The number of data points in matrix and vector forms

Question

Supposed that X contains 1000 rows with m columns, where m equal to 3 as follows:

set.seed(5)  
X <- cbind(rnorm(1000         


        

Answer 1

You can use NROW for both cases. From ?NROW

nrow and ncol return the number of rows or columns present in x. NCOL and NROW do the same treating a vector as 1-column matrix.

So that means that even if the subset is dropped down to a vector, as long as x is an array, vector, or data frame NROW will treat it as a one-column matrix.

sub1 <- X[,2:3]
is.matrix(sub1)
# [1] TRUE
NROW(sub1)
# [1] 1000
sub2 <- X[,1]
is.matrix(sub2)
# [1] FALSE
NROW(sub2)
# [1] 1000

So if(NROW(X) < 1000L) a + b should work regardless of whether X is a matrix or a vector. I use <= below, since X has exactly 1000 rows in your example.

a <- 5; b <- 15
if(NROW(sub1) <= 1000L) a + b
# [1] 20
if(NROW(sub2) <= 1000L) a + b
# [1] 20

A second option would be to use drop=FALSE when you make the variable selection. This will make the subset remain a matrix when the subset is only one column. This way you can use nrow with no worry. An example of this is

X[, 1, drop = FALSE]