In Lua, what is the right way to handle varargs which contains nil?

Question

I'm trying to create a debug print function which takes a file handle as the first argument. First, I write a function like this:

function fprint (f, ...)    for i, v in ipairs{...} do       f:write(tostring(v))       f:write("\t")    end    f:write("\n") end 

This function works as long as I don't pass nil value in arguments. But if I call this with a nil, it doesn't print the nil value and rest of arguments.

fprint(io.stderr, 1, 2, nil, 3) => prints only 1 and 2 

So, what is the right way to fix this problem?

Answer 1

Actually, it's easy to handle nil values in varargs, all you need is to use the select function, which works even with nil (it counts the actual number of parameters). The following idiom is so useful that it is a core library function table.pack in Lua 5.2:

function table.pack(...)   return { n = select("#", ...), ... } end 

The number of arguments is stored in field n, so in order to iterate through them just use this:

function vararg(...)     local args = table.pack(...)     for i=1,args.n do         -- do something with args[i], careful, it might be nil!     end end