How to understand nil vs empty vs blank in Ruby

Question

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

User · Answer

Just extend Julian s table     Ref  empty blank nil

User · Answer

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

User · Answer

I made this useful table with all the cases     blank   present  are provided by Rails

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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

User · Answer

Quick tip   obj blank     obj present   Can be handy easier on the eyes in some expressions

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A special case is when trying to assess if a boolean value is nil   false present     false false blank     true false nil     false   In this case the recommendation would be to use  nil

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nil  is a standard Ruby method that can be called on all objects and returns true if the object is nil   b   nil b nil      gt  true   empty  is a standard Ruby method that can be called on some objects such as Strings  Arrays and Hashes and returns true if these objects contain no element   a      a empty      gt  true  b     2   4   b empty      gt  false   empty  cannot be called on nil objects   blank  is a Rails method that can be called on nil objects as well as empty objects

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Everybody else has explained well what is the difference    I would like to add in Ruby On Rails  it is better to use obj blank  or obj present  instead of obj nil  or obj empty    obj blank  handles all types nil               and returns true if values are not available and returns false if values are  available on any type of object

User · Answer

Everything that is nil  is blank  Everything that is empty  is blank  Nothing that is empty  is nil  Nothing that is nil  is empty    tl dr -- only use blank   amp  present  unless you want to distinguish between    and

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Rails 4  an alternative to  corban-brook  s  Array gotcha  blank   for checking if an arrays only holds empty values and can be regarded as blank  true     nil       all   amp  blank     true   one could also do    nil                   reject  amp  blank   blank     true

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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 false blank     true    blank     true    blank     true    blank     true 5 blank     false 0 blank     false    blank  also evaluates true on strings which are non-empty but contain only whitespace        blank     true      empty     false   Rails also provides  present   which returns the negation of  blank    Array gotcha  blank  will return false even if all elements of an array are blank  To determine blankness in this case  use all  with blank   for example     nil       blank     false   nil       all   amp  blank     true

User · Answer

nil  can be used on any object  It determines if the object has any value or not  including  blank  values   For example   example   nil example nil     true     nil     false   Basically nil  will only ever return true if the object is in fact equal to  nil    empty  is only called on objects that are considered a collection  This includes things like strings  a collection of characters   hashes  a collection of key value pairs  and arrays  a collection of arbitrary objects   empty  returns true is there are no items in the collection   For example      empty    true  hi  empty      false    empty     true       gt      empty      false    empty      true  nil  empty     false  nil empty     NoMethodError  undefined method  empty   for nil NilClass   Notice that empty  can t be called on nil objects as nil objects are not a collection and it will raise an exception   Also notice that even if the items in a collection are blank  it does not mean a collection is empty   blank  is basically a combination of nil  and empty  It s useful for checking objects that you assume are collections  but could also be nil

User · Answer

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

User · Answer

exists  method can be used to check whether the data exists in the database or not  It returns boolean values either true or false

[ruby-on-rails] How to understand nil vs. empty vs. blank in Ruby

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