Supply & Demand

Globals [price producer-surplus consumer-surplus Total-Surplus]
Breed [Buyers Buyer]
Breed [Sellers Seller]
Buyers-own [willingtopay consumersurplus]
Sellers-own [willingtosellfor producersurplus]
turtles-own [partner]

to setup
  
  clear-all                                                     ;clear the world
  create-Buyers 50                                              ;create buyers, put them in random places
  [
  setxy random-xcor random-ycor
  ]
  create-Sellers 50                                                ;create Sellers, put them in random places
  [
   setxy random-xcor random-ycor
  ]
  ask buyers [set shape "buyersleeping" set size 1.6]                        ;give buyers default neutral shapes
  ask Sellers [set shape "sellersleeping" set size 1.6]                            ;give Sellers default neutral shapes        
  ask Buyers [set willingtopay (who + 50 + Change-in-Demand)]    ;set price buyers are willing to pay
  ask Sellers [set willingtosellfor (who + 1 - Change-in-Supply)] ;set price Sellers are willing to sell for
  set price Initial-Price                               ;converts price set by slider into variable that can change
  set consumer-surplus 0                                        ; set surplusses to 0
  set producer-surplus 0
  set total-surplus 0
  ask turtles [set partner nobody] 
end 

to go
  ;these commands repeat most of the startup procedures for when "go" is called after a price change
  update-plot
  ask buyers [set shape "buyersleeping" set size 1.6]
  ask sellers [set shape "sellersleeping" set size 1.6]
  ask turtles [set partner nobody] 
  ask buyers [set consumersurplus (willingtopay - price)]
  ask sellers [set producersurplus (price - willingtosellfor)]
  set consumer-surplus 0
  set producer-surplus 0
  set total-surplus 0
  
  ;if you're a buyer willing to buy and haven't bought, show that you're looking
  ask buyers [if willingtopay >= price and partner = nobody and not any? buyers with [shape = "buyermad"][set shape "buyerlooking"]]
  
  ;if you're a seller willing to sell haven't sold, and there are buyers, show that you're selling.
  ask sellers [if willingtosellfor <= price and partner = nobody and not any? sellers with [shape = "sellermad"][set shape "sellerlooking"]]
  
  ;if you're a buyer willing to buy and haven't bought, find a car to buy
  ask buyers [if shape = "buyerlooking" [findseller]]
  
  ;if you're a seller who wants to sell and there are no buyers left, look angry
  ask sellers [if willingtosellfor <= price and partner = nobody and not any? buyers with [shape = "buyerlooking"] [set shape "sellermad"]]
  ;if nobody is looking to buy or sell, stop and announce the equilibrium price ahs been reached
  if not any? sellers with [shape = "sellerlooking"] 
     and not any? sellers with [shape = "sellermad"]
        and not any? buyers with [shape = "buyerlooking"] 
           and not any? buyers with [shape = "buyermad"]
              [output-print " " output-type "Equilibrium price reached! " output-type count sellers with [partner != nobody] output-type " Cars sold for $" output-type price output-type " each."  update-plot stop]
  
  ;if buyers and sellers are angry, and they are willing to adjust prices, make them adjust their prices
  if any? buyers with [shape = "buyermad"] and price-adjusting [raiseprice]
  if any? sellers with [shape = "sellermad"] and price-adjusting [lowerprice]
  
  ;if buyers and sellers are angry, but they are not willing to adjust prices, stop and announced they are angry
  if any? buyers with [shape = "buyermad"] and not price-adjusting [output-print "Buyers are angry!" stop]
  if any? sellers with [shape = "sellermad"] and not price-adjusting [output-print "Sellers are angry!" stop]
end 

to findseller
  ;if there are buyers who want to buy, but can't, make them mad
  ask buyers [if willingtopay >= price and partner = nobody and not any? sellers with [shape = "sellerlooking"] [set shape "buyermad"]]
  
  ;if there are sellers who want to sell, but can't, make them mad
  ask sellers [if willingtosellfor <= price and partner = nobody and not any? buyers with [shape = "buyerlooking"] [set shape "sellermad"]]
  
  ;if there are sellers left who want to sell, wander around and look for them
  if any? sellers with [shape = "sellerlooking"] and partner = nobody [lt random 40 rt random 40 fd 1 checkseller]
end 

to checkseller
  ;if there is a Seller who hasn't sold who wants to sell on this spot, buy it and make both of you happy.
  if any? other turtles-here with [(partner = nobody) and (shape = "sellerlooking")]
       [set partner one-of other turtles-here with [partner = nobody and shape = "sellerlooking"] 
       set shape "buyerhappy"
       set consumer-surplus consumer-surplus + (consumersurplus)
       ask partner [set partner myself set shape "sellerhappy" set producer-surplus producer-surplus + (producersurplus)]]
  set total-surplus consumer-surplus + producer-surplus
  
  ;if there's not a seller here, look somewhere else
  if not any? other turtles-here with [shape = "sellerlooking"][findseller]
end 

to raiseprice
  ;announce your are raising the price, then raise the price
  Output-print "Buyers are angry there are no cars to buy." output-print "Sellers realize they can raise prices without losing sales." output-type "Sellers raise price to $" output-type price + 1 output-print "." 
         output-print " "
  set price (price + 1)
  wait 2
  go
end 

to lowerprice
  ;announce you are lowering the price, then lower the price
  Output-print "Sellers are angry nobody will buy their car." output-type "Sellers drop price to $" output-type price - 1
       output-print "." output-print " "
  set price (price - 1)
  wait 2
  go
end 

to update-plot
set-current-plot "Price & Quantity"
set-current-plot-pen "Price"
plot price
set-current-plot-pen "Quantity"
plot count sellers with [shape = "sellerhappy"]
set-current-plot "Change in Surplus"
set-current-plot-pen "Total Surplus"
plot Total-Surplus
set-current-plot "Change in Surplus"
set-current-plot-pen "Producer Surplus"
plot Producer-Surplus
set-current-plot "Change in Surplus"
set-current-plot-pen "Consumer Surplus"
plot Consumer-Surplus
end