Skip to content

Latest commit

 

History

History
76 lines (53 loc) · 2.12 KB

POEM_018.md

File metadata and controls

76 lines (53 loc) · 2.12 KB
Raw

POEM ID: 018
Title: indices and src_indices can contain slices
authors: Kenneth-T-Moore (Ken Moore)
Competing POEMs: [N/A]
Related POEMs: [N/A]
Associated implementation PR:

Status:

  • Active
  • Requesting decision
  • Accepted
  • Rejected
  • Integrated

Motivation

OpenMDAO, like other Python-based tools, has much-improved performance when using vectorized calculations. We really make use of this in Dymos, where a calculation on a scalar is performed simultaneously at n points (nodes). A scalar variable is stored in OpenMDAO with shape (n,).

If we want to model a 3-vector at n points, then the shape is (n, 3). A 3x3 matrix would be (n, 3, 3), etc.

Consider having a 3-vector representing position, and another calculation which only relies on the 3rd index (3rd column) of that vector. In straight up numpy, we would do something like:

input[:] = output[:, 2]

In OpenMDAO, src_indices is currentlty required to be a sequence of ints, and building it for a case like this is a non-trivial effort. Unfortunately, we can't pass [:, 2] as src_indices because it isn't legal Python. But Python does have a slice object that allows us to do this.

g.connect('x', 'y', src_indices=(slice(None), 2))

or alternatively, using numpy.s_:

import numpy as np
g.connect('x', 'y', src_indices=np.s_[:, 2])

This is also useful in OpenMDAO's set_val method on Problem. If we want to assign a column of an (n, 3) variable, its much easier to use the dictionary access, but that lacks unit conversion.

p['pos'][:, 2] = 5.0

p.set_val('pos', 5.0, indices=???)

Under the new notation, one could do

p.set_val('pos', 5.0, indices=(slice(None), 2))

or using numpy:

import numpy as np
p.set_val('pos', 5.0, indices=np.s_[:, 2)

Description

Those functions and methods which have arguments for indices or src_indices will allow them to be tuples which contain integers or slice objects. At setup time, these slice objects can be converted to an expanded list of numeric indices.