####2.1.Boolean Expressions（布尔运算）
== <= <= != && || !
####2.2条件表达式
if ... else
unless
boolean ? a : b #三元运算符
Tips:
nil or false are equal to false, object such as 1, 0, “” is equal to true Every other object like say 1, 0, “” are all evaluated to be true.
####2.3循环 loop
Array#each Array#select
例1
[1,2,3,4].select{ |e| e.odd? } #[1,3]
N.times N代表数字
例2: 输出”mlx”五次
5.times do
puts "mlx"
end
