R devel - Oct 2009 - unexpected behaviour of isSymmetric() (PR#14000)

Full_Name: Mike Danilov
Version: 2.9.0
OS: Fedora Core 9
Submission from: (NULL) (142.103.121.198)


When checking for the symmetry of a matrix, function isSymmetric.matrix() gets
confused by the discrepancy of colnames/rownames if its argument. See the code
snippet below. Perhaps it's a problem of the matrix product which copies
colnames of the first argument but not the rownames of the second to its value.
Not sure which one should be fixed but the way it is now it seems illogical that
X'X is deemed to be non-symmetric.

x <- c(1,2,3)
names(x) <- c("v1","v2","v3")
isSymmetric(x%*%t(x)) ## returns FALSE instead of TRUE

On 10/12/2009 02:05 AM, mike at stat.ubc.ca wrote:
>
> Full_Name: Mike Danilov
> Version: 2.9.0
> OS: Fedora Core 9
> Submission from: (NULL) (142.103.121.198)
>
>
> When checking for the symmetry of a matrix, function isSymmetric.matrix()
gets
> confused by the discrepancy of colnames/rownames if its argument. See the
code
> snippet below. Perhaps it's a problem of the matrix product which
copies
> colnames of the first argument but not the rownames of the second to its
value.
> Not sure which one should be fixed but the way it is now it seems illogical
that
> X'X is deemed to be non-symmetric.
>
> x<- c(1,2,3)
> names(x)<- c("v1","v2","v3")
> isSymmetric(x%*%t(x)) ## returns FALSE instead of TRUE
It seems to be concerned with the names

 > y <- x %*% t(x)
 > y
      v1 v2 v3
[1,]  1  2  3
[2,]  2  4  6
[3,]  3  6  9

 > isSymmetric( y )
[1] FALSE

# dropping the dimnames
 > isSymmetric( structure( y, dimnames = NULL ) )
[1] TRUE

# pass the ... along this path : isSymmetric > all.equal >
all.equal.numeric
 > isSymmetric( y, check.attributes = F )
[1] TRUE

# set the dimnames equal
 > dimnames( y ) <- rep( list( names(x) ), 2 )
 > isSymmetric( y )
[1] TRUE

Not sure this is expected behaviour

Romain

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