WolframLang: Association: Filter

By Xah Lee. Date: . Last updated: .

Truncate Association

Take[ assoc, n ]
take first n items. Take 
xx = Association[ a -> 1, b -> 2, c -> 3, d -> 4 ];

Take[ xx, 2 ] === <|a -> 1, b -> 2|>

Filter by List of Keys

KeyTake[assoc, {key1, key2 etc}]
return an association containing only the elements with give keys. KeyTake 
xx = Association[ a -> 1, b -> 2, c -> 3, d -> 4 ];

KeyTake[ xx, {a, c} ] === <|a -> 1, c -> 3|>
KeyTake[{assoc1, assoc2 etc}, keys]
gives a list of associations.

Filter by Function on Value

Select[ assoc, f]
filter assoc by function f applied to values that return true. Select 
xx = Association[ a -> 1, b -> 2, c -> 3, d -> 4 ];

Select[ xx, EvenQ ] === <|b -> 2, d -> 4|>

Filter by Function on Key

KeySelect[assoc, crit]
return a new assoc such that Function crit return True on keys. KeySelect 
xx = Association[ 1 -> a, 2 -> b, 3 -> c, 4 -> d ];

KeySelect[ xx, EvenQ ] === <|2 -> b, 4 -> d|>

KeySelect[ xx, Function[ x, Mod[x,3] === 0 ] ] === <|3 -> c|>

WolframLang: Association (Key Value Pairs)

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