The number of data points in matrix and vector for

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

set.seed(5)  
X <- cbind(rnorm(1000,0,0.5), rnorm(1000,0,0.5), rnorm(1000,0,0.5))

Variable selection is performed, then the condition will be checked before performing the next operation as follows.

if(nrow(X) < 1000){print(a+b)}

,where a is 5 and b is 15, so if nrow(X) < 1000 is TRUE, then 20 will be printed out. However, in case that X happens to be a vector because only one column is selected,

how can I check the number of data points when X can be either a matrix or vector ?

What I can think of is that

  if(is.matrix(X)){
  n <- nrow(X)
  } else {
  n <- length(X)}
  if(n < 1000){print(a+b)}

Anyone has a better idea ?

Thank you

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