[R] Problems with crossprod

[Giovanni Marchetti]

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