[R] Problems with crossprod

[Liaw, Andy]

Somehow R creates `a' as a matrix with 0 rows and 5 columns.  I don't know
how crossprod() or other linear algebra functions deals with such a
degenerate matrix.

I'd suggest R Core to add checks for strictly positive dimensions in such
functions.

(Also, I find it strange that A[1,] is a vector, but A[numeric(0),] is a 0x5
matrix...)

Andy

> From: Giovanni Marchetti [mailto:gmm at ds.unifi.it] 
> 
> Dear R-users, 
> 
> I found a strange problem 
> working with products of two matrices, say: 
>  
> a <- A[i, ] ; crossprod(a)
> 
> where i is a set of integers selecting rows. When i is empty 
> the result is in a sense random.
> 
> After some trials the right answer 
> (a matrix of zeros) appears.
> 
> --------------- Illustration --------------------
> R : Copyright 2003, The R Development Core Team
> Version 1.8.0  (2003-10-08)
> 
> > A  <-matrix(0, 5, 5)
> > i <- c()
> > a <- A[i, ] ; crossprod(a)
>               [,1]          [,2]          [,3]          [,4] [,5]
> [1,] 6.578187e-313           NaN           NaN           NaN  NaN
> [2,]           NaN 1.273197e-313           NaN 1.485397e-313  NaN
> [3,]           NaN 4.243992e-313 2.121996e-314           NaN  NaN
> [4,]           NaN 1.697597e-313           NaN 4.880590e-313  NaN
> [5,] 5.941588e-313           NaN           NaN 1.697597e-313  NaN
> > a <- A[i, ] ; crossprod(a)
>               [,1]          [,2]          [,3]          [,4]  
>         [,5]
> [1,] 2.121996e-314 5.729389e-313           NaN           NaN  
>          NaN
> [2,]           NaN           NaN           NaN           NaN 
> 1.909796e-313
> [3,] 2.970794e-313           NaN           NaN           NaN  
>          NaN
> [4,]           NaN           NaN           NaN 8.487983e-314  
>          NaN
> [5,]           NaN 6.365987e-313 2.546395e-313           NaN  
>          NaN
> > a <- A[i, ] ; crossprod(a)
>               [,1]          [,2] [,3]          [,4]          [,5]
> [1,]           NaN 1.485397e-313  NaN           NaN 2.970794e-313
> [2,] 3.182994e-313           NaN  NaN 1.060998e-313           NaN
> [3,]           NaN           NaN  NaN 1.697597e-313 2.737375e-312
> [4,]           NaN           NaN  NaN           NaN  2.048394e+10
> [5,]           NaN           NaN  NaN           NaN 2.970794e-313
> > a <- A[i, ] ; crossprod(a)
>               [,1]        [,2] [,3] [,4] [,5]
> [1,] 1.591383e-266 20489834629    0    0    0
> [2,] 5.031994e-266           0    0    0    0
> [3,] 1.591205e-266           0    0    0    0
> [4,] 1.264128e-267           0    0    0    0
> [5,] 1.037656e-311           0    0    0    0
> > a <- A[i, ] ; crossprod(a)
>      [,1] [,2] [,3] [,4] [,5]
> [1,]    0    0    0    0    0
> [2,]    0    0    0    0    0
> [3,]    0    0    0    0    0
> [4,]    0    0    0    0    0
> [5,]    0    0    0    0    0
> --------------- End of illustration------------
> 
> The same problem does not appear using the matrix product:
> 
> > a <- A[i, ] ; t(a) %*% a
>      [,1] [,2] [,3] [,4] [,5]
> [1,]    0    0    0    0    0
> [2,]    0    0    0    0    0
> [3,]    0    0    0    0    0
> [4,]    0    0    0    0    0
> [5,]    0    0    0    0    0
> 
> Note that Splus 6 returns an error message:
> 
> > a <- A[i, ] ; crossprod(a)
> 
> Problem in .Fortran.ok.Internal(if(cmplx) "zcrossp1"..: 
> subroutine dcrossp1: 
> Argument 1 has zero length
> 
> 
> Thank you, 
> 
> Giovanni
> 
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