GEOG3150 - GIS, Geocomputation and Geoplanning - Semster 2

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• Lectures
  • Lecture 1
  • Lecture 2
  • Lecture 3
  • Lecture 4
  • Lecture 5
  • Lecture 6
  • Lecture 7
  • Lecture 8
  • Lecture 9
  • Lecture 10
  • Introduction to Semster 2
  • Big Data and Smart Cities
  • Revision Lecture
• Practicals
  • Practical 1 - NetLogo Basics
  • Practical 2 - First NetLogo Model
  • Practical 3 - Advanced Netlogo Model
  • Practical 4 - Festival (1)
  • Practical 5 - Festival (2)
  • Practical 6 - Festival (3)
• Seminars
  • Seminar 1 - GIS and Geocomputation
  • Seminar 2 - Ethics of Individual-Level Modelling
  • Seminar 3 - Modelling Societal Challenges
• Project
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  • Example NetLogo Models
  • Model Thinking
  • Lecture Capture
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Practical 3, part 2 - Grass Grows..

This part is very short - all you need to do is add a few lines so that the grass in the model re-grows after a certain amount of time. Importantly, we are going to create a slider so that the person running the model can experiment with different regrowth times.

1. To begin with, create a slider called grass-regrow-time (if you can't remember how to create sliders, have a look back at practical 2). Give the slider a range of 0 to 100 with a default value of your choice (I chose 80).


Recall that, in NetLogo, sliders represent variables. So, if you have created a slider called grass-regrow-time, NetLogo also makes a variable called grass-regrow-time that you can refer to in your model code. The variable will be part of the observer context, so all turtles and patches can see what value it holds. We are going to use this variable to determine how long it takes for grass to regrow once it has been eaten.

2. Add the following code to the go procedure, once the ask turtles block has finished (after the closing ']'): ask patches[
 if pcolor = brown [
  set time-since-eaten time-since-eaten + 1
  if time-since-eaten > grass-regrow-time [
   set pcolor green
  ]
 ]
]
(see the right image if you're not sure where the code should go).

It might look complicated at first, but the code above doesn't do anything that you haven't seen before. Its overall job is to see if enough time has passed for brown patches to become green again. The following table explains what each line does:

Code	Explanation
ask patches [	This tells NetLogo to run through the patches, one by one, so we can see if they are ready to re-grow.
if pcolor = brown [	This checks the colour of an individual patch. If the patch is brown then it might be time to regrow. If it is green, then we just ignore it.
set time-since-eaten time-since-eaten + 1	This increases the value of the time-since-eaten variable by one unit. This is how we keep track of the amount of time that has passed since the patch became brown (i.e. the grass was eaten).
if time-since-eaten > grass-regrow-time [	Here, we make use of the grass-regrow-time slider. If enough time has passed since the patch become brown, the we can do something to it.
set pcolor green	This simply changes the colour of the patch from brown to green. Importantly, the line above makes sure that this will only happen if enough time has passed. If enough time hasn't passed, then the patch will stay brown (for now).
]	This finishes the if time-since-eaten > grass-regrow-time [ block.
]	This finishes the if pcolor = brown [ block.
]	This finishes the ask patches [ block.

Understanding the logical flow of the program as it compares variables and decides what to do is a common task in computer programming and a very useful skill in general. If you're not sure about how the if commands work, have a look back at the lecture .

3. Finally, go back to the Interface tab and try to run the model. You should see grass be eaten and then re-grow as the turtles move around the environment.

Now you can move on to the next part where we will allow the turtles to give birth.