assoc

Searches through an array whose elements are also arrays comparing obj with the first element of each contained array using obj.==.

Returns the first contained array that matches (that is, the first associated array), or nil if no match is found.

See also Array#rassoc

s1 = [ "colors", "red", "blue", "green" ]
s2 = [ "letters", "a", "b", "c" ]
s3 = "foo"
a  = [ s1, s2, s3 ]
a.assoc("letters")  #=> [ "letters", "a", "b", "c" ]
a.assoc("foo")      #=> nil