Now that we've seen a bit about how

Netlogo works, let's build our own

simple model. This model will consist of

one ant moving around randomly

and that's it.

So, let's start-up Netlogo.

I am going to click on this Netlogo 5.3.1,

you can click on whatever version

that you have.

Now the first thing I am going to do is

create the Setup and the Go buttons.

So, I go to the Interface Builder Menu,

When I click on it, you can see all the

different types of objects I can add to

the interface. So, we're going to

add a button. So, I click on the interface,

I can fill-in that this button is for Setup,

and I can get another button,

Add another button for Go

I can select this by right-clicking on it,

Click Select, I can move it, I can change

its size. If I drag it here, I can adjust

the buttons however I like. So, now we

have the Setup and Go buttons.

They are red because they don't have any

code associated with them yet.

There is nothing that they actually do yet

when they are clicked, so I am going to

have to create the code. Now, let's

create the code that go with the

Setup and Go buttons.

We do that by going to the code tab and

writing the procedures. So, the first one

we will do is Setup.

So, we say to Setup here is what we do

when the button is clicked,

we clear all, here clear all is a command

that clears the world, that is a place

you see ants committing any actions, it

clears that window and anything leftover

from the previous run. We are going to

reset ticks, that is, we are going to set

the ticks, or the time set to zero,

Now we are going to create an ant.

Every agent in Netlogo is called a turtle.

This is for historical reasons, coming

from the original language, Logo, that

Netlogo was based on, all the agents were

turtles. In Logo you just had one turtle.

Here you are eventually going to have

many turtles. For now, we are going to

say 'create turtles'. We are going to say

how many turtles we want. For now, we are

just going to say that we want one.

So, we are going to create one turtle and

then we are going to ask it to do some

things. We say ask turtle and we are

going to give it a list. We are going to

ask it to set its shape to bug; that is

the shape of an ant in Netlogo. We are

going to set its size to 3. That is a

good size in Netlogo for viewing it. We

are going to set its color to red.

Ok, I think that's all that we want to

ask it to do, so we put End. There is

this nice check mark here which checks the

code for bugs. So, click on that, and

it tells me there is a bug, and it tells

me where the bug was, and I notice that

I should have actually said ask "turtles"

I missed an "s" there. The reason is

that Netlogo assumes that there is a

population of turtles or agents and

whenever you ask to do something, you ask

the entire population. We only have one

turtle, but later we are going to ask all

of them to do this. Check this again.

It brings me back to the interface saying

that everything seems to work, so let's

click Setup, and we see our ant, right in

the middle. The Go button is still red

which means that we haven't written the

code for that. Let's write the Go

procedure. Let's go back to the code tab.

We write "to go". We write "ask turtles"

to do a few things. First, we ask them to

turn their bodies to the right 30 degrees,

then we are going to ask them to move

forward 4 steps, write "forward 4", then

we will increase the time steps by one,

that's what the tick command does,

we will reset the ticks to 0, and then

type "tick" which increases the

time step by one each time we go through

the procedure and then we end with "end".

Let's go to the interface, and hit "Go".

You can see that the turtle turned it's

body by 30 degrees and then moved forward

by 4 steps. We keep hitting the "Go"

button over and over again, and it goes

around in a circle. I would like to have

it go on forever, without me having to

continually hit the "Go"button

and I can do that by right-clicking on

"Go"and going to Edit, and we see this

option, forever, which I can click which

means that it will keep repeating "Go"

forever, so let's do that.

If I slow it down a little bit, you can

see the ant going around and around by

itself, and I click on the "Go"button to

stop it. I can click on "Go" to start it

again. If I want to start from scratch, I

click on Setup. It starts in the same

place, in the center, but it points in a

different direction each time. That's

because it puts it in the center, but it

points it in a random direction each time.

We can save the model so far, by going to

the File tab, and click on SaveAs. I am

going to save it as, Ant1.

You see it here on the desktop, Ant1.logo.

Now we can go to Netlogo menu,

quit Netlogo.

If I want to start this up, I just

double-click on this. Netlogo will

start-up and load this model.

There is only one thing wrong with our

model, and that is that it is boring. The

only thing that the ant does is go around

in a circle, so let's make it a little

more interesting. I can do that by going

to the code and adding one simple command,

Instead of going right 30 degrees, we will

add the word, "random", so now the ant

will choose a random number between 0 and

30 minus 1, and go to the right that

number of degrees. We can do the same

thing here, we can have the ant move

forward a random number of steps,

Let's go back to the interface, we click

on Setup and Go, and we can see it makes

the behavior a little more interesting.

You might notice the ant appears to go

through the wall over here and come out

the other side. The wall is wrapped

around. This makes it look a little

strange. I can right-click on the black

area, click on Edit. There is a lot of

information here about the setup of this

world. The number of patches, is the

grid, the world is divided-up into.

Uncheck them, apply, ok.

Let's see what happens then. Now, when

the ant hits the wall, it gets stuck

there, until it gets turned around.

Our ant still just turns to the right.

Let's write code that allows it to choose

at random whether it turns to the right or

to the left. It essentially flips a coin

to decide if it is going to turn to the

right or to the left. I am going to

implement the coin flip procedure.

Something called "to-report" "coin-flip",

"to-report" is different from a "to"procedure,

"to-report" returns a value, here it will

return true or false. We say

"report random 2 = 0"

"end"

what this does is "random 2"

"random" returns a random number between

0 and this number, 2, minus 1,

so this returns 0 half of the time and

1 half of the time, in a random way.

If this is zero, this statement returns

true if the statement = 0, else it returns

false. We can use the value of that

statement, by saying, "if else" coin-flip,

what that does is "if coin-flip is true"

do this first thing that I am going to

put in brackets, otherwise if it is false,

do the second thing that I am going to

put in brackets, "left random 30".

Everytime we run the Go procedure, it will

decide at random whether it will turn

right or left. So, let's see how that

works. Setup and Go.

You can see it is turning a different

direction, it is turning forward, right

and left. The only problem is that it

is getting stuck too often. We can make

that better by changing these 30's to 60's

Have a bigger turning radius, that is, it

turns more each time. It seems to be

getting stuck less often. Whenever you

write a program, it is always good to put

in comments. They are ignored by the

computer, but they are good for people

to read. In Netlogo, a comment is just

text following a semicolon. I can write

anything I want and the computer will

ignore it. Let's write, returns true or

false at random, which is what it does,

Here we can write another comment,

if coin-flip is true, turn right,

else turn left,

There you have little reminders, either

for ourselves, or for others reading the

program. Really helpful if you have a

more complicated program. Especially, if

you come back to it after a few days.

Comments are absolutely essential, even if

no one else is going to read your program.

Now we are going to save our model.

First, I am going to label it.

That is an aesthetic thing, I am going to

the Interface menu and choose Note, I am

going to put up here a little bit of text

that says the name of the model, Ant1.

Let's make the text a little bigger, 18,

and then I can select this and move them

down a little bit, center them, do

whatever I want to make them prettier,

click Save, and the model is saved.

I just wanted to mention that the purpose

of teaching Netlogo is to give you a sense

of how it all works. We are going to be

using a lot of Netlogo models in this

course. We're not going to be writing

many. I wanted to give you a sense of

how it all works. You could be doing some

modifications of what we wrote. If you

are more advanced, you can try and write

some of your own. As an assignment, I

would like you to implement that exact

same model, as I did, Ant1, using the

same commands, and get it to work.

That is going to be ungraded, but it is to

your benefit. Don't hesitate to ask

questions on the student forum when

they come up. That's why they're there.