;global variables
globals
[
  percent-similar  ;; on the average, what percent of a household's neighbors
                   ;; are the same shape as that household?
  percent-unhappy  ;; what percent of the household are unhappy?
]


;turtle variables
turtles-own
[
  happy?           ;; for each turtle (representing a household), indicates whether at least %-similar-wanted percent of
                   ;; that households' neighbors are the same shape as the household
  same-nearby      ;; how many neighboring patches have a household with my shape?
  different-nearby ;; how many have a household of another shape?
  total-nearby     ;; sum of previous two variables
]



to setup

  clear-all

  if number > count patches  ;check we're not try to create more households than there is space for!
  [
    user-message (word "This space only has room for " count patches " turtles.")
    stop
  ]

  ;; create households on random patches
  ask n-of number patches
  [
    sprout 1 [ set shape "circle" set color [117 112 179] ] ]

  ;; turn half the households to stars
  ask n-of (number / 2) turtles
    [ set shape "star" ]

  ;update-happiness

  reset-ticks

end

to go

  update-happiness                     ;execute the update-happiness procedure

  if all? turtles [happy?] [ stop ]    ;stop simulation if everyone is happy

  move-unhappy-households              ;execute the move-unhappy-households procedure

  tick                                 ;increase ticks by 1 (needed for the plots)

end

to move-unhappy-households

  ask turtles with [ not happy? ]      ;if household is not happy
      [ find-new-location ]            ;excute the find-new-location procedure

end

to find-new-location

  ;random walk
  rt random-float 360              ;; turn right a random number of degrees from 0 to 359
  fd random-float 10               ;; move forward a distance of 10 units

  ;assess current location
  if any? other turtles-here       ;; if this location is already occupied
    [ find-new-location ]          ;; repeat this procedure until we find an unoccupied patch

  ;move to the location identified
  move-to patch-here               ;; move to center of patch

end

to update-happiness

  update-households                ;execute the update-households procedure
  update-globals                   ;execute the update-globals procedure

end

to update-households

  ask turtles
  [
    ;; in next two lines, we use "neighbors" to test the eight patches
    ;; surrounding the current patch
    set same-nearby count (turtles-on neighbors) with [shape = [shape] of myself]
    set different-nearby count (turtles-on neighbors) with [shape != [shape] of myself]

    set total-nearby same-nearby + different-nearby  ;set the total-nearby variable to the sum of the same-nearby and different-nearby

    ;calculate if the household is happy
    set happy? same-nearby >= ( %-similar-wanted * total-nearby / 100 )

    ;change the colour of the household depending on whether they are happy or not
    ifelse(happy?)
    [ set color [27 158 119] ]
    [ set color [217 95 2] ]
  ]

end

to update-globals

  let same-neighbors sum [same-nearby] of turtles                                ;for ALL HOUSEHOLDS add up the number of same neighbours
  let total-neighbors sum [total-nearby] of turtles                              ;for ALL HOUSEHOLDS add up the total number of neighbours
  set percent-similar (same-neighbors / total-neighbors) * 100                   ;using last two counts, calculate proportion of same neighbours that are similar
  set percent-unhappy (count turtles with [not happy?]) / (count turtles) * 100  ;use last calculation to calculate % of households unhappy

end
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@#$#@#$#@
## WHAT IS IT?

This model represents the behaviour of two types of 'household' in a hypothetical city space based on their 'happiness' with the composition of their local neighbourhood.

The star households and circle households get along with one another. But each household wants to make sure that it lives near some of "its own." That is, each star household wants to live near at least some star households, and each circle household wants to live near at least some circle households. Households determine if they are happy as in the _Happiness in Schelling's World_ model (Millington 2021). 

This model extends the the _Happiness in Schelling's World_ model (Millington 2021) by enabling households to move if they are unhappy with the composition of their local neighbourhood. It does this by moving unhappy households to randomly selected locations in the city, where the household again assess if they are happy or not. By repating (iterating) this process over and over, the simulation of the model shows how individual preferences ripple through the city, leading to large-scale patterns (of segregation).

The idea was inspired by Thomas Schelling's writings about segregation, such as housing patterns in cities (Schelling 1978, Rauch 2002). The code and project was developed from the _NetLogo Segregation_ model (Wilesky 1997) and builds on the _Happiness in Schelling's World_ model (Millington 2021). Colours have been selected from [ColorBrewer](https://colorbrewer2.org) to be colourblind safe.

## HOW TO USE IT

Click the SETUP button to set up the households. There are equal numbers of star and circle households. 

Click GO to start the simulation. The households move around until there is at most one household on a patch. If households don't have enough same-shape neighbours, they move to a random nearby patch. This repeats until all households are happy (or continues forever!)

Click GO ONCE to simulate the movement of households once (i.e. one iteration of the go procedure). 

The NUMBER slider controls the total number of households. (It takes effect the next time you click SETUP.)  

The %-SIMILAR-WANTED slider controls the percentage of same-shape household that each household wants among its neighbours. For example, if the slider is set at 30, each star household wants at least 30% of its neighbours to be star households.

The % SIMILAR monitor shows the average percentage of same-colour neighbours for each household. It starts at about 50%, since each household starts (on average) with an equal number of star and circle households as neighbors. 

The % UNHAPPY monitor shows the percent of households that have fewer same-shape neighbours than they want (and thus want to move). Both monitors are also plotted through time.

The number of times move-unhappy-households procedure has been repeated is shown as the number of iterations (above the model grid, top centre)

## THINGS TO NOTICE

Households turn a green colour when they are happy, and a red (orange) colour when they are unhappy. When you click SETUP, the star and circle households are randomly distributed throughout the space. But many households are "unhappy" since they don't have enough same-shape neighbours. 

When you click GO ONCE, the unhappy households move to random new locations in the city. They may be happy or unhappy with their new location (i.e. households do not seek locations where they will know they will be happy, but rather move randomly). Regardless of their own happiness in the new locations, households might tip the balance of the nehighbourhood population they have moved to, prompting other households to leave! If a few star households move into an area, the local circle households might leave. But when the cirle households move to a new area, they might prompt star households to leave that area. See if you can observe this happening (although it can be quite difficult in this large model). 

When you click GO, the process above is repeated (iterated) until all households are happy. Notice that sometimes this continues forever! Why is this?

## THINGS TO TRY

1. For 2000 households, if each household wants at least 10% same-shape neighbours, what percentage (on average) do they end up with? How many iterations (repeats) does it take for all households to be happy? 
 
2. Repeat 2. for 33%, 50%, 66% and 90%. How does the overall degree of segregation change? Working through values systematically like this, recording your results, is a scientific approach. 

3. Repeat 1. and 2. for different numbers of households in the space. Does this influence results? Why or why not?

## REFERENCES

Millington, J.D.A. (2021) Happiness in Schelling's World. NetLogo model. http://www.landscapemodelling.net/models/schelling

Rauch, J. (2002). Seeing Around Corners; The Atlantic Monthly; April 2002;Volume 289, No. 4; 35-48. http://www.theatlantic.com/issues/2002/04/rauch.htm

Schelling, T. (1978). Micromotives and Macrobehavior. New York: Norton.

Wilensky, U. (1997).  NetLogo Segregation model.  http://ccl.northwestern.edu/netlogo/models/Segregation.  Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

## COPYRIGHT AND LICENSE

The original work by Willensky (1997) was licensed under the [Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License](http://creativecommons.org/licenses/by-nc-sa/3.0/).

![CC BY-NC-SA 3.0](http://i.creativecommons.org/l/by-nc-sa/3.0/88x31.png)

This work is licensed by [James D.A. Millington](http://www.landscapemodelling.net) under the [Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-nc-sa/4.0/). 
 
![CC BY-NC-SA 4.0](https://i.creativecommons.org/l/by-nc-sa/4.0/88x31.png)

This work should be cited as:
Millington, J.D.A. (2021) Schelling's Segregation Shapes. NetLogo model. http://www.landscapemodelling.net/models/schelling
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