# How to find the location of a value in a vector

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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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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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.

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