Plots as Objects

A plot is a record, and two functions work with it: growPlot and describePlot. Data and the functions that work with them belong together, and that's what a class is for, as in the lesson on objects. In this step, plots become objects of the class Plot. The program will do exactly the same as before, and its output won't change at all. Only its code will be tidier. Changing how a program is written without changing what it does is called refactoring, and programmers do it all the time.

Here's a reminder of how a class works, with one of Emma's hens:

local Hen = {}
Hen.__index = Hen

function Hen.new(name)
    local hen = {name = name, eggs = 0}
    return setmetatable(hen, Hen)
end

function Hen:lay(count)
    self.eggs = self.eggs + count
end

function Hen:__tostring()
    return self.name .. " (" .. self.eggs .. " eggs)"
end

local goldie = Hen.new("Goldie")
goldie:lay(2)
goldie:lay(1)
print(goldie)
print("Best hen: " .. tostring(goldie))

This prints:

Goldie (3 eggs)
Best hen: Goldie (3 eggs)

Hen.new creates a table with the hen's data, gives it the metatable Hen and returns it: setmetatable returns the table it gets. Thanks to Hen.__index = Hen, a hen that doesn't have a field, such as lay, looks for it in Hen. The colon in goldie:lay(2) passes the hen itself as self. And __tostring says what print and tostring make of a hen (%s in string.format uses it too). But .. doesn't, so "Best hen: " .. goldie would end with an error. That's why the example calls tostring(goldie).

The same goes for table.concat. If you add the plots themselves to the day's list instead of their texts, the program stops:

main.lua:108: invalid value (table) at index 1 in table for 'concat'

So add tostring(plot) to the list. Another mistake is a dot instead of the colon. plot.grow(growth[today]) passes the number of days as self, and the method can't read a field of a number:

main.lua:44: attempt to index a number value (local 'self')

The harvest in the main program can stay as it is. An object is still a table, so plot.grown and plot.crop work as before.