Lua array as parameter with null

Question

I have a problem, i need to call a function with variable parameters that i get from a webservice... so for example:

I have a function:

function MyPrint(param1, param2)
   print(param1, param2)
end

I save this function in an array of callbacks:

callback[1] = MyPrint

I get parameters from server:

params = "[2, 88]" --> JSON from server

I do:

params = json.decode(params)
-- so 
-- params[1] = 2
-- params[2] = 88

I tried to pass this parameters to my callback as:

pcall(callback[1], unpack(params))

I got 2 and 88 on MyPrint function...

But if server sends "[null, 88]", I got nil on both values... I have readed that unpack function has problem with null values... but then... how can I call callback[1] with some null values?

Is there a way to pass an array of parameters directly to function without unpack it?

EDIT: I created MyPrint as an example... but really I don't know how many params needs the callback function, i only have a list of functions and needs to call them with a variable number of parameters that i got from server.

pastebin.com/10UuPc6j -- works here. What's your lua version? What's the value of params before unpacking? Also, see here — Diego
– Diego, Commented May 28, 2015 at 22:58
json.decode should create additional field n in the table returned to store the number of arguments. Then your_function(unpack(params,1,params.n)) would solve the problem. — Egor Skriptunoff
– Egor Skriptunoff, Commented May 29, 2015 at 4:53
@EgorSkriptunoff There is no "n" field... print(params.n) --> nil — Pipe
– Pipe, Commented May 29, 2015 at 12:51
@diego It seems the problem is on json.decode ignoring the null parameter thats why your example works... it pass the null directly without parsing the null with json.decode — Pipe
– Pipe, Commented May 29, 2015 at 12:53
If you can't make json.decode create a n field or equivalent then there is a loss of data. You can't very well deal of a json object without know how many elements there are. To move on, you'll have to find a way to carry over the number of elements. — Tom Blodget
– Tom Blodget, Commented May 30, 2015 at 14:45

Alexander Lutsenko · Accepted Answer · 2015-06-06 17:25:14Z

1

If a table has gaps, you should explicitly specify a range of indices to unpack: table.unpack(params, 1, table.maxn(params)). This works well in lua 5.1.5.

answered Jun 6, 2015 at 17:25

Alexander Lutsenko

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