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Clarify that boolean array indexing requirements #164

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Apr 19, 2021
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Clarify that boolean array indexing requirements #164

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4 changes: 2 additions & 2 deletions spec/API_specification/indexing.md
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Expand Up @@ -160,7 +160,7 @@ _Rationale: this is consistent with bounds-checking for single-axis indexing. An

## Boolean Array Indexing

An array must support indexing via a **single** `M`-dimensional boolean array `B` with shape `S1 = (s1, ..., sM)` according to the following rules. Let `A` be an `N`-dimensional array with shape `S2 = (s1, ..., sM, ..., sN)`.
An array must support indexing where the **sole index** is an `M`-dimensional boolean array `B` with shape `S1 = (s1, ..., sM)` according to the following rules. Let `A` be an `N`-dimensional array with shape `S2 = (s1, ..., sM, ..., sN)`.

- If `N >= M`, then `A[B]` must replace the first `M` dimensions of `A` with a single dimension having a size equal to the number of `True` elements in `B`. The values in the resulting array must be in row-major (C-style order); this is equivalent to `A[nonzero(B)]`.

Expand All @@ -184,4 +184,4 @@ The result of an indexing operation (e.g., multi-axis indexing, boolean array in
```{note}

The specified return value behavior includes indexing operations which return a single value (e.g., accessing a single element within a one-dimensional array).
```
```