为什么在使用默认对象初始化时,我的Hash不会被填充

Question

鉴于此代码:

h = Hash.new([])

3.times do |i|
  h[:a] << i
end

我希望h如此{:a => [0, 1, 2]},但它是空的.我究竟做错了什么？

Answer 1

正如API所说:

如果指定了obj,则此单个对象将用于所有默认值.

通过对代码进行少量重写,应该清楚会发生什么:

a = []
h = Hash.new(a)
3.times { |i| h[:a] << i }

# This is like:
# 3.times { |i| a << i }
# because `h` does not respond to your key :a

h
# => {} 
a
# => [0, 1, 2]

你想做什么,是这样的:

h = Hash.new { |h, k| h[k] = [] }
3.times { |i| h[:a] << i }

h
# => {:a=>[0, 1, 2]}