Equivalent of .Try() for a Hash to Avoid "Undefined Method" Errors on Nil

Equivalent of .try() for a hash to avoid undefined method errors on nil?

You forgot to put a . before the try:

@myvar = session[:comments].try(:[], @comment.id)

since [] is the name of the method when you do [@comment.id].

`try` method when trying to fetch hash value

If you need to allow for object.content.nil?, then you'd use try. If you want to allow for a missing key then you don't want fetch (as Priti notes), you want the normal [] method. Combining the two yields:

object.content.try(:[], 'en')

Observe:

> h = { :a => :b }
 => {:a=>:b} 
> h.try(:[], :a)
 => :b 
> h.try(:[], :c)
 => nil 
> h = nil
 => nil 
> h.try(:[], :a)
 => nil

You could also use object.content.try(:fetch, 'en', nil) if :[] looks like it is mocking you.

How to avoid NoMethodError for nil elements when accessing nested hashes?

If I understand your question correctly i.e. make it forgiving in case an attribute value is missing, then you could try the following:

@param_info.try(:fetch, :drug).try(:fetch, :name)

This might return nil as well, but this will get rid of the error undefined methods '[]' for nil:NilClass

Update:

In order to handle keys that do not exist, you could try the following. (Got this hint from Equivalent of try for a hash):

@param_info.try(:[], :drug).try(:[], :name)

Getting undefined method `[]=' for nil:NilClass error when injecting array with hash

Since puts is the last statement in the block and it returns nil, nil is returned causing the accumulator hash to be nil in the second iteration. You should return the hash. And you should pass the array as an argument to the inj_hash method to make it reusable.

def inj_hash(arr)
  arr.inject({}) do |hash, element|
    hash[element.first] = element.last
    hash
  end
end

inj_hash([[:key1, "value1"], [:key2, "value2"]])
#=> {:key1=>"value1", :key2=>"value2"}

The simplest solution to create a hash from an array of key-value pairs is to use Hash::[].

arr = [[:key1, "value1"], [:key2, "value2"]]
Hash[arr]
#=> {:key1=>"value1", :key2=>"value2"}

How to avoid NoMethodError for missing elements in nested hashes, without repeated nil checks?

Ruby 2.3.0 introduced a new method called dig on both Hash and Array that solves this problem entirely.

name = params.dig(:company, :owner, :name)

It returns nil if the key is missing at any level.

If you are using a version of Ruby older than 2.3, you can use the ruby_dig gem or implement it yourself:

module RubyDig
  def dig(key, *rest)
    if value = (self[key] rescue nil)
      if rest.empty?
        value
      elsif value.respond_to?(:dig)
        value.dig(*rest)
      end
    end
  end
end

if RUBY_VERSION < '2.3'
  Array.send(:include, RubyDig)
  Hash.send(:include, RubyDig)
end

Ruby: undefined method `[]' for nil:NilClass when trying to get Enumerator on an Array of Hashes

undefined method `[]' for nil:NilClass says you tried to do something[index] but something is nil. Ruby won't let you use nil as an array (ie. call the [] method on it).

The problem is not on the attributs.each line but on the line following which calls the [] method on attribut.

typeAttribut = attribut['objectTypeAttribute']

This indicates something in attributs is nil. This could happen if attributsParam is a list that contains nil like so.

attributsParam = [nil];
attributs = Array(attributsParam);

# [nil]
puts attributs.inspect

Simplest way to debug it is to add puts attributs.inspect just before the loop.

Also consider if you really need the attributs = Array(attributsParam) line or if it's already something Enumerable.

How to access an element deep in an array of arrays without getting 'undefined method' error

Depending on your array content, you can omit the .present?. Ruby will also just take the last value in such a construct, so you can omit the if statement.

@foursquare['response']['groups'][0] &&
@foursquare['response']['groups'][0]['items'] &&
@foursquare['response']['groups'][0]['items'][42]

More elegant solutions for this problem are the egonil (blog post), the andand gem (blog post), or even Ruby 2.3's safe navigation operator.

Update: Recent Rubies include the #dig method, which might be helpful in this case. See user513951's answer for more details.

httparty NoMethodError: undefined method `[]' for nil:NilClass

This is one of most common errors regarding what Rollbar says

The problem must be than json_stats['Facebook'] is nil, so accessing to ['share_count'] produces an error. I recommend you to use dig to prevent this undesired error and decide what to do when json_stats['Facebook'] is nil.

    self.share_count = json_stats.dig('Facebook', 'share_count')

Why is safe navigation better than using try in Rails?

(1) &. is generally shorter than try(...)

Depending on the scenario, this can make your code more readable.

(2) &. is standard Ruby, as opposed to try

The method try is not defined in a Ruby core library but rather in a Rails library. When you are not developing a RoR web app but instead are writing e.g. little helper scripts, this will get relevant pretty fast.
(I prefer Ruby over Bash, for example.)

(3) &. makes debugging easier

The safe traversal operator will throw an error if a nonexistent method is being invoked.

>> "s"&.glubsch
NoMethodError: undefined method `glubsch' for "s":String

Only on nil it will behave leniently:

>> nil&.glubsch
=> nil

The try method will always return nil.

>> "s".try(:glubsch)
=> nil

Note that this is the case with most recent versions of Ruby and Rails.

Now imagine a scenario where a method named glubsch exists. Then you decide to rename that method but forget to rename it in one place. (Sadly, that can happen with ruby...) With the safe traversal operator, you will notice the mistake almost immediately (as soon as that line of code is executed for the first time). The try method however will happily provide you with a nil and you will get a nil related error somewhere downstream in program execution. Figuring out where such a nil came from can be hard at times.

Failing fast and hard with &. makes debugging easier than blithely returning nil with try.

EDIT: There is also the variant try! (with a bang) that behaves the same as &. in this regard. Use that if you don't like &..

(4) What if I don't care if a method is implemented or not?

That would be strange. Since that would be an unexpected way of programming, please make it explicit. For example by fleshing out the two cases (implemented or not) using respond_to? and branch off of that.

(5) What about try's block form?

Instead of a method name, a block can be passed to try. The block will be executed in the context of the receiver; and within the block there is no leniency applied. So with just a single method call, it will acutally behave the same as &..

>> "s".try{ glubsch }
NameError: undefined local variable or method `glubsch' for "s":String

For more complex computations, you might prefer this block form over introducing lots of local variables. E.g. a chain of

foo.try{ glubsch.blam }.try{ bar }.to_s

would allow foo to be nil but require foo.glubsch to return a non-nil value. Then again, you can do the same with the safe traversal operator in a more concise fashion:

foo&.glubsch.blam&.bar.to_s

Using try's block form for complex computations IMHO is a code smell, though, because it impedes readability. When you need to implement complex logic, introduce local variables with descriptive names and maybe use an if to branch off a nil case. Your code will be more maintainable.

HTH!

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