[R] Apply command on vectors
Elliot Williams
ewilliams at ucsd.edu
Tue May 29 02:38:48 CEST 2001
Hi all,
I keep running into a small problem in my programming, and I'm sure there's
an elegant way around it...
I often want to use apply() on a matrix with a variable number of columns. It
works just fine unless the number of columns is one, in which case the matrix
becomes a vector, and apply() complains. Example below:
-----------------------------
a.matrix <- matrix(rnorm(6), nc=3)
matrix.mask <- c(1,2)
apply(a.matrix[,matrix.mask], 1, mean) # Works just fine
matrix.mask <- 1
apply(a.matrix[,matrix.mask] ,1, mean) # Fails
Error in apply(a.matrix[, matrix.mask], 1, mean) :
dim(X) must have a positive length
-----------------------------
So what I usually do is explicitly cast the thing as a matrix using,
as.matrix(), but it's unnecessary in most cases (when the thing is a
full-fledged matrix)-- extra computation, moving memory around, etc.
Is there an elegant R fix? I guess I just want the subset of the matrix to
retain its matrixiness.
Thanks,
Elliot.
--
Elliot Williams (ewilliams at ucsd.edu)
Economics Department, UC San Diego
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