array_keys — 配列のキーすべて、あるいはその一部を返すarray array_keys ( array $array [, mixed $search_value = "" [, bool $strict = false ]] ) PHP: array_keys - Manual arra = [7,5,4,1,4,5,6,5] arrb = Array.new (0...arra.size).each {|i| arrb…