KeySelect¶

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description¶

KeySelect[assoc, pred]

Keeps the entries whose key satisfies pred.

Examples (3)¶

Every input below was run against the current Mathilda build and its output recorded.

Basic examples (1)¶

In[1]:= KeySelect[<|1 -> 10, 2 -> 20, 3 -> 30|>, EvenQ]
Out[1]= <|2 -> 20|>

Applications (2)¶

Keep even-numbered keys

In[2]:= KeySelect[<|1 -> a, 2 -> b, 3 -> c, 4 -> d|>, EvenQ]
Out[2]= <|2 -> b, 4 -> d|>

In[3]:= KeySelect[<|1 -> a, 2 -> b, 3 -> c, 4 -> d|>, OddQ]
Out[3]= <|1 -> a, 3 -> c|>

Implementation notes¶

Algorithm. builtin_keyselect keeps the entries whose key passes a predicate. For each entry it forms pred[key] (apply1), evaluates it, and keeps the whole entry only when the verdict is literally the symbol True; any other result drops the entry. The surviving entries, in their original order, form a fresh Association.

Data structures. A single Expr** output array sized to the entry count; the predicate application is a throwaway evaluated per key.

Complexity / limits. O(n), one predicate evaluation per entry. The companion KeySelect tests keys; Select (and Discard) over an association test the values instead. A non-True verdict — including an unevaluated predicate call — counts as rejection.

Attributes: Protected.

References¶

Source: src/assoc.c
Specification: docs/spec/builtins/data-structures.md
Tests: tests/test_assoc_forms.c
Tests: tests/test_association.c

Notes & additional examples¶

Notes¶

The predicate is applied to each key; an entry is kept only when the result is exactly True. Order is preserved. To filter by the values rather than the keys, use Select over the association.