Multidimensional slicing

[Joseph Rushton Wakeling]

Hello all,

Suppose I have some data in an 2-dimensional associative array (it could be 
larger dimension, but let's limit it to 2d for now).  I'm using an associative 
array because the underlying data is sparse, i.e. for any given index pair (i, 
j) there's most likely not an entry.

It's trivial to take a slice of this data using the first dimension, i.e. if 
a[i][j] gives the value for the key pair (i, j), a[i] will give the array of 
values for all values of j, and a[i].keys will give me all the values of j that 
are "connected" to i.

... however, I can't do this the opposite way, i.e. to slice it a[][j] -- I get 
an error message, e.g.:

     Error: ulong[ulong][ulong] cannot be sliced with []

and similarly there is no way to identify a[][j].keys i.e. all the i indexes 
associated with j.

So, I'm curious: is there any practical way to slice across the 2nd (or higher) 
dimension in this way, so that I get access to both values and keys?

The application here is a large (but sparse) dataset of links forming a 
bipartite graph of user-object weighted links.  The goal would be for the 2d 
array to store those weights, and the slices would allow you to look at that 
collection from one of two viewpoints -- i.e. all the objects a given user is 
linked to, or all the users a given object is linked to.

Of course, it's trivial to have two associative arrays, say userLinks and 
objectLinks, where the keys are respectively object and user IDs and the values 
are the weights, but that means storing the weight values twice.  I'd like to be 
able to have one entity that stores the links and weights, and which can be 
sliced either way without needing to copy data and without any added CPU 
overhead (the planned simulation and data sizes are large, so something that is 
costly CPU-wise or that requires additional allocation is unlikely to scale).

Thanks in advance for any advice,

Best wishes,

     -- Joe