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Summary

Adds pandas-style boolean mask indexing to Deedle, addressing the long-standing feature request in #318.

Closes #318.

New Functions

Series.filterByMask

// Filter a series using a boolean mask series
let s = series [ "a" => 1.0; "b" => 2.0; "c" => 3.0; "d" => 4.0 ]
let mask = series [ "a" => true; "b" => false; "c" => true; "d" => false ]
let result = s |> Series.filterByMask mask
// result: series [ "a" => 1.0; "c" => 3.0 ]

Frame.filterRowsByMask

// Filter frame rows using a boolean Series<'R, bool>
let adults = df.GetColumn(int)("Age") |> Series.mapValues (fun age -> age >= 18)
let filtered = df |> Frame.filterRowsByMask adults

Frame.filterColsByMask

// Filter frame columns using a boolean Series<'C, bool>
let wantedCols = series [ "A" => true; "B" => false; "C" => true ]
let filtered = df |> Frame.filterColsByMask wantedCols

All three functions exclude keys missing from the mask (treated as false).

Design Notes

• Composable: works naturally with Series.mapValues, Series.map, or any expression that produces a Series<'K, bool>
• Missing-safe: mask entries with OptionalValue.Missing are treated as false
• Consistent with existing API: follows the same conventions as filterRows, filterValues, etc.
• C# compatible: [(CompiledName)] attributes provide idiomatic PascalCase names: FilterByMask, WhereRowsByMask, WhereColsByMask

Test Status

✅ 7 new tests added, all 501 tests pass (previously 494 tests).

Tests cover:

• Filtering series by mask (all true, mixed, all false)
• Filtering with mask that has fewer keys than the series
• Filtering frame rows by mask
• Filtering frame columns by mask
• Derived boolean mask from column predicate (the idiomatic pandas-style usage)

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