2

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

newbie
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1 Answers1

4

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]
Rich Scriven
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