Python for Absolute Beginners Transcripts
Chapter: Problem solving techniques for writing software
Lecture: Demo: Find the winner, naive version

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0:00 Our tic-tac-toe is almost done
0:02 we just have to figure out how to find a winner.
0:05 Again, this comes back to our data structure
0:07 which is really, really helpful in putting this together.
0:09 So let's sort of sketch this out here.
0:12 We're going to do win by rows
0:16 win by columns, and win by diagonals.
0:22 Now, what I'm going to do
0:23 is I'm going to write this in the most straight forward
0:25 immediate, obvious way I can think of it
0:27 solving these three problems.
0:29 But what we're going to find out, is
0:31 that this is actually not the best way to do it.
0:34 There's sort of a couple of things
0:37 we're going to have to do that will make this a lot better.
0:39 So let's first write out what the rows are.
0:41 Well, the rows, we want this as a list
0:44 or a sequence we can go through
0:46 that's exactly what the board is
0:48 though the board is going to be the rows
0:50 like row 1, row 2, row 3
0:52 that's what the board holds.
0:54 The cells, or the columns, rather
0:57 this is going to take some work.
0:58 So what we need to do
1:00 let's just go through them as numbers.
1:01 It's going to be easier that way.
1:02 So if we say for col
1:06 how do we go through just numbers?
1:07 We could use a while loop
1:08 but there's a cool thing we can do called range.
1:11 And if we go here
1:12 we can say start = 0
1:16 and then not sure it's actually
1:19 going to not stop at the right spot
1:21 so we can go click on this
1:22 and get the quick documentation.
1:24 So it returns an object that produces
1:25 a sequence of integers
1:27 from start inclusive to stop exclusive.
1:29 So that's a little annoying
1:31 you got to remember to put a 3 here
1:33 so it goes 0, 1, 2.
1:35 Do we call them 0, column 1 and column 2.
1:38 And then what is one of the columns
1:42 let's call index, so we're going to create a column
1:47 it's going to be a list
1:48 and what goes into this list?
1:50 Just like each row is a list
1:51 we want to list here.
1:53 So the way we're going to get it
1:56 is we're going to go row 0, row 1, row 2
1:59 so like this, this is close
2:03 not quite done, but two
2:05 and then for highest middle, bottom
2:08 and then here we need to know which column are we in.
2:10 Are we in the first column
2:11 which means it's always like 0
2:14 no column index here
2:16 and here, and here.
2:17 Is it always the first one in row 1, 2, and 3?
2:20 Is it the second one in row, 2, or 3, and so on?
2:24 Okay, so that's going to do that.
2:25 And then the diagonals
2:27 the diagonals are a little bit funky as well.
2:31 This is going to be an empty list
2:32 and there's just going to be, well
2:33 actually, let's not make it empty
2:34 we'll just do it like this.
2:35 This is going to be board of 0, 0
2:40 with a forward that's called a forward diagonal.
2:43 And then 1, 1, and 2, 2.
2:46 Well, that one's easy
2:48 but now we got to switch it.
2:49 So let's go from the front row
2:53 it's going to be the end coming backwards
2:56 and then here like this.
2:58 Yeah, that should do it.
3:00 For all of these
3:01 we're going to have to go through
3:03 and then loop over them
3:04 and see if there's a winner.
3:06 Now, how do we know there's a winner?
3:07 There's a winner if it's the same symbol
3:10 across the board, right.
3:12 If there's a row of X, X, X
3:15 or if there's a column of O, O, O
3:17 or there's a diagonal of O, O, O, or X, X, X.
3:20 Alright, so that's going to be a little bit tricky here.
3:23 And what we need to do is loop over all of these ideas.
3:27 So just do it here for the win by row first.
3:32 Let's say we're looking for four each
3:36 or row in rows
3:38 what we need to know is what symbol is in the first place
3:42 if there's a symbol in the first place at all.
3:45 So we'll say symbol one, I guess
3:48 we'll call this, I'll say, row of 0.
3:50 It's going to be the first cell in there.
3:52 Now, there's a couple of things we can do
3:54 we can say, if not symbol
3:57 right, so that means there's nothing here, right
3:59 we don't want to win because it's None, None, None
4:02 'cause then we'll just say continue.
4:04 I'm going to make this nicer in a second
4:05 but if it's all that
4:06 then we can say four cell in row
4:11 we're going to go across
4:12 we'll say, if not cell
4:16 cell not equal to
4:18 see how you're saying not equal to, symbol one.
4:21 So if there's some other symbol
4:22 including potentially none, then we're going to continue.
4:27 And here we're going to say return True.
4:30 Now, that is a mess.
4:31 Let's go down to the bottom
4:32 and say, yeah, it still says returns False.
4:35 We can do a lot better
4:36 and I'll show you here in just a second.
4:37 But this is the most naive way of writing this.
4:40 Go through each row
4:41 and we're going to see if it's a winner.
4:43 Now, the first symbol
4:45 got to make sure it's something, not nothing
4:47 and that's what this test is
4:49 and then we go through every cell
4:50 and just make sure that they're all the same.
4:52 If there's any that are not the same
4:54 one was an X, another is a None
4:56 then we don't carry on down here
4:58 and we keep going.
5:00 Whoo, okay, let's run it for the moment
5:02 and just see that we can win across the board.
5:05 So I'm going to play one-one
5:06 nobody's won yet, the game's not
5:08 oh, Michael has won the game.
5:11 Actually that would be wrong.
5:14 All right, rather than figuring this out
5:15 let me just rewrite this in the way
5:16 that I wanted to write it in the first place.
5:18 So what we're going to do over here is we're going to replace
5:21 all of this stuff here with something better.
5:23 I'm going to say if symbol
5:25 that means it's not empty
5:28 and we're going to do one other test.
5:31 There's a really cool way that we can write it
5:32 we haven't talked a lot about it
5:34 but I think you'll dig it.
5:35 So what we can do is we can use this all function
5:37 and we can call all
5:39 and what it's going to do
5:40 is it's going to check that something is True
5:44 for every item in a collection.
5:46 And then see where it says arable here?
5:48 And by them we can write little tiny expressions.
5:52 This one will be called a generator expression
5:55 and what we can do is we can just say something like
5:58 the value for some collection.
6:01 So what we want to do is we want to say
6:04 symbol is equal to cell for symbol
6:09 I guess it's cell in row, like that.
6:12 Let's go over symbol one.
6:13 There we go.
6:14 So here what we can say is
6:15 if this is not nothing
6:18 and for every one in the row
6:20 every cell in the row
6:22 it's equal to the first one
6:24 and we're going to return true.
6:27 Otherwise we're going to return, you know
6:29 we're never going to hit this
6:30 it's going to go down and check the others
6:31 and then check the others
6:32 and then eventually return False.
6:34 So if I've got everything right this time
6:37 I should have this working.
6:38 Alright, 1, 1, that didn't work
6:40 let's put 1, 2 for the computer
6:44 and I'll put 1, 3 for me.
6:47 Let's have the computer take the second row
6:49 so 2, 1, I'll do 3, 1
6:53 2, 2, we're getting close
6:56 I'm going to play 3, 2.
7:00 Now, the computer is going to play second row, third column
7:04 it should win right here.
7:05 Boom, process is finished
7:07 the game over, computer has won with this board right there.
7:11 Perfect, so now we got this check working.
7:14 But I hope, hopefully you can tell
7:16 this is starting to get kind of gross over here.
7:19 So we did this for the rows
7:23 I haven't checked the columns yet, have we?
7:25 Oh yeah, we haven't checked the diagonals yet, either.
7:27 So while this works
7:29 I'm going to just sort of pause this for a minute
7:31 and we're going to come back and say, well, I guess
7:33 I could put this down here for you, couldn't I?
7:36 So let's just come down here and say columns.append
7:39 and I'm going to pass in a column
7:40 so this is going to be a list that has all the columns
7:43 and then, again, I guess the diagonals
7:46 they already got that.
7:47 So let's go down here
7:48 and we're going to do this check again
7:50 but it's going to be col in columns
7:53 and start to feel this is not great.
7:56 Col zero, column, and guess what?
7:59 I'm going to do the same for the diagonal.
8:05 Call it diag, I don't know what you want to call it.
8:12 Now, whenever you have code that looks like this
8:16 there's this bit, and then there's this bit
8:19 and there's bit, and it's the same, right
8:21 it's exactly the same
8:23 it's just a matter of what goes in here
8:25 what goes in here
8:26 the variable doesn't really matter.
8:28 It's clear that we should be doing something different.
8:30 But let's just show that this is going to work.
8:34 Oh, we didn't quite build our diagonals right.
8:36 This has to be that list
8:38 and this has to be that list, okay.
8:42 Alright, so now we should be able to win diagonal.
8:45 I guess I'll win diagonal this time.
8:47 Oops, that's not going to work, is it?
8:49 And play two-two, computer can play 1, 3
8:54 alright, I'll play 3, 3
8:55 this should win the game.
8:56 Boom, game over, make that a little more obvious.
8:59 But that's good, and then let's have
9:01 the computer win by column.
9:04 What am I going to play
9:05 play that one, the computer can play 2, 2
9:08 and then, let's see
9:10 one-three, the computer will play 3, 2
9:13 and it should win. It does, allright
9:15 so we've got winning the game working
9:18 but here, this is a lot of a mess
9:20 and this duplication, and not a little bit
9:23 this is complicated duplication
9:25 this is not good
9:26 so we're going to come back and fix this.
9:28 But here's our naive version of winning the game.
9:31 Technically, tic-tac-toe is over
9:33 but we can do better.