[R] sweep() and recycling

[Prof Brian Ripley]

The issue here is that the equivalent command array(1:5, c(6,6)) (to 
matrix(1:5,6,6)) gives no warning, and sweep uses array().

I am not sure either should: fractional recycling was normally allowed in 
S3 (S4 tightened up a bit).

Perhaps someone who thinks sweep() should warn could contribute a tested 
patch?

On Mon, 20 Jun 2005, Heather Turner wrote:

>
>
> Dr H Turner
> Research Assistant
> Dept. of Statistics
> The University of Warwick
> Coventry
> CV4 7AL
>
> Tel: 024 76575870
>
>>>> Robin Hankin <r.hankin at noc.soton.ac.uk> 06/20/05 10:32am >>>
> Hi
>
>
> I had a hard-to-find bug in some of my code the other day, which I
> eventually
> traced to my misusing of sweep().
>
> I would expect sweep() to give
> me a warning if the elements don't recycle nicely, but
>
>  X <- matrix(1:36,6,6)
>  sweep(X,1,1:5,"+")
>      [,1] [,2] [,3] [,4] [,5] [,6]
> [1,]    2    9   16   23   30   32
> [2,]    4   11   18   25   27   34
> [3,]    6   13   20   22   29   36
> [4,]    8   15   17   24   31   38
> [5,]   10   12   19   26   33   40
> [6,]    7   14   21   28   35   37
>
> gives no warning, even though (5,36)=1.
>
> Also,
>
>
> sweep(X,1,c(1,1000),"+")
>
> and
>
> sweep(X,2,c(1,1000),"+")
>
>
> behave as expected,   But
>
>
> sweep(X,1,1:5,"+")
>
> and
>
>  sweep(X,2,1:5,"+")
>
>
>
> are identical!
>
> ...which is also expected since the matrix is square. Perhaps the
> following will help you see why:
>> matrix(1:5,6,6)
>     [,1] [,2] [,3] [,4] [,5] [,6]
> [1,]    1    2    3    4    5    1
> [2,]    2    3    4    5    1    2
> [3,]    3    4    5    1    2    3
> [4,]    4    5    1    2    3    4
> [5,]    5    1    2    3    4    5
> [6,]    1    2    3    4    5    1
> Warning message:
> data length [5] is not a sub-multiple or multiple of the number of rows
> [6] in matrix
>> matrix(1:5,6,6, byrow = TRUE)
>     [,1] [,2] [,3] [,4] [,5] [,6]
> [1,]    1    2    3    4    5    1
> [2,]    2    3    4    5    1    2
> [3,]    3    4    5    1    2    3
> [4,]    4    5    1    2    3    4
> [5,]    5    1    2    3    4    5
> [6,]    1    2    3    4    5    1
> Warning message:
> data length [5] is not a sub-multiple or multiple of the number of rows
> [6] in matrix
> i.e. recycling a vector of length 5 down the columns gives the same
> result as recycling the same vector along the rows.
>
> I agree with your main point however, it would be useful if 'sweep'
> would give a warning when the length of the vector of statistics to be
> swept out was not a sub-multiple or multiple of the corresponding array
> dimension.
>
> Heather
>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595