Rui Rui - 1 month ago 9
R Question

Return a matrix with `ifelse`

I have two matrices:

mat <- matrix(1:6, 2, 3)
mat2 <- matrix(1:2, 2, 3)


and a parameter

a <- 1


using
ifelse
, is it possible to return a matrix when
a
is a certain value?
the code that I am using, does not work. For example:

mat.new <- ifelse(a == 1, mat, mat2)

Answer

The length of the return is completely decided by length(a == 1). See also the helpfile with ?ifelse. Your code will only return a single value.

ifelse targets vector input / output. Even if you get the length correct, say: ifelse(rep(TRUE, 6), mat, mat2), you get a vector rather than a matrix output. So an outer matrix call to reset dimension is necessary.


Tip 1:

For your example, looks like a simple result <- if (a == 1) mat else mat2 is sufficient. No need to touch ifelse.

Tip 2:

It is not impossible to ask ifelse to return a matrix, but you have to protect it by a list (remember a list is a vector):

ifelse(TRUE, list(mat), list(mat2))

But, this is inconvenient.