[R] Problems with crossprod

[Spencer Graves]

I still have R 1.7.1, and the problem appears there as well: 

 > a <- A[i, ] ; crossprod(a)
              [,1]          [,2]          [,3]          [,4]          [,5]
[1,] 1.195616e-301 7.042305e-302 9.563047e-302 2.281448e-302 2.198017e-302
[2,] 6.905419e-302 1.204915e-301 3.382433e-302 2.398701e-302 2.358828e-302
[3,] 1.194968e-301 7.039991e-302 1.384628e-302 2.446584e-302 4.507199e-302
[4,] 7.046400e-302 1.204416e-301 2.444003e-302 2.357136e-302 2.446228e-302
[5,] 1.205363e-301 1.204367e-301 2.445605e-302 3.979963e-302 7.861951e-302
 > 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
 > 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
 > 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
 > a <- A[i, ] ; crossprod(a)
     [,1] [,2]          [,3]          [,4] [,5]
[1,]    0    0 5.092790e-313 4.372522e-111    0
[2,]    0    0 5.708624e-307  0.000000e+00    0
[3,]  NaN    0 4.516565e-300  0.000000e+00    0
[4,]    0    0 8.997262e-312  0.000000e+00    0
[5,]    0    0 1.086462e-311  0.000000e+00    0
 >
hope this helps.  spencer graves

Giovanni Marchetti wrote:

>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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