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KeyUnion[{assoc1,assoc2,…}]

generates a list of associations in which each association has the union of the keys of the associ, padding by inserting values of Missing[…] if necessary.

KeyUnion[{assoc1,assoc2,…},f]

uses f[key] as the value associated with a missing key.

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KeyUnion

KeyUnion[{assoc1,assoc2,…}]

generates a list of associations in which each association has the union of the keys of the associ, padding by inserting values of Missing[…] if necessary.

KeyUnion[{assoc1,assoc2,…},f]

uses f[key] as the value associated with a missing key.

Details

  • KeyUnion returns a list of associations in which all the associations have keys ordered in the same way.
  • If f is an association containing all the keys to be returned by KeyUnion, then the order of keys in f will determine the order of keys in the associations returned.
  • In other cases, the keys will appear in order of their first occurrence in the sequence of associ.
  • KeyUnion can be used not only on Association objects, but also on lists of rules.

Examples

Basic Examples  (4)

Make a list of associations with the same keys:

Wolfram Language code: KeyUnion[{<|a -> 1, b -> 2|>, <|b -> 3|>}]

Reorder the keys of all associations:

Wolfram Language code: KeyUnion[{<|a -> 1, b -> 2, c -> 3|>, <|c -> z, a -> x, b -> y|>}]

Use a function to provide missing values:

Wolfram Language code: KeyUnion[{<|a -> 1, b -> 2, c -> 3|>, <|b -> y|>}, # ^ 2&]

The missing function can be an association:

Wolfram Language code: KeyUnion[{<|a -> 1, b -> 2|>, <|c -> 3|>}, <|a -> x, b -> y, c -> z|>]
Wolfram Research (2014), KeyUnion, Wolfram Language function, https://reference.wolfram.com/language/ref/KeyUnion.html.

Text

Wolfram Research (2014), KeyUnion, Wolfram Language function, https://reference.wolfram.com/language/ref/KeyUnion.html.

CMS

Wolfram Language. 2014. "KeyUnion." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/KeyUnion.html.

APA

Wolfram Language. (2014). KeyUnion. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/KeyUnion.html

BibTeX

@misc{reference.wolfram_2026_keyunion, author="Wolfram Research", title="{KeyUnion}", year="2014", howpublished="\url{https://reference.wolfram.com/language/ref/KeyUnion.html}", note=[Accessed: 13-June-2026]}

BibLaTeX

@online{reference.wolfram_2026_keyunion, organization={Wolfram Research}, title={KeyUnion}, year={2014}, url={https://reference.wolfram.com/language/ref/KeyUnion.html}, note=[Accessed: 13-June-2026]}