问题描述
限时送ChatGPT账号..给定以下代码:
a = true # let's assign `a` a value
# and let's test if calling `b`, an unassigned variable, throws an error
begin
puts "The value of b is: #{b.inspect}"
rescue NameError => e
puts "Caught an error: #{e}"
end
a || b = true # the assignment should never be executed because `a` is `true`
puts "The value of b is: #{b.inspect}" # will calling `b` still raise an error?
我们得到以下结果:
Caught an error: undefined local variable or method `b' for main:Object
The value of b is: nil
即使我们期望第二次调用 b
会引发错误,但我们看到 b
现在实际上是 nil
.
Even though we expected calling b
to raise an error the second time, we see that b
is now, in fact, nil
.
这是为什么?为什么 b
被赋值为 nil
?由于 ||
从未达到分配,我希望 b
保持未定义.如何定义但不赋值?
Why is that? Why does b
get assigned nil
? Since the ||
never reached the assignment, I would expect b
to remain undefined. How can it be defined, but not assigned a value?
推荐答案
docs 解释了变量是如何创建的;据我了解,这就是解析器的工作方式:
Some of the docs explain how variables are created; the explanation as I understand it is that's just how the parser works:
局部变量在解析器遇到赋值时创建,而不是在赋值发生时创建:
The local variable is created when the parser encounters the assignment, not when the assignment occurs:
a = 0 if false # does not assign to a
p local_variables # prints [:a]
p a # prints nil
您可以看到其他示例:
b = true if false # b is nil
"test" || c = true # c is nil
还有其他人没有被分配:
And others it doesn't get assigned:
puts d if false # d generates a NameError
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