WolframLang: Association, Check Key Exist

Check Key Exist

KeyExistsQ[ assoc, key ]

returns True if the key exist, else False.
KeyExistsQ

KeyExistsQ[ Association[ a -> 3,  b -> 2 ], b ]

Check Key Exist by Pattern

KeyMemberQ[ assoc, pattern]

return True if pattern match any key in assoc, else False. [see WolframLang: Pattern Syntax] KeyMemberQ

(* check key exist by pattern *)

xx = Association[ a -> 3, b -> 2 ];
KeyMemberQ[ xx, _Integer ] === False

yy = Association[ a -> 3, 2 -> b ];
KeyMemberQ[ yy, _Integer ] === True

KeyMemberQ[ assoc, pattern]

opposite of KeyMemberQ KeyFreeQ

Check Value Exist by Pattern

MemberQ[assoc, pattern]

check value exist by Pattern. Return True if pattern matches a value.

(* check value exist by pattern *)

xx = Association[ a -> 3, b -> 2 ];

MemberQ[ xx, 3 ] === True
MemberQ[ xx, _Integer ] === True

MemberQ[ yy, 9 ] === False

xx = Association[ a -> {9, 1}, b -> { 3, 6} ];

(* check if there's a value that's list of pairs with second part a even number *)
MemberQ[ xx, {_, _?EvenQ} ]