Lua sort array by key values

Question

I've got an array of 32 elements set up like so:

racers[i] = {plCP = 0, plPos = 0}

I'd like to sort this array by the value plCP, and set plPos to be the value that the sort came up with. Something like this:

racers[1] = {plCP = 3, plPos = 3}
racers[2] = {plCP = 2, plPos = 4}
racers[3] = {plCP = 6, plPos = 2}
racers[4] = {plCP = 12, plPos = 1}
racers[4] = {plCP = 6, plPos = 2}

Also note that it is possible for 2 items to have the same plCP value. I'd like for them to have the same plPos value in these cases.

This is for a racing game I'm working on, and I'm trying to calculate the position of the player in the race based upon which checkpoint they're currently driving towards.

Answer 1

Please refer to the Lua reference manual:

https://www.lua.org/manual/5.3/manual.html#pdf-table.sort

table.sort (list [, comp])

Sorts list elements in a given order, in-place, from list[1] to list[#list]. If comp is given, then it must be a function that receives two list elements and returns true when the first element must come before the second in the final order (so that, after the sort, i < j implies not comp(list[j],list[i])). If comp is not given, then the standard Lua operator < is used instead. Note that the comp function must define a strict partial order over the elements in the list; that is, it must be asymmetric and transitive. Otherwise, no valid sort may be possible. The sort algorithm is not stable: elements considered equal by the given order may have their relative positions changed by the sort.

table.sort(racers, comp) will do the trick if you implement a function comp that tells Lua which of two elements comes first.

Simple example:

local animals = {
     {name = "bunny", size = 4},
     {name = "mouse", size = 1},
     {name = "cow", size = 30}
}
-- sort animals by size descending
table.sort(animals, function(a,b) return a.size > b.size end)

for i,v in ipairs(animals) do print(v.name) end

print()

-- sort animals by size ascending
table.sort(animals, function(a,b) return a.size < b.size end)
for i,v in ipairs(animals) do print(v.name) end

All you need to do is to copy a few values around, once you know the order. Or you write your own sorting algorithm.