How to generate knock-out vectors in Chapel?

Question

I have a matrix, yes, A. Operating on her rows, I often need to create "knock-out" vectors. Basically

var v = [5, 4, 3, 2, 1];
v_{-2} = [5, 3, 2, 1]; // e.g. v[2] is removed

I don't want to remove it permanently, just for this calculation, and I want to do it along the rows of A.

var knockouts: [A.dim(1)] int;  // list of knockout dims, as tall as A
for i in A.dim(1) {
  var w = ||v_{-knockouts[i]}|| / ||v||
}

== UPDATE ==

More on A to keep it general. It is very large and (as is my wont) sparse. The elements knocked out are expected to be within the populated subdomain, but may not be in some cases. The entries are often probabilities, as in a stochastic matrix, so a few common row operations are

r = A[i,..]
s = r[3] / sum(r_{-3})
s = sum(r[3] log(r_{-3}))
s = sum(log (r_{-3})) / sum (log (r_{-5}))

With all the logging going on, it may not be safe to set r[3] = 0. But if that is the solution, it would still be nice to have a convenience function to do it under the covers. I don't recall seeing one but, maybe sum(r.except(3)) or another syntax.

Could you clarify what you mean by "just for this calculation"? Is it your intention to create temporary arrays for each knockout, or to modify `A` for each knockout? — ben-albrecht, Sep 01 '17 at 00:36
How about using an idea similar to a boolean overlay mask(-ing-)-{ vector | array }, which helps to "hide" those temporarily knock-out-ed ( zones-of-)-values in **`A`**? — user3666197, Sep 01 '17 at 00:58
@bencray I just need to get the value, next time I look at the row I will need the whole row. So for instance, compare the norm with/without the 3rd entry. — Brian Dolan, Sep 01 '17 at 01:58
@user3666197 I don't know what that is, can you point me to something? Thx — Brian Dolan, Sep 01 '17 at 03:02
For the norm case, I would think that temporarily setting those elements to `0` would be more efficient than popping / inserting elements. Would a solution like that work for you, or are there other use-cases that make removing the element necessary? — ben-albrecht, Sep 01 '17 at 05:15

score 1 · Accepted Answer · answered Sep 01 '17 at 23:19

I don't know of a good way to do this "in-place" such that no temporary arrays are created.

For now, here's an approach that knocks out indices by creating a temporary array:

var v = [5, 4, 3, 2, 1];
writeln(exclude(v, 2));     // 5 3 2 1
writeln(exclude(v, 3));     // 5 4 2 1


/* Returns array with element at idx excluded */
proc exclude(A: [], idx) {
  var v1 = A[..idx-1];
  v1.push_back(A[idx+1..]); // See NOTE
  return v1;
}

NOTE: Passing arrays to push_back() is not supported in Chapel 1.15. It has been added in #7180.

How to generate knock-out vectors in Chapel?

1 Answers1