Introduction to Complexity

Homework 1 Advanced Option

This is a tutorial that will walk through

the advanced option of the homework.

Please open up the Netlogo model, labeled

multipleants.nlogo, which is located in

the course materials section of the

web site. When it opens the interface

should look like this. We've done a

couple of modifications already in the

first two sections of the homework and we

haven't eliminated that. So, the first

thing that I would like to do is re-color

the ants so they always stay red.

I am just going to eliminate these two

lines of code. That will make it less

confusing. Let's go back to the interface

Now the homework asks us to

create a pheromone trail that the ants

leave behind that the other ants can

follow once they find food. To test that

to see if it's any faster than the present

method, let's do that, I'll go to the code

section. First, let's create a patch

variable, call it pheromone, do that here,

Next, scroll down to the setup,

we need to setup the patches to not only

have a nest, but to also have pheromone.

I am going to change this setup nest,

just to setup patches, we'll ask patches

to initialize the pheromone to be zero,

so set pheromone 0, and let's check that,

Next we would like the ants to leave a

pheromone trail behind them after they

eat the food and make their way back to

the nest. And then we would like other

ants to follow this pheromone trail.

To do this I will scroll down to the

return to nest procedure and we will add

a line to set the pheromone level of the

patch. So set pheromone to be the present

level of pheromone + 1. To visualize the

amount of pheromone level on each patch

I am going to set the patch label, so

plabel to simply be numeric value of its

pheromone. You could also use a different

color for this if you want but I think it

is more interesting to use the label.

So, let's check that.

Let's see if it works.

It looks like the ants are gathering food

returning it to the nest and they are

laying down a trail of pheromone as they

do so which increases numerically.

Now we would like the pheromone to

evaporate over time. We go back to the

code to do that next. Let's create a line

in the go procedure, we'll call it,

evaporate pheromone. Let's write that

procedure, to evaporate pheromone,

we would like the pheromone to have a

probability of evaporating, which means a

number between 0 and 1. Let's setup a

temporary variable, call it x, which will

be a random decimal between 0 and 1, so

random float 1, and then we will ask

patches with pheromone greater than zero,

so any patch that has pheromone,

if the x variable is less than the

probability that we decide, which will be

a slider we setup on the interface, and

we will call that slider,

probability to evaporate, then we will set

the pheromone to be its current value -1,

We would like to see that, so we set the

plabel is the new number, and we need to

add that slider that I mentioned, to do

that, we go to the Interface, to this

button, choose Slider, and we will put it

over here for a second, and we will call

it, probability to evaporate, and it will

be a number between zero and one, and we

will increment by .01, and

let's set it initially to be 0.1.

I lost an "e" there. Press OK.

I would like to move this slider to be

under this other slider,

so I am going to move this plot window

down and line it up with the worldview

which always looks nice, and then move

this slider over. Let's make it the same

width as the other sliders. We may be ok.

I am going to check.

Go back to the Interface and let's run it.

These pheromone levels should be

decreasing, and it looks like they are,

so that's good, Now the final problem is

to have the other ants follow the

pheromone trail, so let's do that next,

By the way, the way to select different

windows on the Interface is to click an

open space with the cursor and then drag

over the button or the slider or whatever,

you can also right-click after you select

the button, and then edit it in different

ways, now let's go to the code and we will

change the way the ants move about their

world. We will go to, to look for food,

and we're going to add some code here,

We're going to setup some new variables

under, to look for food, we will say,

let pheromone ahead be sent at angle 0,

and let pheromone to the right be sent

at angle 45, and let pheromone to the left

be sent at angle -45. This code is very

similar to the ant model in the NetLogo

models library, so we are basically asking

each ant to look ahead and a little bit to

the right and a little bit to the left, to

determine if there is any pheromone there,

and if there is to follow the pheromone,

and if there isn't, to randomly walk,

which will keep this code the same down

below. Next we will write a series of

if-else statements; if else open

parentheses, if pheromone right is greater

than pheromone ahead, or if pheromone left

is greater than pheromone ahead, close

parentheses, if else pheromone right is

greater than pheromone left, we will go

right 45, and if not, we will go left 45,

and if neither of those is true then there

is either pheromone ahead or there is no

pheromone at all. So, let's say, if

pheromone ahead equals 0, then we will go

ahead with the rest of this. We will need

an opening bracket and a closing bracket.

Let's check that. It is telling us that

there is nothing named sent-at-angle,

that's true, we haven't setup the

sent-at-angle yet, let's scroll all the

way to the end of the code.

We will put in a reporter.

to report sent-at-angle, and then we will

input angle, let p be a patch right and

ahead. These are primitives built into

Netlogo. To check what a primitive means,

you can highlight it and right-click it,

and go to Quick Help, and this gives a

synopsis on what that primitive is.

So, right and ahead. We will be using

the angles we already inputted and the

distance ahead which is also 1.

If p equals nobody, so there is no

pheromone, so report zero, and otherwise

report what the pheromone is, report

pheromone of p, and that should do it.

Let's check the code now.

No closing bracket for this open bracket.

So we have an error message. Looks like I

forgot to put a closing bracket over here.

I also notice that there are a couple

brackets missing for these lines.

I am going to add that. Sorry about that.

Now we will check it.

Looks like everything is ok.

Let's go back to the Interface.

It looks like the behavior is pretty good.

We could analyze this a little more, and

make some better statements about how it

is working, but I will leave that to you.

I do notice that a lot of ants are

collecting around the edges of the world

view. We can fix this by going to

settings and click on

"world wraps horizontally" and

"world wraps vertically" and click on OK.

Now, it looks like the ants are

very quick and eating all the food,

about 73 ticks, and in the earlier model,

before the pheromone was added,

we had 128 ticks.

So, that's it for homework #1 tutorial,

I hope you enjoyed it.

I look forward to talking to you next time