具有常量的奇怪的ruby行为

Question

有没有人能解释一下，为什么我最初的常量LIST从一开始就被操纵了？我认为常量可以只初始化一次。我希望在不影响原始数组(LIST)的情况下，将操作存储在新的数组(new_list)中。

$ned = "foo"
$med = ""

print LIST = [:nrd, :mrd_y] # -> [:nrd, :mrd_y]


list = LIST

new_list = list.delete_if { |element|
  case element
  when :nrd then $ned.empty?
  when :mrd_y then $ned.empty? || $med.empty?
  end
}

print new_list # -> [:nrd]

print LIST # -> [:nrd] instead of [:nrd, :mrd_y]

Answer 1

Array#delete_if ->删除块计算为true的self的每个元素。

$ned = "foo"
$med = ""

LIST = [:nrd, :mrd_y]
p LIST.object_id #=> 84053120
list = LIST
p LIST.object_id #=> 84053120
new_list = list.delete_if { |element|
  case element
  when :nrd then $ned.empty?
  when :mrd_y then $ned.empty? || $med.empty?
  end
}

List和list持有相同的Array对象，object_id如上所述。因此，对于来自块的每个true求值，delete_if都会从84053120引用的对象中删除项。由LIST和list保存。所以你可以使用下面的代码：

$ned = "foo"
$med = ""

LIST = [:nrd, :mrd_y] 
list = LIST
new_list = list.dup.delete_if { |element|
  case element
  when :nrd then $ned.empty?
  when :mrd_y then $ned.empty? || $med.empty?
  end
}

p new_list #=>[:nrd]
p LIST #=>[:nrd, :mrd_y]

或(更好的方法使用Array#reject)，

$ned = "foo"
$med = ""

list = [:nrd, :mrd_y]  
new_list = list.reject { |element|
  case element
  when :nrd then $ned.empty?
  when :mrd_y then $ned.empty? || $med.empty?
  end
}
p new_list #=>[:nrd]
p list #=>[:nrd, :mrd_y]