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All right guys. In this lesson, we're going to be building out our turtle race.

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And through building this race,

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we're going to get more familiar with these concepts of state and instances.

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And that's because we need to create multiple turtles to join our race.

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This is what we're aiming for. In the beginning,

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there's going to be a little popup that will ask us to bet who will win the

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race. And it asked me to choose a color.

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Now the turtles are in the color of the rainbow,

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so I'm just going to pick the red one. Once I hit okay,

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we have all of our turtles lined up in the starting position and they will start

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making random steps towards the right edge of the screen. Now,

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the first hurdle that reaches past the right edge of the screen is going to be

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the winner.

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And at the moment it looks like the green turtle is probably going to get there

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first. There we go. So as soon as a turtle crosses the finish line,

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then it will print out a line telling us whether if we won or whether

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if we lost our bets and which turtle won the game.

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So this is the goal.

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Now I'm going to get started by deleting all of the code inside our previous

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Etch-A-Sketch project other than these few lines.

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So I've got Tim my turtle, screen

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which is a screen object that is going to hold the screen until I click on it so

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that I can see what's going on. Now, because in this game

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the dimensions of the screen or of the window is really crucial,

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I'm not just going to get the default screen size show up. Instead

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I'm actually going to use one of the methods that is in the screen object

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which is called setup. And setup, as you can see,

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allows me to set up the width and the height of this window

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that's going to show up.

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So I'm going to set the width of my screen to 500 and the height to 400.

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So I should end up with a 500 pixel wide, 400 pixel high screen.

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And once I hit run, you can see this is what it gets resized to.

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Now, if you came across this code and it was somebody else's code,

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then these numbers and their positions don't actually make a lot of sense,

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which one's the height, which one's the width. So in these situations,

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I recommend using keyword arguments rather than positional arguments. Oh,

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let's write out the parameters explicitly. Our width

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to be 500 and our height to be 400. This way

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when somebody else comes along and reads your code,

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it's a lot easier to understand rather than just 500 and 400,

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cause it could be both ways.

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So now that we've got our screen set up,

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the next thing I want to do is to bring up that popup and ask the user to make a

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bet. Now, if we take a look at the Python documentation,

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um, where we've been spending a lot of time recently,

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you can see that there is a method on the screen

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which is called textinput.

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And this is going to show up a popup window and it allow the user to look at the

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prompt and the title and to enter a piece of text.

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And if you want the user to enter a number, then you would use numinput.

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So let's go ahead and add that here. So it will be a method from screen

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which is called textinput and then it's got two parameters.

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One is the title which in my case

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I'm just going to say 'Make your bet'.

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And then the second parameter is going to be the prompt. And in my case,

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I will probably say, um,

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which turtle win the race and tell them to enter a color.

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Now, this works really similarly to our input

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which we've been really used to because it will return the string and we can

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catch that string inside a variable.

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So we'll call it user_bet and we'll set it to equal the output from this

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method.

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Now let's give this a shot.

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Let's run the code and then let's enter a color, hit okay.

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And if we take a look in our console,

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you can see that that bet string is printed in here.

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So now the next thing we're going to do is we need to do something with our

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turtle. I'm going to move it down a little bit.

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And what I want my turtle to do is I want it to go to the start of the line.

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So the very left edge of my screen. Now I could,

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of course, just say tim.backward by, um,

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however many paces,

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but thinking forward and knowing that I'm going to have six or seven of these

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turtles,

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I can't just tell them all to go backwards because they're going to go to the

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same position. Instead, we're going to use a method that the turtle has

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which is called goto.

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And this allows us to define an X value and a Y value.

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But how do we know what X and Y values to give it? Well,

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we first have to understand how the Python turtle coordinate system works.

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So if you imagine your program window as a graph where the center of it is at

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the coordinate (0, 0). Well

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then if you have a window that has a height of 400,

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then that graph will have a Y-axis that extends from the center at zero all the

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way up to the top edge and then from the middle

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it goes down to the very bottom edge, which is going to be -200.

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So 200 plus 200 makes up 400 and the same happens with the X-axis.

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So if the width of the screen is 500, then

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the X-axis goes from the center 0 to positive 250,

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so half of 500. And then from zero to -250.

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So here's a question. Let's say we wanted to move our turtle here.

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What do you think would be the X and Y coordinate of that point?

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Okay. So let's say

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that this is roughly halfway on the X-axis and about halfway on the Y-axis.

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Then half of 250 is 125 and half of 200 is 100.

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So at this point would have the coordinate of 125 by 100.

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If we wanted our turtle to go all the way to the left side of the screen

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say maybe starting over here or starting over here, well,

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then we would have to use that

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goto method and then specify a X-value and a Y-value. Following the logic of the

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total coordinate system then in order to move our turtle to the very left edge,

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then we should supply an X-value of minus 250

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which is half of the width of 500.

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Now the Y-value will determine where on the Y-axis our turtle moves to.

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So if we had it at zero

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then it would basically just go straight backwards and it would not go up or

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down. But let's say that we put it at, um,

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-100. So now if I run this code,

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you can see that it goes in the right direction,

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but it's actually gone off the screen.

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So what's happened here is that -250 is on the very,

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very edge of the window.

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And once our turtle moves over there and the arrow is at the back,

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then you can't actually see it at all.

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So let's try changing that X-value and shifting our turtle a little bit more to

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the right. Then you can see it moves to pretty much the start of the window.

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So I recommend using this goto method and trying out some different numbers

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just to see where it ends up on the screen.

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And once you put in some different numbers in here,

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you'll start to get the hang of how this coordinate system actually works.

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But just remember that the X-axis is along the horizontal.

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It goes from zero to positive and zero to negative.

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And then the Y-axis is along the vertical,

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going from zero to positive and zero to negative.

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So we want to get rid of this line and we don't actually want our turtle to draw

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at all. So to do that, we're going to do tim.penup,

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and we're actually never going to put the pen down because we're going to be

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moving the turtle itself.

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And the other thing is that it would be really nice if instead of an arrow,

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we actually got a turtle.

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So we can of course say tim.shape and set it to a turtle shape,

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but here's an even easier way. When we create a new turtle,

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look at the prompt that it's giving us.

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It's actually giving us a way to initialize a new turtle object already with a

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shape set up.

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Now this shape is set to have a default value

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which is basically the arrow.

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But we can also specify the shape and give it the turtle shape to begin with.

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Now, when I run the code you can see that firstly,

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I get a turtle shape and then the pen is up and I'm not drawing

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and then it moves to this place on the graph; -230, -100.

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So now the next thing we want to do is to be able to create lots of turtles,

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right?

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What if we had all of the colors in the rainbow and we create a turtle for each

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color?

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So there's six colors in this list and these of course,

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correspond to the colors which turtle will recognize.

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So here's a challenge for you.

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I want you to create six turtles,

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one for each of the colors in this list of colors.

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You're aiming for the turtles to all go to the starting line in a distribution

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that looks something like this. It doesn't have to be precise,

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you don't have to get it perfectly right.

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But just make sure that they're sort of evenly spaced out and they are at the

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starting line. So this is what you're aiming for; six turtles, six colors,

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all starting at the starting point along a different point on the Y axis.

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Pause the video and give that a go.

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So we know that in order to create lots of turtles,

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we're going to need some sort of a loop.

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So let's go ahead and use a for loop to say turtle_index

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and we're going to create a range to specify how many turtles we need.

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Our range function will go from 0 to 6.

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And remember that the range function actually doesn't include the number six,

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so it'll create a range from 0 to 5. And once we've done that,

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then we can indent this block of code so that we create six turtles.

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But at the moment, they're all going to the same position.

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So how can we change this? Well,

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we want the X-position to always be the same for all the turtles,

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because the X-position is along the horizontal axis

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and we want all the turtles to start at the starting point.

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Nobody gets a headstart. But the Y-position is the thing that we want to change.

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A really simple way of doing this is to simply create a list of Y-positions

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and you can work out some sort of reasonable position that you want to take.

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So let's say we start out at -70 and then we just increased by 30 each

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time.

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So then that becomes -40 and then -10 and then 20,

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50 and 80.

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So it's roughly distributed somewhere along the middle. Now,

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instead of using this Y-position

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which is hardcoded -100,

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we're going to take the Y-positions and then pass in our turtle_index like

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this. So now when we rerun this code,

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you can see that for each of the turtles

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they're all going to get an individual Y-position

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and they're all separated by 30 in distance.

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Now we've got our turtles neatly lined up at the starting point.

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The next thing is to give them a different color.

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So we're going to use the list of colors and then picking out the color using

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the turtle_index.

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So now we should get some multicolored turtles and it will be much easier to bet

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on a turtle. So now that we have our turtle race

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all set up and ready to go,

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the next step is to actually get the turtles to start moving.

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But before we can do that,

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I really want you to get a good grasp of how the coordinate system works in

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turtle. So in the next lesson,

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I've got a quick quiz for you just to make sure that you really understand

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what's going on here.