Association

Association[key1val1,key2val2,…] or key1val1,key2val2,…

represents an association between keys and values.

Details

  • The value associated with a given key can be extracted using assoc[key].
  • An Association acts like a symbolically indexed list. The value associated with a given key can be extracted by using the part specification Key[key]. If key is a string, Key can be omitted.
  • If key is not present in assoc, assoc[key] yields Missing["KeyAbsent",key].
  • Typical list operations (such as Map, Select, and Sort) apply to the values in an association, leaving the keys unchanged.
  • Association[{key1->val1,…}] gives <|key1->val1,…|>.
  • If there are multiple elements with the same key, all but the last of these elements are dropped. Merge yields instead a list of values for repeated keys.
  • Association can be input using the characters \[LeftAssociation] and \[RightAssociation].
  • Normal converts an Association to a list of rules.
  • KeyValuePattern can be used to represent a pattern for associations that include particular elements.

Examples

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Basic Examples  (8)

An association in which key a is associated with value x, key b with value y, etc.:

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Extract the value associated with key b:

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Convert a list of rules to an association:

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Convert an association to a list of rules:

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Keys are "transparent" for many operations:

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Position gives keys in an association:

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Append to an association:

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Delayed rules can be used to construct associations whose values are stored unevaluated:

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The value in the association is evaluated when it is extracted:

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Values in associations can be extracted like parts:

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If the key in an association is a string, no explicit Key is needed to extract it as a part:

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Numerical part specifications work structurally on associations:

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Lookups in associations interoperate with parts in lists and other expressions:

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The explicit Key can be omitted when the keys are strings:

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Keys extracts the list of keys:

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Associations can be modified by resetting values:

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Scope  (11)

Properties & Relations  (5)

Possible Issues  (2)

See Also

List  Dataset  Normal  AssociationQ  Lookup  KeyExistsQ  Keys  Values  KeyValuePattern  SparseArray  CompoundElement

Introduced in 2014
(10.0)
| Updated in 2016
(10.4)