function test_tableremove(mylist)
    for i=#mylist,1,-1 do
        if mylist[i] == 13 then
            table.remove(mylist, i)
        end
    end
end -- func

function test_mitch(mylist)
    -- NOTE: This is a messy, hardcoded version of my algorithm, for test-purposes.
    -- Go to https://stackoverflow.com/a/53038524/8874388 for the actual, clean version.
    local j, n = 1, #mylist;

    for i=1,n do
        local value = mylist[i]
        if value ~= 13 then -- Keep everything except "13"
            -- Move i's kept value to j's position, if it's not already there.
            if (i ~= j) then
                mylist[j] = mylist[i];
                mylist[i] = nil;
            end
            j = j + 1; -- Increment position of where we'll place the next kept value.
        else -- Delete "13"
            mylist[i] = nil;
        end
    end
end

function build_tables(howMany)
    local ofs = {}
    for i=1,howMany do
        ofs[i*50] = true;
    end

    local tables = {}
    for i=1, 1 do
      tables[i] = {}
      for j=1, 2000000 do -- 2 million items
        tables[i][j] = (ofs[j] and 13 or 100) -- put a "13" (deleted) in the slots to be deleted, otherwise "100" (kept)
      end
    end

    return tables
end

function time_func(func, name, howMany)
    local tables = build_tables(howMany)
    time0 = os.clock()
    for i=1, #tables do
        func(tables[i])
    end
    time1 = os.clock()
    print(string.format("[%13s] elapsed time (deleting %.0f items): %.3f\n", name, howMany, time1 - time0))
end

-- The number below (i) controls how many items that will need to be deleted from the array.
-- This test checks if table.remove() is ever faster if you only need to remove a few entries.
-- The test arrays contain 2 million items, and the items that need to be removed at all at the start,
-- hence triggering the need to re-index the subsequent array entries.
for i=1,6 do
    time_func(test_mitch, "mitch", i)
    time_func(test_tableremove, "table.remove", i)
end


--[[

LuaJIT 2.0.5 -- Copyright (C) 2005-2017 Mike Pall. http://luajit.org/
JIT: ON CMOV SSE2 SSE3 SSE4.1 fold cse dce fwd dse narrow loop abc sink fuse

[        mitch] elapsed time (deleting 1 items): 0.008

[ table.remove] elapsed time (deleting 1 items): 0.010

[        mitch] elapsed time (deleting 2 items): 0.008

[ table.remove] elapsed time (deleting 2 items): 0.019

[        mitch] elapsed time (deleting 3 items): 0.008

[ table.remove] elapsed time (deleting 3 items): 0.022

[        mitch] elapsed time (deleting 4 items): 0.008

[ table.remove] elapsed time (deleting 4 items): 0.028

[        mitch] elapsed time (deleting 5 items): 0.008

[ table.remove] elapsed time (deleting 5 items): 0.031

[        mitch] elapsed time (deleting 6 items): 0.008

[ table.remove] elapsed time (deleting 6 items): 0.039



Lua 5.3.4  Copyright (C) 1994-2017 Lua.org, PUC-Rio

+ $ lua test.lua
[        mitch] elapsed time (deleting 1 items): 0.306

[ table.remove] elapsed time (deleting 1 items): 0.121

[        mitch] elapsed time (deleting 2 items): 0.309

[ table.remove] elapsed time (deleting 2 items): 0.164

[        mitch] elapsed time (deleting 3 items): 0.313

[ table.remove] elapsed time (deleting 3 items): 0.237

[        mitch] elapsed time (deleting 4 items): 0.326

[ table.remove] elapsed time (deleting 4 items): 0.262

[        mitch] elapsed time (deleting 5 items): 0.326

[ table.remove] elapsed time (deleting 5 items): 0.304

[        mitch] elapsed time (deleting 6 items): 0.334

[ table.remove] elapsed time (deleting 6 items): 0.341



]]--