called a
25:01membership test so sets do this a lot
25:03more efficiently than list and tupal so
25:06what I mean by this is that within our
25:08print statement if I was to say um math
25:13in CS courses and I run that then when
25:17you can see that it prints out true now
25:19we could do that with list and tups also
25:22but sets are optimized for this okay and
25:25lastly something else really useful that
25:27sets can quickly do is determine what
25:30values they either share or don't share
25:32with other sets so for example let me
25:35create another set called Art courses
25:38and I'll create this pretty similar to
25:40CS courses here um but instead of
25:44physics and compi I'll instead say Art
25:47and Design and with both of these I'm
25:49going to go ahead and take out that
25:51extra uh math on the end just so it's a
25:54little bit more clear and instead of
25:55this being CS courses I'm going to call
25:57this set art courses so now we have two
26:00different sets here and some of them
26:02have different courses and some of the
26:04courses are the same so let's say that I
26:06wanted to see what courses these sets
26:09had in common so to do this we could use
26:11the intersection method so we can say CS
26:16courses.
26:19intersection and we will pass in the art
26:22courses into that so if we run that then
26:25we can see that it shows us that the
26:27history and math coures are in both of
26:29those sets and if I wanted to see what
26:31courses are in the Cs courses but not
26:35the art courses then I could use the
26:37difference method so instead of
26:39intersection I will instead say
26:41difference so if I run that then now we
26:43can see that it shows that the uh
26:46physics and compsite courses weren't in
26:48the art courses now if I wanted to
26:51combine both of these sets and print all
26:54of the courses offered then I could use
26:56the union method so so I'll say CS
26:59courses. Union and we want to Union with
27:02the art courses so if I run that then we
27:05can see that now we get all of the
27:06courses printed out from both sets so
27:09sets can definitely be useful for these
27:11kinds of use cases and for these
27:13particular problems they're much more
27:15performant than lists or tupal okay so
27:18we are basically finished up with this
27:19video but let me show you one last thing
27:21before we close this out and that's
27:23going to be how to create empty list
27:25tupes and sets so I have these over here
27:28and my Snippets so let me grab these
27:31real quick and copy and paste these over
27:35here and bring this down a line so I
27:37wanted to show this because there's a
27:39small gotcha when it comes to creating
27:41an empty set so to create an empty list
27:45we can either set it equal to empty
27:47square brackets or we can use this
27:49built-in list class and to create an
27:52empty tupal we can use these empty
27:54parentheses or this built-in tupal class
27:57now to create an empty set we actually
27:59can't use these empty curly braces um so
28:03this line right here is wrong that's not
28:05an empty set this is actually going to
28:08create an empty dictionary so to create
28:10an empty set the way to properly do this
28:13is to use the built-in set class with no
28:16values and speaking of dictionaries
28:18that's actually what we're going to
28:19cover in our next video but I hope
28:22everyone feels comfortable now with
28:23working with lists and tupal and sets
28:26but if anyone does have any questions
28:27about about what we covered in this
28:29video then feel free to ask in the
28:30comment section below and I'll do my
28:31best to answer those now if you enjoy
28:33these tutorials and would like to
28:34support them then there are several ways
28:36you can do that the easiest way is to
28:38Simply like the video and give it a
28:39thumbs up and also it's a huge help to
28:41share these videos with anyone who you
28:42think would find them useful and if you
28:44have the means you can contribute
28:45through patreon and there's a link to
28:47that page in the description section
28:48below be sure to subscribe for future
28:50videos and thank you all for watching

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