Re: how to work with tables with holes
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- Subject: Re: how to work with tables with holes
- From: Sean Conner <sean@...>
- Date: 2013年9月26日 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