;; text, like this, that start with semi-colons are comments and do not effect anything
;;
;; First we have lists of general and individual properties/slots

globals [global-sales]

breed [sellers seller]
breed [buyers buyer]

;; only attribute is age, all agents automatically have the attribute of color and size
sellers-own [money food price sales]
buyers-own [money food]

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Next we have the procedure to initialise the simulation
;; this is executed when one presses the "setup" button

to setup 
  clear-all ;; this clears everything at the start - a clean slate
  
  ;; first create the sellers people, shown as circles, put them on a random patch
  ;; the parameter "population" is determined by a slider
  create-sellers number-sellers [
    ;; set initial characteristics
    set money random 500
    set food random 100
    ;; random price to start with
    set price 5 + random 10
    
    ;; set appearence 
    set shape "truck"
    set size food / 500
    set xcor 0 + random-normal 0 0.15
    set ycor random-float 1
  ]
  
  ;; do the same for the buyers 
  create-buyers number-buyers [
    set money random 20
    set food random 10
    
    ;; set appearence 
    set shape "person"
    set size food / 100
    set xcor 1 + random-normal 0 0.15
    set ycor random-float 1
  ]
  
  reset-ticks                            ;; this initialises the simulation time system and graphs
end

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Next we have the procedure to progress the simulation one time step
;; this is executed when one presses the "step" button or repeatedly
;; if one presses the "go" button

to go
  let chosen-seller nobody
  let possible-sellers nobody
  let desired-amount 0
  
  set global-sales 0
  
  ;; things that happend to all sellers each time period
  ask sellers [
    
    ;; reset sales counter
    set sales 0
    
    ;; add to food to sell from production and subtract costs
    set food food + production-rate
    set money money - seller-costs

  ]
  
  ;; things that happen to all buyers each time period
  ask buyers [
    ;; buyers income
    set money money + buyer-income
    
    show (word "Food of " self " is " food)

    
    ;; if have enough food, eat some, if none die
    set food food - usage-rate
    
    ;; find possible sellers
    set possible-sellers sellers with [food >= 1]
    
    if any? possible-sellers  [     
      ;; find seller with lowest price
      set chosen-seller min-one-of possible-sellers [price] 
      
      ;; determine how many units could buy from this seller
      set desired-amount floor (money / ([price] of chosen-seller))
      
      ;; if can buy one unit or more buy it, adjust food and money 
      if desired-amount >= 1 [
        set food food + desired-amount
        set money money - desired-amount * [price] of chosen-seller
        
        ;; adjust food and money for seller
        ask chosen-seller [
          set food food - desired-amount
          set money money + desired-amount * [price] of chosen-seller
          
          ;; record sales
          set sales sales + desired-amount
          set global-sales global-sales + sales
        ]
      ]
    ]
    
  ]
  
  ;; how sellers change their price
  ask sellers [
    if sales < 3 
      [set price price * 0.75]
    if sales > 6
      [set price price / 0.75]
  ]
  
  ;; adjust the size of icons depending upon their stock of food
  ask sellers [ set size food / 500]
  ask buyers [ set size food / 100]
  
  tick                                   ;; this progresses the tick counter by one
end
@#$#@#$#@
GRAPHICS-WINDOW
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SLIDER
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SLIDER
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0
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0
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BUTTON
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NIL
go
T
1
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OBSERVER
NIL
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MONITOR
8
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468
Tot.Sales
global-sales
1
1
11

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

This is an example model, used as part of the "2-day Introduction to Agent-Based Modelling".  

This is a simple agent-based market for food with buyers and sellers.  Buyers use a given amount of food each time unit, sellers produce different amounts of food.  Buyers have an income with which to buy food, sellers have a certain level of costs that they have each time unit.


## HOW IT WORKS

There are two dynamic processes.  

Buyers try to spend what money they have on food each time.  If there are any buyers with a unit of food, they choose the one with the minimum price and buy what units of food they can afford from them.

Sellers keep track of the number of units they sell, if they sell less than 3 they lower their price, if they sell more than six they raise it.

## HOW TO USE IT

Press...
"setup" to initialise the simulation
"step" to progress the simulation one step
"go" to repreatedly step through the simulation


## THINGS TO NOTICE

It is very difficult to balance such a market.

## THINGS TO TRY

Use commands to interogate the model, e.g. show [price] of sellers

Change the display of buyers and sellers to indicate their current stock of food.

Add monitors and graphs to understand what is happening in the market.

## EXTENDING THE MODEL

Think about the decision from the point of view of the buyers and sellers (each).  What limitations would they have?  What aspects of their environment might they know about?  What additional factors might they use in their decision making?  How might they realistically make their decisions?

## NETLOGO FEATURES

Note the use of local variables within the "go" procedure.  They are declared using "let".

## CREDITS AND REFERENCES

Model by Bruce Edmonds, Feb 2013
http://cfpm.org/simulationcourse
@#$#@#$#@
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