gives an association that groups the elemi into lists associated with distinct keys f[elemi].


groups the fv[elemi] according to the fk[elemi].


groups into nested associations using fsi at level i.


applies the function red to reduce lists of values that are generated.


represents an operator form of GroupBy that can be applied to an expression.


  • GroupBy provides a generalization of the map reduce operation.
  • GroupBy[list,f] gives an association whose keys are the distinct f[elemi] and whose values are sublists of the list list.
  • GroupBy[assoc,f] gives an association whose keys are the distinct f[elemi] and whose values are subassociations of the association assoc.
  • GroupBy[spec][expr] is equivalent to GroupBy[expr,spec].


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Basic Examples  (3)

Group pairs of items according to the first element of the pair:

Group values of an association:

Group by the first element and compute the mean of the corresponding last elements:

Scope  (12)

Group the elements repeatedly by using several functions:

Group last parts of pairs according to the first part:

Group values of an association by using several functions:

Use a symbolic selector for grouping:

Pairs for which the selector yields the same value are grouped under the same key:

Use different functions to extract keys and values:

Use the operator form of Extract to specify the key or the value:

Use a combiner function to combine the values:

Group associations according to the value of their first element:

Group associations according to the value of the given key:

Specify keys and values using Key:

Missing keys are replaced with Missing:

Group a list of integers by positivity and parity, displaying the groups as column vectors:

Use the operator form of GroupBy:

Group results of applying a function to values of an association:

Applications  (1)

Use GroupBy to group rows in a dataset, introducing new levels of association:

Group the passengers by class:

Group the passengers by sex and then class:

Properties & Relations  (2)

GroupBy returns an association, while GatherBy returns a list:

Grouping by f is equivalent to grouping by fIdentity:

Introduced in 2014