Allow empty sub-containers in reductions #129
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majosm
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This has picked up some merge conflicts. Could you take a look? Also, ready to un-draft? |
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| if result is None: | ||
| raise ValueError("cannot reduce empty array container") |
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Personally, I think reducing over an empty container is well-defined: just return the neutral element. reduce_func should be trusted to do the right thing. What was the reasoning for putting in this error message? Basically, what I'm pushing for is to get rid of it and return reduce_func([]) instead.
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If reduce_func worked for an empty list, I think this PR wouldn't be needed, right?
The motivation for this was for things like actx.np.max where reduce_func doesn't (currently) behave nicely for empty inputs. Should that maybe be fixed instead?
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For reference, calling np.max on an empty list raises
>>> np.max([])
ValueError: zero-size array to reduction operation maximum which has no identity
and requires specifying an initial value with np.max([], initial=np.inf). We should probably do something like that in actx.np too.
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The motivation for this was for things like
actx.np.maxwherereduce_funcdoesn't (currently) behave nicely for empty inputs.
That's a great point. I had not thought far enough to realize that, but I agree now.
Should that maybe be fixed instead?
I think that's exactly what we should do.
We should probably do something like that in
actx.nptoo.
I agree. While I wasn't loving numpy's interface of "no implicit neutral" element at first, I think it makes sense from the perspective of integers. I think we should aim to replicate this, both at the level of actx.np, and for the array container reductions under discussion here.
One subtlety is the ValueError for empty arrays. For symbolically-shaped pytato arrays, it may not be possible to do a perfect job.
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Did some work towards that:
- Introduce
ReductionOperationclass, accept 'initial' in reductions pytato#238 - Match numpy interface for empty reductions #136
That latter PR needs some more work done to it. @majosm, could you take a look?
| if result is None: | ||
| raise ValueError("cannot reduce empty array container") |
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Allows some (but not all) of the sub-containers in an array container to be empty in reductions. This allows reductions to be used on single-species CV objects in mirgecom.
Depends on #128 (not in an essential way, just avoiding dealing with merge conflicts).