[DF] Add median support for RDataFrame - #23079
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August 17, 2026 15:39
Cover the Median action in the existing action-API tests for scalar and collection columns, and add dedicated cases for: - odd and even numbers of entries, with distinct middle values to check that the two central values are averaged - empty datasets for both scalar and collection columns, which return NaN - a skewed distribution in the multithreaded test fixture, verifying that per-slot buffers are merged correctly - cloning the action via MakeNew, checking that the original and the clone both produce the correct result within a single event loop Each case uses data where the mean and median differ, so a regression cannot pass by coincidence.
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August 17, 2026 16:42
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This Pull request:
Changes or fixes:
Adds a
Medianaction to RDataFrame, computing the exact median of a column.Draft - implementation is working; Google Test coverage and doxygen documentation still to come.Implemented as a
MedianHelperinActionHelpers.hxxfollowing the structure ofMeanHelper:values are buffered per slot in
Exec(with anIsDataContaineroverload for collection columns),and
Finalizemerges the per-slot buffers and usesstd::nth_elementto select the middleelement. Even entry counts return the average of the two middle values.
Scope, per the discussion in the issue: exact mode only.
GetMergeableValueis not implemented,so distributed execution is not supported - a median cannot be merged from a summary the way
RMergeableMeanmerges(mean, count), so that deserves separate discussion. Memory cost isO(N) doubles, which is inherent to an exact result.
Manually verified so far via the templated and jitted C++ paths, from Python, on
RVeccolumns,and with implicit multithreading enabled.
Checklist:
This PR is part of #22794