Adding variables to a basic NetLogo model

Objective: Students will learn how to improve the basic turtle NetLogo model to make it more realistic.

Materials: Each student will need

• A computer with NetLogo

Directions

Add and Energy Variable

1. Add a turtles-own declaration at the top of the Procedures tab:
   turtles-own [ energy ]

Questions to ask:

• Why is an energy variable necessary to having the turtles eat, reproduce, and die properly?
• When should turtles gain energy? When should they lose energy?

Make the Turtles Eat Grass

1. Add a procedure called "to eat-grass" to the Procedures:
   to eat-grass
     ask turtles [       if pcolor = green [
         set pcolor brown
         set energy (energy + 10)
       ]
     ]
   end
2. To make this happen when you click the go button, add this function to the "to go" procedure:
   to go
     move-turtles
     eat-grass
   end
3. Run your model.

Questions to ask:

• After seeing the model run, what happens when the turtles "eat-grass"?
  If the grass under them is green, it turns brown.
• What is the grass called in NetLogo models? Patches
• What do you think pcolor is? What does "set pcolor brown" do?
  Patch color. set pcolor brown makes the grass turn brown.
• When does the grass turn brown(Look at the if-statement)?
  When the grass color (pcolor) is green and a turtle is standing on it.

Make the Turtles Lose Energy and Die

1. Rewrite move-turtles so the turtles lose energy as they walk:
   to move-turtles
     ask turtles [
       right random 360
       forward 1
       set energy energy -1
     ]
   end
2. Add a procedure called check-death:
   to check-death
     ask turtles [
       if energy < = 0 [ die ]
    ]
   end
   
   And as always, for this procedure to happen, you must add it to the "go" procedure:
   to go
     move-turtles
     eat-grass
     check-death
   end
3. Run your model. Your turtles should eat the grass and eventually they should all disappear.

Questions to ask:

• Why did the turtles all die? What part of the code caused this?
• What should we add so all the turtles do not die?
  Make the grass grow back!
• How often should the grass grow back? What percentage of the time?

Make the Grass Grow

1. Add a procedure for regrow-grass:
   to regrow-grass
     ask patches [
       if random 100 < 3 [ set pcolor green ]
     ]
   end
2. Add this to the go procedure:
   to go
     move-turtles
     eat-grass
     check-death
     regrow-grass
   end

Questions to ask:

• What do you think "if random 100 < 3" means?
  Lead them to the idea that NetLogo uses a random number generator. random 100 < 3 means that each time this command is called a number between 1 and 100 will be generated. If 1,2, or 3 the generated number, then the grass grows.

Make the Turtles Reproduce

1. Make the turtles reproduce:
   to reproduce
     ask turtles [
       if energy > 50 [
         set energy (energy - 50)
         hatch 1 [ set energy 50 ]
       ]
     ]
   end
   
2. And add this to the go menu:
   to go
     move-turtles
     eat-grass
     reproduce
     check-death
     regrow-grass
   end

Questions to ask:

• What has to be true for a turtle to reproduce?
  Their energy must be greater than 50
• What happens when a turtle reproduces?
  It loses 50 energy. It produces one turtle.
• How much energy does a turtle have when it is first produced? How do you know that?
  50 energy