Repeat a vector n times

General
1

Hello,

Question 1: I’m trying to repeat a row_vector n times using:

model {
//...
row_vector[2] myvector = [1,2];
row_vector[6] myvector2;

myvector2 = rep_row_vector(myvector, 3)
//...
}

I get:
No matches for:

rep_row_vector(row vector, int)

Available argument signatures for rep_row_vector:

rep_row_vector(real, int)

Question 2: I suppose the above performs the equivalent in R to rep(c(1,2),times = 3). How about rep(c(1,2), each = 3) in Stan?

Thanks in advance.

2

There isn’t a rep_ function to copy a (row) vector into a longer (row) vector. You could do something like

row_vector[6] = append_col(append_col(my_vector, my_vector), my_vector);
1 Like
3

Thank you @bgoodri for your quick answer. Any alternative to repeat each element of vector n times as in R rep(my_vector, each = n) ?

4

You would have to just do a loop.

1 Like
5

@bgoodri what would such a loop look like (in this case) ?

6

Here’s one approach. The defined function can then be used in any other block.

(Please note: There may very well be more efficient ways of doing this - I’m not a programmer by training or trade - as should be evident from the number of edits…).

functions{

  vector repeat_vector(vector input, int reps){

    int index[rows(input) * reps];

    for (i in 1:reps){
      for (j in 1:rows(input)){       
        index[j + rows(input) * (i-1)] = j;
      }
    }    
    return(input[index]);
  }
}

Edit: Doh, saw now that you wanted the rep() function with the “each” argument. Then you have to switch the construction of the index vector around, like this:

functions{

  vector repeat_vector(vector input, int reps){

    int index[rows(input) * reps];

    for (i in 1:rows(input){
      for (j in 1:reps){       
        index[j + rows(input) * (i-1)] = i;
      }
    }    
    return(input[index]);
  }
}
2 Likes
7

Thank you @erognli, I could work around the rep(x, each = n) function, by defining an index in R. But this may be helpfull in the future!

8

Here’s code for repeating a vector K times, and an N*M matrix K times:

functions {
  vector repeat_vector(vector input, int K) {
    int N = rows(input);
    vector[N*K] repvec; // stack N-vector K times

    for (k in 1:K) {
      for (i in 1:N) {
        repvec[i+(k-1)*N] = input[i]; // assign i-th value of input to i+(k-1)*N -th value of repvec
      }
    }
    return repvec;
  }

  matrix repeat_matrix(matrix input, int K) {
    int N = rows(input);
    int M = cols(input);
    matrix[N*K,M] repmat; // stack N*M matrix K times

    for (k in 1:K) {
      for (i in 1:N) {
        repmat[i+(k-1)*N] = input[i]; // assign i-th row of input to i+(k-1)*N -th row of repmat
      }
    }
    return repmat;
  }
}
1 Like