How to find the location of a value in a vector

General
1

I want to find the location of a value in a vector. E.g.,
In R
x <- c(1, 5, 9, 10)
y <- 5
which(x==y) would give me the answer 2. is there a corresponding function in Stan?

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2

There’s some discussion of this in this post:

I made a few edits to the functions @Bob_Carpenter suggested in that post, and I think the following should work:

int num_matches(vector x, real y) {
  int n = 0;
  for (i in 1:rows(x))
    if (x[i] == y)
      n += 1;
  return n;
}
  
int[] which_equal(vector x, real y) {
  int match_positions[num_matches(x, y)];
  int pos = 1;
  for (i in 1:size(x)) {
    if (x[i] == y) {
      match_positions[pos] = i;
      pos += 1;
    }
  }
  return match_positions;
}

To use it you would do something like this:

int ids[num_matches(x, y)] = which_equal(x, y);

and ids would then contain the indexes of the elements in x that equal y. If you know in advance that there can only be one value of x that equals y then you can simplify all this because num_matches wouldn’t be necessary.

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3

Thanks! I ended up doing something similar, but you reminded me that I can define such as function. I guess Stan doesn’t have any existing function for this.

1 Like
4

Glad you figured something out.

Not yet unfortunately.