[Kanish2002]

When no observations remain, valueCounts rebuilds its observed-value column from an empty list. For a ColumnGroup, this loses the child columns. DataFrame counting uses a temporary group, so df.take(0).valueCounts("city") can return [rows, count] rather than [city, count].

Use an empty slice for structured columns when there are no distinct values. Value columns keep the existing type-aware construction, preserving their runtime nullability according to dropNA. Nonempty counting and public signatures are unchanged.

Regression coverage includes empty all-column/selected-column frames, all rows excluded by dropNA using a genuinely nullable String fixture, an empty ColumnGroup with non-nullable children, and empty nullable value columns with dropNA both enabled and disabled. The website description documents the empty result.

Fixes #2167.

Validation:

• Isolated Kotlin/JUnit: all 32 ValueCountsTests pass. The new empty nullable value-column regression fails on the previous PR head and passes with this update.
• Compiled the actual API source, unchanged current annotations/Plugin.kt, and unchanged current impl/columns/ValueColumnImpl.kt with Kotlin 2.4.21, targeting JVM 8, against published dataframe-core 1.0.0-rc01. Including the current implementation exercises its current slicing/nullability behavior rather than the older published implementation. Tests target JVM 11; friend paths expose library internals.
• Standalone ktlint and git diff --check pass.
• The repository debug test command was attempted, but the Gradle 9.8.1 distribution download is blocked by network access. Full-module/debug validation and KDoc processing remain unverified locally.

Found while preparing #2165, but independent of that API proposal. AI-assisted.

[Jolanrensen]

@Kanish2002 thanks for the bugfix! I'll take it from here :)

[Jolanrensen]

Made some tiny fixes, but we need to merge the fix for #2034 first so the tests pass as expected :)

[jetbrains-air]

Nice, targeted fix — just two test expectations to sort out before this can go green. 🙂

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[jetbrains-air]

Thanks for digging into this one — the structure-preserving slice is the right call; just one blocking note inline before merge.

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[Jolanrensen]

@Kanish2002 thanks, I'll run the CI