Ok, the reproduction procedure

is going to ask the turtles

to create copies of themselves

going to use the hatch procedure

ask each turtle to hatch 2 turtles

by default those turtles will have

all the attributes of their parents

and to keep track of generations

that involves giving the turtles variables

I am going to call one of those variables

a generation.

So, now when we create a turtle, it will

automatically have a variable

called generation.

These little hatchlings,

that arrive on the scene,

they are going to automatically have that

variable too.

They are the next generation, so we want

them to set their generation to

Here we have to use a generation

myself plus one.

I think this might have to be

in square brackets.

Basically, we're setting

the commands apply to the newly arrived

hatchlings. Setting myself plus one.

Basically, just incrementing that.

I don't know if these square brackets

need to be there.

I could look at quick help for myself.

Yeah, sure enough.

It does have to be in square brackets.

This explains the difference

between self and myself.

We're going to hatch two and set them.

Just so we can see them.

Set their color to red.

Let's try going back here.

I am going to comment this out for now.

So we can test the code we've got so far.

Now we set up, and we step.

It looks to me like, every turtle has

Let's do a smaller population, just so

we can see. Let's start with 5 turtles.

We don't, ..., oh, I see, yeah,

I am guessing that these inherited

the heading of the parent too.

Sometimes it is confusing

when you are doing these kinds of models

because you don't know how many turtles

you have. You could do a simple thing,

show count of turtles.

It looks to me like there is just

10 turtles or so, but when we look, indeed

there are 15, there is overlap.

Another way you can do this is,

click on that,

turtles 11 and 12 are coincident.

The heading from their parent is causing

overlap. We don't need to worry about

that for now. So, reproduction works.

We might want to see if the generation

business works. Take a look back here at

the interface. This, if we inspect

this turtle, we should see that it has a

generation variable.

I don't see it.

Let's see if this one does.

Well, well, it's not there.

Something has gone awry.

Let's comment this out again.

Run setup.

Inspect this turtle.

Sure enough, we have the generation.

This is generation 0.

Turtle 3 is of generation 0.

When we step, with any luck,

it will be of generation 1.

So we are incrementing generation.

Our generation feature is working.

Ok, we're going to ask the parents to die.

It occurs to me we are going to have to

keep track of what generation we're on.

That is a global variable.

Every time we step we are going to

increment the current generation.

This will allow us to keep track of who

should die when.

Start out at zero, add one to it.

Make the parents die.

Ask turtles with generation less than

or equal to current generation

Create to partial agent sets.

Want to talk only with the turtles whose

generation is less than, maybe I just

mean less than, to die.

Let's go back here.

Setup, step, looks like, let's instead of

setting to red, set to a random number,

up to 100. That will mean they are

different colors. Just temporarily, so

we can see them.

We're seeing the different generations.

You can see the colors are changing.

The different generations are coming,

we can inspect these turtles,

we're only at generation 12,

but we've hatched over 20,000 turtles,

that's how exponential population growth

works, right?

We've got the parents dying.

We need some kind of constraint.

Something that makes it difficult,

so we don't have unlimited growth.

One easy way to do that

is to create a situation where

individuals die depending on how crowded

This is an idea from evolutionary biology,

individuals compete with each other,

and also compete with the environment

in some way.

This is a case where they are kind of

competing with each other for space, and

kind of the environment too.