AssociationQ¶

Status: Stable

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

Description¶

AssociationQ[expr]

Gives True if expr is a valid Association (every entry a Rule or RuleDelayed), else False.

Examples (5)¶

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

Basic examples (3)¶

In[1]:= AssociationQ[<|"a" -> 1|>]
Out[1]= True

In[2]:= AssociationQ[{1, 2, 3}]
Out[2]= False

In[3]:= AssociationQ[Association[f[a -> 1]]]
Out[3]= False

Applications (2)¶

In[4]:= AssociationQ[<|a -> 1, b -> 2|>]
Out[4]= True

A list of rules is not an association

In[5]:= AssociationQ[{a -> 1, b -> 2}]
Out[5]= False

Implementation notes¶

Algorithm. builtin_associationq takes a single argument and returns the symbol True or False according to assoc_is_wellformed (assoc_struct.h). Having the Association head is not sufficient: the constructor leaves a malformed Association[1, 2], or a Map/Apply result such as Association[f[a -> 1]], unevaluated, and such a node is an ordinary expression, not an association. The well-formedness test therefore requires every entry to be a two-argument Rule or RuleDelayed.

Data structures. A pure structural scan over the argument's child array; no hash index is built and the argument is only read, never copied.

Complexity / limits. O(n) in the number of entries (a bare <|...|> literal that already carries a validated shape still re-scans). A bare list of rules {a -> 1} answers False — it is a List, not an Association. The head is a *Q predicate: it always returns a Boolean and never stays unevaluated.

Attributes: Protected.

References¶

See also: Rule, RuleDelayed

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

Notes & additional examples¶

Notes¶

AssociationQ[expr] returns True only for a well-formed association — one with the Association head whose every entry is a two-argument Rule or RuleDelayed. A bare list of rules such as {a -> 1, b -> 2} is a List, not an association, so it answers False; likewise a malformed Association[1, 2] left unevaluated by the constructor. Like every *Q predicate it always returns a Boolean, never staying unevaluated.