• Subject: Re: how to work with tables with holes
• From: Sean Conner <sean@...>
• Date: Thu, 26 Sep 2013 16:06:21 -0400

```It was thus said that the Great Rafis Ganeyev once stated:
> Tables can't be used so easily as array() in PHP. All Lua developers should
> know their peculiarities to use them properly: distinctions between vector
> and hash parts, no count() function, maybe vector and hash parts grew
> strategy, etc.
>
> My question is simple. If I have some collection of objects in vector part
> and I create holes by setting nil to middle elements, length operator and
> ipairs begin work improperly, how to I traverse this collection? Only with
> pairs() or should I use table.getn(), which seems have been removed in Lua
> 5.2? Or should I create element called "n" in hash part and create
> add/remove functions for collection? What are best practices for that case?

1) Don't set an element to nil.  Use table.remove() instead.  That way, #
will alway work:

t = { 1 , 2 , 3 , 4 }
print(#t)
table.remove(t,2)
print(#t)

2) Use a sentinel value to mark removal.

empty = {}
t = { 1 , 2 , 3 , 4 }
print(#t)
t[2] = empty
print(#t)

for i = 1 , #t do
if t[i] ~= empty then
print(t[i])
end
end

3) Keep track of both the size and the maximum index.

m = { __newindex = function(t,i,v)
if v == nil then
print(">>>")
t.N = t.N - 1
t.P[i] = v
return
end

if i > t.MAX then
t.MAX = i
end

t.N = t.N + 1
t.P[i] = v
end
}
t = { P = { 1 , 2 , 3 , 4 } , N = 4 , MAX = 4 }
setmetatable(t,m)
print(t.N)
t[2] = nil
print(t.N)

In my opintion, the best practice is #1.

-spc

```