Rails 3.1：Ruby 成语防止.each 如果为零则抛出异常？

Question

Is there a way to use .each so it does not throw an error if the object is nil or empty (without adding an additional nil/blank test?

It seems that if I say phonelist.each do |phone| that if phonelist is empty, then the block should not be executed.

But in my view (haml) I have - @myvar.phonelist.each do |phone| and if phonelist is empty, it throws a NoMethodError.

I run into this a lot, and always workaround by adding an explicit check/branch for.blank? but it seems there should be an easier way to tell.each that empty means do nothing.

Answer 1

You can use the try method to call .each on a nil so it does not throw an error if the object is nil or empty.

phonelist = nil
phonelist.try(:each){|i| puts i}

Answer 2

Simply do the following:

Array(phonelist).each do |phone|
  #deal with your phone
end

Array(my_variable) will ensure to return an array if my_variable is nil.

It doesn't create a new Array if my_variable is already an array, so it is safe and light to use it wherever you want !

Answer 3

You're attempting to smack a band-aid on a larger problem.

Ruby has a concept of nil; can't get around it. If you are calling a method on nil , then you are assuming it is valid, ie, your design assumes it to be valid. So the question really is: where is the hole in your design? Why is your assumption incorrect?

The problem here is not that you cannot call arbitrary methods on an object which does not support it; the problem is that your data is assumed to be valid when obviously that is not always the case.

But in my view (haml) I have - @myvar.phonelist.each do |phone| and if phonelist is empty, it throws a NoMethodError.

No. If phonelist is not an object which implements .each it throws an error. Very different.

You can always initialize it to an empty array if null, ie, phonelist ||= [] , but I would prefer a design which ensures valid data whenever possible.

Answer 4

Can't believe no one has suggested this yet:

(@myvar.phonelist || []).each do |phone|
   ...

If phonelist is nil , the each will loop on the empty array, executing the block zero times.

HOWEVER, this will still throw an exception if phonelist is not an enumerable (eg an array).

Answer 5

如果您从散列（例如解析的 JSON 文件）中获取phonelist列表，您可能希望使用带有[]作为默认值的fetch 。

phonelist = my_data.fetch('phonelist', [])

Answer 6

Simply make sure empty phonelist is a [] , instead of a nil value.

Alternatively, a nil value is falsey in Ruby, so you can use nil-punning

if phonelist
   phonelist.each do |phone|
     ...

Answer 7

what i've seen done a lot is:

@myvar.phonelist.each do |phone|
  # ...
end unless @myvar.phonelist.nil?