[R] Matrix indexes
Prof Brian Ripley
ripley at stats.ox.ac.uk
Mon Jan 12 11:17:03 CET 2004
On Mon, 12 Jan 2004, W. Beldman wrote:
> Two questions about matrix indexing:
Not about `matrix indexing', which may have confused you. Matrix indexing
of a matrix is when the index is a two-column matrix of row-column number
pairs.
> Is is correct that V <- V[lower.tri(V, diag=TRUE)] returns the lower
> triangular of matrix V, that is: all elements above diagonal are set to zero? I
No, omitted.
> understand that the triangle of matrix elements of V for which lower.tri is
> TRUE are returned while the others (above diagonal) are set to zero (or NA ???).
So this is vector indexing by a logical vector. It does no harm to try
it:
> V <- matrix(1:16, 4,4)
> V
[,1] [,2] [,3] [,4]
[1,] 1 5 9 13
[2,] 2 6 10 14
[3,] 3 7 11 15
[4,] 4 8 12 16
> lower.tri(V, diag=TRUE)
[,1] [,2] [,3] [,4]
[1,] TRUE FALSE FALSE FALSE
[2,] TRUE TRUE FALSE FALSE
[3,] TRUE TRUE TRUE FALSE
[4,] TRUE TRUE TRUE TRUE
> V[lower.tri(V, diag=TRUE)]
[1] 1 2 3 4 6 7 8 11 12 16
so the result is the lower triangle read out as a vector in the usual
first-index-varies fastest order.
> If D and B are vectors of logicals,
> what does V contain after V <- v[D, B, drop=FALSE] ?
> I guess that elements are returned if both indexes D and B are TRUE, but I'm
> not really convinced... (And again, what about the other elements?)
Yes. Again, simple example:
> B <- c(T, F, T, F)
> D <- c(T, T, F, F)
> outer(B, D)
[,1] [,2] [,3] [,4]
[1,] 1 1 0 0
[2,] 0 0 0 0
[3,] 1 1 0 0
[4,] 0 0 0 0
> V[B, D]
[,1] [,2]
[1,] 1 5
[2,] 3 7
And finally an example of matrix indexing
> U <- matrix(c(1,1,2,3), 2, 2, byrow=T)
> U
[,1] [,2]
[1,] 1 1
[2,] 2 3
> V[U]
[1] 1 10
> These are probably tutorial questions, but I'm still not sure after reading "R
> Language Definition (draft): Evaluation of expressions" and applicable sections
> of "The R Reference Manual". Thanx!
Some of the books in the FAQ, notably `S Programming', will help a lot.
--
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
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