在 變數 中談過，在Ruby中有五種變數，包括區域變數、全域變數、實例變數、類別變數與常數，目前已經談過的有區塊變數、全域變數與常數，在看過 def 定義方法 與 迭代器與程式區塊 後，這邊要來談談區塊變數、全域變數與常數的範圍問題。
全域變數比較不需要討論，全域變數的範圍為整個程式可見，比較要討論的是區塊變數，在Ruby中，區域變數的可見範圍真的就是「區域」，變數在哪個範圍中宣告，就在哪個範圍中可見。例如：
>> x = 10
=> 10
>> def some
>> puts x
>> end
=> nil
>> some
NameError: undefined local variable or method `x' for main:Object
from (irb):3:in `some'
from (irb):5
from C:/Winware/Ruby192/bin/irb:12:in `<main>'
>>
=> 10
>> def some
>> puts x
>> end
=> nil
>> some
NameError: undefined local variable or method `x' for main:Object
from (irb):3:in `some'
from (irb):5
from C:/Winware/Ruby192/bin/irb:12:in `<main>'
>>
上例於some方法外定義了x區域變數（實際上是Object類別本體中的區域變數，之後還會談到），some方法中是看不到該變數的。類似地，
>> def some
>> x = 10
>> def inner
>> puts x
>> end
>> inner
>> end
=> nil
>> some
NameError: undefined local variable or method `x' for main:Object
from (irb):15:in `inner'
from (irb):17:in `some'
from (irb):19
from C:/Winware/Ruby192/bin/irb:12:in `<main>'
>>
>> x = 10
>> def inner
>> puts x
>> end
>> inner
>> end
=> nil
>> some
NameError: undefined local variable or method `x' for main:Object
from (irb):15:in `inner'
from (irb):17:in `some'
from (irb):19
from C:/Winware/Ruby192/bin/irb:12:in `<main>'
>>
some方法中宣告了x區域變數，inner是看不到該變數的。要小心以下：
>> x = 10
=> 10
>> def some
>> x = 20
>> puts x
>> end
=> nil
>> some
20
=> nil
>> x
=> 10
>>
=> 10
>> def some
>> x = 20
>> puts x
>> end
=> nil
>> some
20
=> nil
>> x
=> 10
>>
上例中some中的x指定，其實是建立了新的區域變數，而不是將值指定給some方法外的x變數。
在 變數 中談過，Ruby區域變數有個特性，直譯器只要看到程式碼中有「變數=值」的語句，就會為建立該變數，這會造成有以下的結果：
>> if false
>> a = 10
>> end
=> nil
>> a
=> nil
>> b
NameError: undefined local variable or method `b' for main:Object
from (irb):5
from C:/Winware/Ruby192/bin/irb:12:in `<main>'
>>
>> a = 10
>> end
=> nil
>> a
=> nil
>> b
NameError: undefined local variable or method `b' for main:Object
from (irb):5
from C:/Winware/Ruby192/bin/irb:12:in `<main>'
>>
在上例中，雖然if的區塊不會執行，但直譯器看到a = 10，就會建立a變數，因此之後嘗試顯示a的值，是有a變數但預設為nil，然後b變數是不存在的，因此出現錯誤。
如果在方法外定義常數的話，方法內是可見的：
>> X = 10
=> 10
>> def some
>> puts X
>> end
=> nil
>> some
10
=> nil
>>
=> 10
>> def some
>> puts X
>> end
=> nil
>> some
10
=> nil
>>
如果使用程式區塊，程式區塊外的區域變數，程式區塊中是看得到的：
>> sum = 0
=> 0
>> [1, 2, 3].each do |element|
?> sum += element
>> end
=> [1, 2, 3]
>> sum
=> 6
>>
=> 0
>> [1, 2, 3].each do |element|
?> sum += element
>> end
=> [1, 2, 3]
>> sum
=> 6
>>
如果怕區塊中使用的變數，剛好與區塊外的變數同名，可以特別在分號之後宣告，告知區域中這個變數必須是區域變數：
>> x = 10
=> 10
>> [1, 2, 3].each do |element; x|
?> x = element + 1
>> end
=> [1, 2, 3]
>> x
=> 10
>>
=> 10
>> [1, 2, 3].each do |element; x|
?> x = element + 1
>> end
=> [1, 2, 3]
>> x
=> 10
>>
定義程式區塊時，若區塊參數湊巧與區塊外的區域變數同名，則只會看到區塊參數：
>> sum = 0
=> 0
>> [1, 2, 3, 4, 5].reduce { |sum, element|
?> sum + element
>> }
=> 15
>> sum
=> 0
>>
=> 0
>> [1, 2, 3, 4, 5].reduce { |sum, element|
?> sum + element
>> }
=> 15
>> sum
=> 0
>>
想得知變數是哪個範圍的變數，可以使用defined?方法：
>> x = 10
=> 10
>> X = 10
=> 10
>> defined? x
=> "local-variable"
>> defined? X
=> "constant"
>> defined? xyz
=> nil
>>
=> 10
>> X = 10
=> 10
>> defined? x
=> "local-variable"
>> defined? X
=> "constant"
>> defined? xyz
=> nil
>>