Re: Depth indexing of nil

["Soni L." <fakedme@...>]

On 08/07/15 10:10 PM, Simon Green wrote:
> Hi all,
>
> I’m relatively new to Lua and I feel like this should have already been answered, but if so I cannot find the right combination of words to type into Google to find it.
>
> Suppose I have a nested structure of tables a to e such that I could have a.b.c.d.e. However, anything from b down might not actually exist. So, ‘a’ could be empty, or ‘b’ could be … all the way down to ‘d’ not having an ‘e’.
>
> I want to use the value of ‘e’, if it exists. If it doesn’t, that’s fine, I do something else. This:
>
> if a.b.c.d.e == something then
>      something-else
> end
>
> only works if everything down to e exists. If anything else doesn’t, then Lua spits the dummy and tells me I’m indexing a nil value. This is technically correct, but actually, I don’t care. So what I end up having to do is:
>
> if a and a.b and a.b.c and a.b.c.d and a.b.c.d.e and a.b.c.d.e == something then
This is SLOW! the fast way would be:

local x = a and a.b
x = x and x.c -- x = a.b; x.c = a.b.c
x = x and x.d
x = x and x.e
if x and x == something then
  -- do stuff
end

Another SLOW option is to wrap it in a function like so:

local function f(a) return a.b.c.d.e end -- cache this somewhere so you 
avoid creating/looking up a closure

if pcall(f, a) and f(a) == something then
  -- do stuff
end
>      something-else
> end
>
> In Python, if I referenced a.b.c.d.e and b didn’t exist I’d simply get an IndexError or KeyError exception. Noting that there are no exceptions in Lua, is there some other way I can avoid having thousand-word-long lines every time I want an if statement? Basically, what I'm asking is, if 'b' doesn't exist, is there a way to have a.b.c.d.e == x just return false instead of break my entire program?
>
> Cheers
> Simon
>
>
>
>
> Simon Green
> Senior VAS Engineer
> Two Degrees Mobile Limited
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> www.2degreesmobile.co.nz
>
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