Hi,

You can set the size of the vector and use indexing to index and change the vector. Or you can use the function append(), to append a value to a vector. In any case you need to iterate over a sequence.

n <- 100

ctrl <- c(rep(NA, n)) # a vector with NAs across 100 positions 
trt <- c(rep(NA, n)) # a vector with NAs across 100 positions 

for ( i in seq(n) ) {

   ctrl[i] <- i # add 1, 2, 3 .... in position 1, 2, 3 .....

   trt[i] <- i # add 1, 2, 3 .... in position 1, 2, 3 .....
}

If the vector does not have a predefined length you can use the append function, like:

n <- 100

ctrl <- c() 
trt <- c() 

for ( i in seq(n) ) {

   if ( n < 50) {
        ctrl <- append(ctrl, i) # add 1, 2, 3 .... 49
   } else {
        trt <- append(trt, i) # add 50, 51, 52 .... 100
   }

}

You need to define conditions, but it is not clear to me from your description.

António