[R] matrix indexing

[Marc Schwartz (via MN)]

On Wed, 2006-03-15 at 06:03 -0500, tom wright wrote:
> Can someone please give me a pointer here.
> I have two matrices
> 
> matA
> 	A	B	C
> 1	5	2	4
> 2	2	4	3
> 3	1	2	4
> 
> matB
> 	A	B	C
> 1	TRUE	FALSE	TRUE
> 2	FALSE	TRUE	TRUE
> 3	FALSE	FALSE	FALSE
> 
> how do I extract all the values from matA where the coresponding entry
> in matB == TRUE (or FALSE), perferably in vector form.
> 
> Many thanks
> tom

The subsetting/indexing is premised on the index values being TRUE,
thus:

> matA[matB]
[1] 5 4 4 3

> matA[!matB]
[1] 2 1 2 2 4

HTH,

Marc Schwartz