Summing values within group

Algorithms
1

Hi,

I am trying to write a function that does simple aggregation by group.

Suppose I have two variables, groupid and values:

groupid values
1 3
1 2
1 4
2 2
2 2
2 3

The output I want is 2x1 vector,

9
7

Is there any easy way to go about this?

Thank you!

2

Are you trying to do this in Stan? It’s easy to work on rows or columns by slicing in Stan, but I’m not sure what 9 and 7 are supposed to represent. In general, if you have two variables:

array[N] int x;
array[N] int y;

vector[2] = [foo(x), bar(y)]';

Where foo and bar are the functions to apply to x and y.