rails nil blank

A concise explanation of nil v. empty v. blank in Ruby on Rails

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I find myself repeatedly looking for a clear definition of the differences of nil?, blank?, and empty? in Ruby on Rails. Here's the closest I've come:

blank? objects are false, empty, or a whitespace string. For example, "", " ", nil, [], and {} are blank.

nil? objects are instances of NilClass.

empty? objects are class-specific, and the definition varies from class to class. A string is empty if it has no characters, and an array is empty if it contains no items.

Is there anything missing, or a tighter comparison that can be made?

ruby-on-rails ruby empty blank nil

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asked May 19 '09 at 22:51 Arrel 184 11 100% accept rate

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5 Answers

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.nil?

can be used on any object and is true if the object is nil

.empty?

can be used on strings, arrays and hashes and returns true if:

• String length == 0
• Array length == 0
• Hash length == 0

running .empty? on something that is nil will throw a NoMethodError

That is where .blank? comes in. It is implemented by Rails and will operate on any object as well as work like .empty? on strings, arrays and hashes.

nil.blank? == true
[].blank? == true
{}.blank? == true
"".blank? == true
5.blank? == false

.blank? also evaluates true on strings which are non-empty but contain only whitespace.

"  ".blank? == true
"  ".empty? == false

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edited May 21 '09 at 19:48

answered May 20 '09 at 16:14 Corban Brook 1,474 2 12

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As mentioned in the question, some non-empty strings count as blank. – Andrew Grimm May 20 '09 at 23:20

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Thanks for catching that. – Corban Brook May 21 '09 at 19:47

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One difference is that .nil? and .empty? are methods that are provided by the programming language Ruby, whereas .blank? is something added by the web development framework Rails.

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answered May 19 '09 at 23:42 Andrew Grimm 2,544 6 17

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Don't forget any? which is generally !empty?. In Rails I typically check for the presence of something at the end of a statement with if something or unless something then use blank? where needed since it seems to work everywhere.

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answered May 19 '09 at 23:51   Andy

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.any? doesn't work with strings in ruby 1.9, as .any? requires enumerable, and string#each by itself doesn't work in ruby 1.9. – Andrew Grimm May 20 '09 at 0:59

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Just a little note about the any? recommendation: He's right that it's generally equivalent to !empty?. However, any? will return true to a string of just whitespace (ala " ").

And of course, see the 1.9 comment above, too.

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answered Jul 13 '09 at 19:25 orangechicken 525

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Quick tip: !obj.blank? == obj.present? Can be handy/easier on the eyes in some expressions link|flag