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Town simulation - town type definitions
this example from a Well House Consultants training course
Source code: towns.lua Module: U050
--[[
This file defines the table types and functions that can run on those tables for two types of town - a growth town and a stable town. Unlike Java, you can define multiple types in the same file, and you would logically do so if they are always going to be used in the same appliaction and maintained by the same person / team. In both table types in this file, we have provided functions of the same name, meaning that our main code can simply ask for (example) the growth factor for a town, and know that the program itself will know WHICH growth factor algorithm to use for that particular town - known in OO as "polymorphism" ]] growth = growth or {} stable = stable or {} local meta_g = {} local meta_s = {} -- Define a new growth town function growth.new(place,pop,tph, houseprice) -- save the data local town = {} town["place"] = place town["pop"] = pop town["tph"] = tph town["houseprice"] = houseprice -- The following add functionallity to a growth town -- Some of these routines are specific to a growth town -- but others are shared with a static town town["getfact"] = growth.getfactor town["addpeeps"] = addpeeps town["wastage"] = wastage town["change_house_prices"] = growth.change_house_prices -- As these are just references being copied, we do -- NOT have an efficiency problem! -- Also set the metatable so that our application can -- pick up underlying code to define other non-standard -- functionallity, such as how to add (merge) two towns -- as the grow into each other, and how to format them -- as a string when they are printed out setmetatable(town,meta_g) return town end function stable.new(place,pop,tph, houseprice) -- A lot of common code with growth.new ... -- So perhaps a case for an internal shared function? local town = {} town["place"] = place town["pop"] = pop town["tph"] = tph town["houseprice"] = houseprice town["getfact"] = stable.getfactor town["addpeeps"] = addpeeps town["wastage"] = wastage town["change_house_prices"] = stable.change_house_prices setmetatable(town,meta_s) return town end -- Differing algorithms defined function growth.getfactor(self) return self.pop / self.houseprice / self.houseprice * (6 + self.tph) end function stable.getfactor(self) return self.pop / self.houseprice / self.houseprice * (3 + self.tph) end function growth.print(self) return string.format("%12s - %6d - %6s - %7d", self.place, self.pop, "FAST", self.houseprice); end function stable.print(self) return string.format("%12s - %6d - %6s - %7d", self.place, self.pop, "STABLE", self.houseprice); end function stable.change_house_prices(self,inc,out) factor = (out/inc) * (out/inc) + 0.02 self.houseprice = self.houseprice * factor end function growth.change_house_prices(self,inc,out) factor = out/inc - 0.01 self.houseprice = self.houseprice * factor end -- Shared algorithms defined -- (but these should really be in a table of their own to avoid -- namespace clutter!) function addpeeps(self,extra) self.pop = self.pop + extra end function wastage(self) self.pop = self.pop * 0.99 end -- Defining operations in the metatables - saying what to -- run when we 'print' a town. meta_g.__tostring = growth.print meta_s.__tostring = stable.print Learn about this subject
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