奇怪的紅寶石行爲與恆定

Question

有人可以解釋我，爲什麼我的原始常量LIST從一開始就被操縱在最後？我認爲常量可能只是一次初始化。我想將操作存儲在新陣列中（new_list），而不影響原始操作（LIST）。

$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 - >刪除自的每個元素爲哪個塊評估爲真。

$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保持相同Array對象，如上述object_id講述。因此，對於來自塊delete_if的每個true評估，刪除84053120引用的對象中的項目。它由LIST和list保存。所以你可以用下面的：

$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]