(1) Using 'C == TRUE' (when you know C is logical)
is equivalent to just plain C, only obscure.
Similarly, 'C == FALSE' is more confusing than !C.
(2) Consider B[C]. The rows of C have 2, 1, 1, 2, 1 TRUE.
entries, so the result here *cannot* be a rectangular array.
And whatever it is, it contains only the elements where C
is true.
(3) You probably already knew that 'ifelse' is vectorised.
What you may not have realised is that it preserves
array dimensions as well.
> A <- cbind(c(1,2), c(3,4))
> B <- cbind(c(5,6), c(7,8))
> C <- cbind(c(FALSE,TRUE), c(TRUE,FALSE))
> ifelse(C, A, B)
[,1] [,2]
[1,] 5 3
[2,] 2 8
> ifelse(C, A, 0)
[,1] [,2]
[1,] 0 3
[2,] 2 0
Isn't it nice when the obvious code just works?
On Sun, 6 Sep 2020 at 06:18, Vivek Sutradhara <viveksu...@gmail.com> wrote:
> Hi
> I would like to get help in combining two matrices. Here is my example:
> A <- 1:20
> B <- matrix(A,nrow=5,ncol=4)
> # B is a numerical matrix
> C <- B<7
> C[4,4] <- TRUE
> # C is a logical matrix
> # if I combine A and C, I get a vector
> D1 <- A[C==TRUE]
> D1
> D2 <- A[C==FALSE]
> D2
>
> I want to get a matrix with the same dimensions as matrix A. At the
> coordinates given by the vector D1, I want to retain the values in
> matrix A. At the locations in D2, I want a zero value.
> I want to know if I can do this without using any loops.
> Thanks, Vivek
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
[[alternative HTML version deleted]]
______________________________________________
R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
