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$ irb
irb(main):001:0> a = 1
=> 1
irb(main):002:0> if a == 1 || a == 2 || a ==3 ; put 'a < 4' ; end
NoMethodError: undefined method `put' for main:Object
from (irb):2
from /home/wangzheng/.rbenv/versions/1.9.3-p484/bin/irb:12:in `<main>'
irb(main):003:0> if a == 1 || a == 2 || a ==3 ; puts 'a < 4' ; end
a < 4
=> nil
irb(main):004:0> [1,2,3,4].include?(1)
=> true
irb(main):005:0> [1,2,3,4].indexOf(1)
NoMethodError: undefined method `indexOf' for [1, 2, 3, 4]:Array
from (irb):5
from /home/wangzheng/.rbenv/versions/1.9.3-p484/bin/irb:12:in `<main>'
irb(main):006:0> [1,2,3,4].include?(1)
=> true
irb(main):007:0> require 'active_support/all'
=> true
irb(main):008:0> 1.in? [1,2,3,4]
=> true
irb(main):009:0>
一定要注意，in?后面有一个空格