• Subject: Re: Zero-based arrays with the FFI
• From: Romulo <romuloab@...>
• Date: Fri, 18 Mar 2011 02:08:11 -0300

```Couldn't you just ignore the zero'th element in the case of ffi array?
In other words, create an array with size n+1?

On 2011-03-18, Geoff Leyland <geoff_leyland@fastmail.fm> wrote:
> Hi,
>
> Mike's scimark.lua contains a trick that makes working with the FFI arrays
> in Luajit and with tables in vanilla Lua almost seamless.
>
> However, FFI arrays are zero-based, and Lua tables are one-based.  Here's a
> modified version of Mike's array_init() and an illustration of the problem:
>
> local darray
>
> local function array_init()
>  if jit and jit.status and jit.status() then
>    local ok, ffi = pcall(require, "ffi")
>    if ok then
>      darray = ffi.typeof("double[?]")
>      return
>    end
>  end
>  darray = function(n, ...)
>    local a = {}
>    for i = 1, select('#', ...) do a[i] = select(i, ...) end
>    return a
>  end
> end
>
> array_init()
>
> a = darray(3, 1, 2, 3)
>
> print(a[0], a[1], a[2], a[3])
>
> Output with Lua is: nil	1	2	3
> and with LuaJIT: 1	2	3	4.268776586633e-319
>
> What's best practice for working around this?  I could change the vanilla
> darray to
>
>  darray = function(n, skip, ...)
>    local a = {}
>    for i = 1, select('#', ...) do a[i] = select(i, ...) end
>    return a
>  end
>
> and always initialise arrays with an extra initial element.  array_init
> could set an OFFSET variable that I could use in loop counters?
>
> Anyone got any better ideas?
>
> Cheers,
> Geoff
>
>

--
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