> 80.to_s(2) => "1010000" > 80[0, 4] => 0 > 80[4, 4] => 5 参考 プロと読み解くRuby 2.7 NEWS - クックパッド開発者ブログ https://techlife.cookpad.com/entry/2019/12/25/121834

irb(main):027:0> str = "0123456789" => "0123456789" irb(main):028:0> str.slice(0, 5) => "01234" irb(main):029:0> str.slice(5, 5) => "56789"