Associations
Along with lists, associations are fundamental constructs in the Wolfram Language. They associate keys with values, allowing highly efficient lookup and updating, even with millions of elements. Associations provide generalizations of symbolically indexed lists, associative arrays, dictionaries, hashmaps, structs, and a variety of other powerful data structures.
Association ( <|…|> ) — an association between keys and values
<|…|>[key] — extract the value associated with any given key
Associations and Parts
<|…|>[["str"]] — extract a value corresponding to a key that is a string
Key — indicate a key within a part specification
Missing — default value if a key is not found
#name — a slot in a pure function that picks out value with key "name" in an association
AssociationQ — test if an expression is a valid association
Elements of Associations
Keys — list of keys
Values — list of values
Normal — convert to a list of rules
Lookup — perform a lookup of a value by key, returning a specified default if it is not found
KeyExistsQ — test whether a key exists in an association
Functions That Apply to Values »
Map ▪ Select ▪ Sort ▪ DeleteDuplicates ▪ ListPlot ▪ Plus ▪ ...
Functions That Apply to Keys
KeySort, KeySortBy — sort an association by its keys
KeyTake, KeyDrop — take, drop particular keys in an association
KeySelect — select elements based on a criterion on their keys
KeyMap — map a function over the keys in an association
KeyValueMap — map a function over keys and values in an association
Modifying Associations
<…>[key]=val — change an element of an association
AssociateTo — add elements to an association
KeyDropFrom — drop elements from an association
Pattern Matching with Associations
KeyValuePattern — elements that can appear anywhere in an association
Functions That Create Associations
Association — turn a list of rules into an association
AssociationMap — create an association by applying a function to a list of keys
AssociationThread — create an association from a list of keys and a list of values
Counts, CountsBy — associate values with the number of times they occur
GroupBy — group values by collecting those sharing a criterion ("map reduce")
PositionIndex — build an index of positions at which values occur
Functions Operating on Lists of Associations
KeyUnion ▪ KeyIntersection ▪ KeyComplement
Catenate — catenate elements from multiple associations
Merge — merge associations using a function to combine elements with common keys
JoinAcross — do the analog of a database join on multiple associations
Dataset — representation supporting general structured data queries