[Swig-user] argout typemap for dynamically sized array

[<tn...@fa...>]

Hello,

Say you have a function takes values from an array, transforms the value
and stores it in another array, like 

    void transform(const int k, const in[], int out[])
    {
        int i;
        for (i=0; i<k; i++) 
            out[i]=in[i]*2; // or whatever...
    }

I want to be able to call this function as (from perl in my case):

    my $x = swigmod::transform([1,2,3,4]); # $x is now [2,4,6,8]

I can't figure out how to accomplish this without having to manually
write a C wrapper function, or forcing the scripting language user to
pass an array preallocated with k elements.  I thought I could write
something analogous to non-array argouts, like this:

    %typemap(argout) double *OUTPUT {
        $result = sv_2mortal(newSVnv(*$1));
        argvi++;
    }
    
    %typemap(in,numinputs=0) double *OUTPUT(double junk) {
    	$1 = &junk;
    }

But for arrays, I would need to allocate an appropriately sized 'double
* junk', which I can't do b/c I don't have access to the length from
within the typemap code (I know I can access the length variable k with
something like 'k_0' where 0 is the position of 'k', but that seems
extremely dirty).  I would also need to know the length in the argout.  

Any ideas?

Or am I approaching this the wrong way?

-Tom