Ruby-on-rails 如何在没有“未定义的局部变量或方法”的情况下检查变量是否存在？
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How to check if a variable exists with a value without "undefined local variable or method"?
提问by Michael Durrant
This is a common pattern: If a variable doesn't exist I get an undefined local variable or methoderror.
这是一个常见的模式：如果变量不存在，我会收到undefined local variable or method错误消息。
The existing code has if variable_name.present?but this didn't account for the variable not existing.
现有代码有，if variable_name.present?但这并没有考虑到不存在的变量。
How can I check the value of the variable and also account for it not existing at all?
如何检查变量的值并说明它根本不存在？
I've tried:
我试过了：
if (defined? mmm) then
if mmm.present? then
puts "true"
end
end
but Ruby still checks that inner mmm.present?and throws "no such variable" when it doesn't exist.
但是 Ruby 仍然会检查内部mmm.present?并在它不存在时抛出“没有这样的变量”。
I'm sure there's a common pattern/solution to this.
我确信对此有一个共同的模式/解决方案。
回答by Michael Durrant
Change the present?to != ''and use the && operator which only tries to evaluate the seond expression if the first one is true:
更改present?to!= ''并使用 && 运算符，如果第一个表达式为真，它只会尝试评估第二个表达式：
if defined?(mmm) && (mmm != '') then puts "yes" end
But actually as of 2019 this is no longer needed as both the below work
但实际上从 2019 年开始，这不再需要，因为下面的工作
irb(main):001:0> if (defined? mm) then
irb(main):002:1* if mm.present? then
irb(main):003:2* p true
irb(main):004:2> end
irb(main):005:1> end
=> nil
irb(main):006:0> if (defined? mm) then
irb(main):007:1* p mm
irb(main):008:1> end
=> nil
回答by Komsun K.
On Ruby on Rails
在 Ruby on Rails 上
if defined?(mm) && mm.present?
puts "acceptable variable"
end
On IRB
在 IRB 上
if defined?(mm) && !mm.blank? && !mm.nil?
puts "acceptable variable"
end
It can make sure you won't get undefined variable or nil or empty value.
它可以确保您不会得到未定义的变量或 nil 或空值。
