Comp function in table.sort

Questions
1

I don’t get how the comp function works as the second parameter of table.sort. I want to call my own custom function instead of the standard function, but I don’t know how to set up this function. Could someone explain what exactly the comp function does, and how I could make my own function to use in place of the original one?

2

Hello. Here is an example i wrote to sort t , a table of vec3

    local s = {}
    local i
    for i=1,#t do
        s[i] = vec4(t[i].x,t[i].y,t[i].z,i)
    end
    local function compVec3(a,b)
        local out = false
        if floor(a[1])<floor(b[1]) then out = true
        elseif floor(a[1])==floor(b[1]) then
            if floor(a[2])<floor(b[2]) then out = true
            elseif floor(a[2])==floor(b[2]) then
                if floor(a[3])<floor(b[3]) then out = true end
            end
        end
        return out
    end
    table.sort(s,compVec3)

the last line applies the the sort to table s.
the comVec3 returns true when a is strictly below b.
If a=b it returns false.
That’s all about ‘comp’.
Because i want to keep what is the original position in of each vector in table t, i have added a 4rth column to s that contains the index i. After the sort i scan the table s to get the indexes i which are now in the good order . Is that clear now?

3

table.sort sorts a table using the quicksort algorithm (which is not important here).

It iterates over the table, at each step taking two elements from it and comparing them. The comparison function tells the sorting algorithm if the two elements are already in order (true) or not (false).

This is an equivalent of the standard comparison rule:


t = {1, 8, 5, 4}

# Return true if a < b
function lesserthan(a, b)
    return a < b
end

table.sort(t, lesserthan)

print(table.concat(t, ", "))
>> 1, 4, 5, 8

The other way round:


t = {1, 8, 5, 4}

# Return true if a > b
function greaterthan(a, b)
    return a > b
end

table.sort(t, greaterthan)

print(table.concat(t, ", "))
>> 8, 5, 4, 1

Now invent a predicate (means comparison function) yourself.


t = {1, 8, 5, 4}

# Return true if a is odder than b.
# This is only possible if a is odd and b is even.
function odderthan(a, b)
    odd_a = math.mod(a, 2)
    odd_b = math.mod(b, 2)
    return odd_a > odd_b
end

table.sort(t, odderthan)

print(table.concat(t, ", "))
>> 1, 5, 8, 4

You will have the “oddest” values listed first, then the even values. Note that no statement was made about the order of the numbers within their repective part, just even or odd.