How to handle ragged-list type argument in LibraryLink?

Question

Given that I have a ragged list as below,

raggedList = {U1, U2, ..., Un}

where, ui is a sublist($i=1,2,\cdots,n$) and their length are distinct. For example,

raggedList = 
  {{0.25, 0.25, 0.5, 0.5, 0.75, 0.75, 0.8}, 
   {0.21, 0.25, 0.3, 0.6, 0.7, 0.8}, 
   {0.25, 0.3, 0.7, 0.8};

Namely, raggedList is not a tensor. For this case, obviously, I cannot use MArgument_getMTensor(Args[0]) to get that input.

Answer 1

You can't. I mean you can't directly. This is not yet supported in LibraryLink, there is a method to parse it via MathLink(now called WSTP), but that's another story. You can however always flatten your ragged array, store the lengths of each sub-array and pass that as another argument too for later formatting.

Another method would be to pad your array with symbols that you know should not appear as elements of your sub-list, things like -1 if you know the matrix is non-negative.

A lot of things are not supported in LibraryLink others include lists of strings and images.