Question

I am running a loop in R to find indices of a vector when its elements are equal to elements of a reference vector.

As far as I know R, I need to declare the variable before the for-loop, but in this case I do not know the final length of my indices vector (see code below).
How can I create a variables that allows R to change its size during the for loop?

extract of my code:

k <- 1
for(i in 1:length(Lid.time)){
  ind <- which(Net.time==Lid.time[i])
  if(length(ind)>0){
    ind.Net[k] <- ind
    k <- k+1
  }
}

Notes about the code:
Lid.time is a vector of a different lenght than Net.time.
I need to find an array of indices that tells me where Net.time is equal to Lid.time. I do not know in advance how long will the ind.Net vector will be, so how can I declare the vector ind.Net?

Thanks for your help

Answer 1

As Dason stated, match will work just fine for that specific task:

>a <- seq(2,20,2)
   #[1]  2  4  6  8 10 12 14 16 18 20
>b <- c(4,14,18)
>match(b,a)
   #[1] 2 7 9  # The indices!
>a %in% b #shorthand logical version of match
   #[1] FALSE  TRUE FALSE FALSE FALSE FALSE  TRUE FALSE  TRUE FALSE

But to answer your question of a vector of unknown length within a loop:

Vector <- c()
for(i in sample(1:100,20)) {
    if(i<50) {Vector <- append(Vector, i)}
    }
length(HowLongIsThisVector)

It will be different every time you run it because of sample.