The Trader

Every day, Hugo the trader drives his cart past Emma's farm and buys crops. His prices change every day. When his cart is empty, he pays 2 coins more for each crop than the market. The next day, when it's half full, he pays 1 coin more. On the third day, it's full, and he pays only what the market pays. Then he takes everything to town, and it starts again: 2, 1, 0, 2, 1, 0…

Emma's rule is simple: if Hugo pays more than the market, she sells him everything in the barn. Otherwise, she keeps it, and at the end of the season, it goes to the market.

Hugo's days are a job for a coroutine, as in the lesson on coroutines: a function that runs a bit every day and pauses at coroutine.yield. coroutine.wrap turns it into a function that continues it: each call runs it until the next yield and returns the value given to yield. For example, what Rex the dog does all day:

local function rexScript()
    while true do
        coroutine.yield("guards the barn")
        coroutine.yield("sleeps by the well")
    end
end

local rex = coroutine.wrap(rexScript)
for day = 1, 3 do
    print("Day " .. day .. ": Rex " .. rex())
end

This prints:

Day 1: Rex guards the barn
Day 2: Rex sleeps by the well
Day 3: Rex guards the barn

The loop while true never ends, but that's fine here. At every yield, the coroutine pauses, and the program goes on. It runs again only when it's called, and it continues where it stopped, here in the middle of its loop. Hugo's script will be the same, only with a for loop from 2 down to 0 inside. Don't forget its step, -1. Without it, the for loop runs zero times, while true never reaches a yield, and the program runs until the site stops it after 5 seconds.

Hugo's script must run as a coroutine, through coroutine.wrap (or coroutine.create and coroutine.resume). If you call hugoScript() directly, it isn't a coroutine, and yield has nowhere to return to: attempt to yield from outside a coroutine (this time without a line number at the start). And create the coroutine only once, above the loop over the days. Created inside the loop, it would start from the beginning every day, and Hugo would always pay 2 coins more.

The function sell needs two more parameters: who buys (buyer) and how many coins more than the market he pays (extra). Give them default values, as in the lesson on functions: buyer = buyer or "Market" and extra = extra or 0. Then the call sell(barn) at the end of the season works as before.

sell must also empty the barn: barn[name] = nil removes a crop once it's sold. Without it, Hugo would buy the same wheat again on another day, the market once more at the end, and Emma would end the season with 390 coins. That's too good to be true. Removing the barn's keys inside the loop is fine, because the loop goes through the separate list from sortedNames.

On the first day, Hugo pays 2 coins more, but the barn is still empty, and sell would print Hugo: = 0. So let it print its line only when the total is greater than 0.