[R] Unexpected errors in sparse Matrix arithmetic with zero-length dimensions

[Martin Maechler]

>>>>> Aaron Lun 
>>>>>     on Sun, 10 Feb 2019 15:22:17 +0000 writes:

    > Dear list,
    > The Matrix package exhibits some unexpected behaviour in its arithmetic
    > methods for the edge case of a sparse matrix with a dimension of zero
    > length. The example below is the most illustrative, where changing the
    > contents of the vector causes the subtraction to fail for a sparse
    > matrix with no columns: 
    >     
    >> library(Matrix)
    >> x <- rsparsematrix(10, 0, density=0.1)
    >>  
    >> x - rep(1, nrow(x)) # OK 
    >> x - rep(0, nrow(x)) # fails
    > Error in .Ops.recycle.ind(e1, len = l2) : 
    >   vector too long in Matrix - vector operation

This is indeed clearly a lapsus of us / mine  as well as the
next examples:  Will all be fixed  "around" .Ops.recycle.ind() 

    > This is presumably because Matrix recognizes that subtraction of zero
    > preserves sparsity and thus uses a different method in the second case.
    > However, I would have expected subtraction of a zero vector to work if
    > subtraction of a general vector is permissible. This is accompanied by
    > a host of related errors for sparsity-preserving arithmetic:

    >> x / 1 # OK
    >> x / rep(1, nrow(x)) # fails 
    > Error in .Ops.recycle.ind(e1, len = l2) : 
    >   vector too long in Matrix - vector operation
    >>  
    >> x * 1 # OK
    >> x * rep(1, nrow(x)) # fails
    > Error in .Ops.recycle.ind(e1, len = l2) : 
    >   vector too long in Matrix - vector operation
    >       
    > A different error is raised for a sparse matrix with no rows:

    >> y <- rsparsematrix(0, 10, density=0.1)
    >>  
    >> y - numeric(1) # OK
    >> y - numeric(0) # fails
    > Error in y - numeric(0) : <Matrix> - numeric(0) is undefined

Thank you, that's another lapsus, I will fix before the next
release of Matrix.

    > I would have expected to just get 'y' back, given that the same code
    > works fine for other Matrix classes:

    >> z <- as(y, "dgeMatrix")
    >> z - numeric(0) # OK

sure.

    > Correct behaviour of zero-dimension sparse matrices is practically
    > important to me; I develop a number of packages that rely on Matrix
    > classes, and in those packages, I do a lot of unit testing with zero-
    > dimension inputs. This ensures that my functions return sensible
    > results or fail gracefully in edge cases that might be encountered by
    > users. The current behaviour of sparse Matrix arithmetic causes my unit
    > tests to fail for no (obvious) good reason.

Interesting that you need 0-dim sparse matrices.  I agree they
should work, too... and will fix
(but it seems they haven't been used much by others;  else I
would have expected these cases to have been reported long ago).

Further note that in R,

  maintainer("Matrix")

gives a nice address to send such findings.
(similarly for all other R packages !)

Thank you very much once more!
Martin

    > Best,
    > Aaron Lun

    > Research Associate
    > CRUK Cambridge Institute
    > University of Cambridge

--
Martin <Maechler at ....>   
Seminar für Statistik, ETH Zürich   HG G 16         Rämistrasse 101
CH-8092 Zurich, SWITZERLAND