--[[
This example shows user defined sorting in Lua, using an extra
function provided by us to compare two values. The difficult
bit of doing all the admin work is done by Lua's sort routine
which calls back to the function we supply

In this example, we are sorting numbers so that we go up the
even numbers in our table, then back down the odd numbers; I
call this a "postman" sort due to the British convention of
having odd numbered houses on one side of a road and even on
the other, and the desirablility of dropping off the post
without the constant need to keep crossing!
]]

function postman(thistable)
        -- psord is a closure in a closure ... but as we
        -- don't share local variables, that doesn't make
        -- any difference!
        function psord(a,b)
                -- uncomment next line to show how sorting works
                -- print (a,b)
                if a % 2 == 0 and b % 2 ~= 0 then
                        return true
                        end
                if a % 2 ~= 0 and b % 2 == 0 then
                        return false
                        end
                if a % 2 == 0 and a < b then
                        return true
                        end
                if a % 2 ~= 0 and b < a then
                        return true
                        end
                return false
                end
        table.sort(thistable,psord)
        report = "Delivery order:"
        for k,v in ipairs(thistable) do
                report = report .. " (" .. k .. "," .. v .. ") "
                end
        return report
end

deliveries = {4,7,3,12,43,23,22,12,34,54,12,32,44,1,5}
hegoes = postman(deliveries)
print (hegoes)