AssociationThread

Status: Stable

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

Description

AssociationThread[{k...}, {v...}] | AssociationThread[keys -> values]

Builds \<|k1 -> v1, ...|> from parallel key and value lists.

Examples (4)

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

Basic examples (2)

In[1]:= AssociationThread[{"a", "b"}, {1, 2}]
Out[1]= <|"a" -> 1, "b" -> 2|>

In[2]:= AssociationThread[{"a", "b"} -> {1, 2}]
Out[2]= <|"a" -> 1, "b" -> 2|>

Applications (2)

In[3]:= AssociationThread[{a, b, c}, {1, 2, 3}]
Out[3]= <|a -> 1, b -> 2, c -> 3|>

The keys -> values rule form

In[4]:= AssociationThread[{a, b} -> {1, 2}]
Out[4]= <|a -> 1, b -> 2|>

Implementation notes

Algorithm. builtin_associationthread accepts either the two-argument form AssociationThread[keys, values] or the single rule form AssociationThread[keys -> values], both requiring two equal-length Lists. It zips them into Rule[key_i, value_i] nodes and canonicalises the result through assoc_from_rules, so a repeated key keeps its first position and takes its last value (the same de-duplication rule as Association).

Data structures. The temporary rule array is handed to assoc_from_rules, which drives a transient KeyIndex open-addressing hash set over the keys to de-duplicate in one pass; the rules are deep-copied into the result and the temporaries freed.

Complexity / limits. O(n) amortised. Returns NULL (unevaluated) unless both arguments are Lists of the same length, matching Wolfram's shape requirement.

Attributes: Protected.

References

• T. H. Cormen, C. E. Leiserson, R. L. Rivest and C. Stein, Introduction to Algorithms, 3rd ed. (MIT Press, 2009), §11 (hash tables, open addressing).
• Source: src/assoc.c
• Specification: docs/spec/builtins/data-structures.md
• Tests: tests/test_association.c
• Tests: tests/test_graph.c

Notes & additional examples

Notes

AssociationThread[keys, values] zips two equal-length lists into an association, pairing keys[[i]] with values[[i]]. The single-argument rule form AssociationThread[keys -> values] is equivalent. It is the inverse of taking Keys and Values apart, and the quickest way to build an association from two parallel lists. Duplicate keys collapse with the usual first position, last value rule; the two lists must have the same length.