Checking if a variable is defined

Question

How can I check whether a variable is defined in Ruby   Is there an isset-type method available

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You can try   unless defined  var     ruby code goes here end   gt  true   Because it returns a boolean

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Use the defined  keyword  documentation   It will return a String with the kind of the item  or nil if it doesn   t exist    gt  gt  a   1    gt  1  gt  gt  defined  a    gt   local-variable   gt  gt  defined  b    gt  nil  gt  gt  defined  nil    gt   nil   gt  gt  defined  String    gt   constant   gt  gt  defined  1    gt   expression    As skalee commented   It is worth noting that variable which is set to nil is initialized     gt  gt  n   nil    gt  gt  defined  n    gt   local-variable

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This is useful if you want to do nothing if it does exist but create it if it doesn t exist   def get var    var     SomeClass new   end   This only creates the new instance once  After that it just keeps returning the var

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defined  your var  will work  Depending on what you re doing you can also do something like your var nil

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Also  you can check if it s defined while in a string via interpolation  if you code    puts  Is array1 defined and what type is it    defined   array1      The system will tell you the type if it is defined  If it is not defined it will just return a warning saying the variable is not initialized   Hope this helps

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This is useful if you want to do nothing if it does exist but create it if it doesn t exist   def get var    var     SomeClass new   end   This only creates the new instance once  After that it just keeps returning the var

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Use the defined  keyword  documentation   It will return a String with the kind of the item  or nil if it doesn   t exist    gt  gt  a   1    gt  1  gt  gt  defined  a    gt   local-variable   gt  gt  defined  b    gt  nil  gt  gt  defined  nil    gt   nil   gt  gt  defined  String    gt   constant   gt  gt  defined  1    gt   expression    As skalee commented   It is worth noting that variable which is set to nil is initialized     gt  gt  n   nil    gt  gt  defined  n    gt   local-variable

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This is useful if you want to do nothing if it does exist but create it if it doesn t exist   def get var    var     SomeClass new   end   This only creates the new instance once  After that it just keeps returning the var

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defined  your var  will work  Depending on what you re doing you can also do something like your var nil

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WARNING Re  A Common Ruby Pattern  This is the key answer  the defined  method  The accepted answer above illustrates this perfectly    But there is a shark  lurking beneath the waves     Consider this type of common ruby pattern     def method1      x     method2  end   def method2     nil  end   method2 always returns nil  The first time you call method1  the  x variable is not set - therefore method2 will be run   and method2 will set  x to nil  That is fine  and all well and good  But what happens the second time you call method1   Remember  x has already been set to nil  But method2 will still be run again   If method2 is a costly undertaking this might not be something that you want   Let the defined  method come to the rescue - with this solution  that particular case is handled - use the following     def method1     return  x if defined   x      x   method2   end   The devil is in the details  but you can evade that lurking shark with the defined  method

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As many other examples show you don t actually need a boolean from a method to make logical choices in ruby  It would be a poor form to coerce everything to a boolean unless you actually need a boolean    But if you absolutely need a boolean  Use     bang bang  or  falsy falsy reveals the truth        irb  gt  gt  a   nil   gt  nil  gt  gt  defined  a    gt   local-variable   gt  gt  defined  b    gt  nil  gt  gt    defined  a    gt  true  gt  gt    defined  b    gt  false   Why it doesn t usually pay to coerce    gt  gt     defined  a     var is defined  colorize  green     var is not defined  colorize  red       defined  a     var is defined  colorize  green     var is not defined  colorize  red     gt  true   Here s an example where it matters because it relies on the implicit coercion of the boolean value to its string representation    gt  gt  puts  var is defined      defined  a   vs   defined  a    var is defined  true vs local-variable   gt  nil

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defined  is great  but if you are in a Rails environment you can also use try  especially in cases where you want to check a dynamic variable name   foo   1 my foo    foo  my bar    bar  try  foo             gt  1 try  bar             gt  nil try my foo           gt  1 try my bar           gt  nil

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Here is some code  nothing rocket science but it works well enough  require  rubygems  require  rainbow  if defined  var  nil      nil  is optional but might make for clearer intent   print  var is not defined n  color  red  else  print  car is defined n  color  green  end   Clearly  the colouring code is not necessary  just a nice visualation in this toy example

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Also  you can check if it s defined while in a string via interpolation  if you code    puts  Is array1 defined and what type is it    defined   array1      The system will tell you the type if it is defined  If it is not defined it will just return a warning saying the variable is not initialized   Hope this helps

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It  should be mentioned that using defined to check if a specific field is set in a hash might behave unexpected   var      if defined  var  unknown     puts  this is unexpected  end   will output  this is unexpected    The syntax is correct here  but defined  var  unknown   will be evaluated to the string  method   so the if block will be executed  edit  The correct notation for checking if a key exists in a hash would be   if var key   unknown

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It  should be mentioned that using defined to check if a specific field is set in a hash might behave unexpected   var      if defined  var  unknown     puts  this is unexpected  end   will output  this is unexpected    The syntax is correct here  but defined  var  unknown   will be evaluated to the string  method   so the if block will be executed  edit  The correct notation for checking if a key exists in a hash would be   if var key   unknown

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This is useful if you want to do nothing if it does exist but create it if it doesn t exist   def get var    var     SomeClass new   end   This only creates the new instance once  After that it just keeps returning the var

User · Answer

defined  is great  but if you are in a Rails environment you can also use try  especially in cases where you want to check a dynamic variable name   foo   1 my foo    foo  my bar    bar  try  foo             gt  1 try  bar             gt  nil try my foo           gt  1 try my bar           gt  nil

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Leaving an incredibly simple example in case it helps  When variable doesn t exist  if defined  a then  quot hi quot  end     gt  nil  When variable does exist  a   2 if defined  a then  quot hi quot  end     gt   quot hi quot

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Try  unless  instead of  if   a    apple    Note that b is not declared c   nil  unless defined  a     puts  a is not defined  end  unless defined  b     puts  b is not defined  end  unless defined  c     puts  c is not defined  end

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You can try   unless defined  var     ruby code goes here end   gt  true   Because it returns a boolean

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The correct syntax for the above statement is   if  defined  var   nil    will now return true or false  print  var is not defined n  color  red  else  print  var is defined n  color  green  end   substituting  var  with your variable  This syntax will return a true false value for evaluation in the if statement

User · Answer

WARNING Re  A Common Ruby Pattern  This is the key answer  the defined  method  The accepted answer above illustrates this perfectly    But there is a shark  lurking beneath the waves     Consider this type of common ruby pattern     def method1      x     method2  end   def method2     nil  end   method2 always returns nil  The first time you call method1  the  x variable is not set - therefore method2 will be run   and method2 will set  x to nil  That is fine  and all well and good  But what happens the second time you call method1   Remember  x has already been set to nil  But method2 will still be run again   If method2 is a costly undertaking this might not be something that you want   Let the defined  method come to the rescue - with this solution  that particular case is handled - use the following     def method1     return  x if defined   x      x   method2   end   The devil is in the details  but you can evade that lurking shark with the defined  method

User · Answer

defined  your var  will work  Depending on what you re doing you can also do something like your var nil

User · Answer

Use defined  YourVariable Keep it simple silly

User · Answer

Use the defined  keyword  documentation   It will return a String with the kind of the item  or nil if it doesn   t exist    gt  gt  a   1    gt  1  gt  gt  defined  a    gt   local-variable   gt  gt  defined  b    gt  nil  gt  gt  defined  nil    gt   nil   gt  gt  defined  String    gt   constant   gt  gt  defined  1    gt   expression    As skalee commented   It is worth noting that variable which is set to nil is initialized     gt  gt  n   nil    gt  gt  defined  n    gt   local-variable

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Please note the distinction between  defined  and  assigned      ruby -e  def f  if 1 gt 2  x 99  end p x  defined  x  end f  nil  local-variable    x is defined even though it is never assigned

User · Answer

Use the defined  keyword  documentation   It will return a String with the kind of the item  or nil if it doesn   t exist    gt  gt  a   1    gt  1  gt  gt  defined  a    gt   local-variable   gt  gt  defined  b    gt  nil  gt  gt  defined  nil    gt   nil   gt  gt  defined  String    gt   constant   gt  gt  defined  1    gt   expression    As skalee commented   It is worth noting that variable which is set to nil is initialized     gt  gt  n   nil    gt  gt  defined  n    gt   local-variable

User · Answer

Use defined  YourVariable Keep it simple silly

User · Answer

Leaving an incredibly simple example in case it helps  When variable doesn t exist  if defined  a then  quot hi quot  end     gt  nil  When variable does exist  a   2 if defined  a then  quot hi quot  end     gt   quot hi quot

User · Answer

As many other examples show you don t actually need a boolean from a method to make logical choices in ruby  It would be a poor form to coerce everything to a boolean unless you actually need a boolean    But if you absolutely need a boolean  Use     bang bang  or  falsy falsy reveals the truth        irb  gt  gt  a   nil   gt  nil  gt  gt  defined  a    gt   local-variable   gt  gt  defined  b    gt  nil  gt  gt    defined  a    gt  true  gt  gt    defined  b    gt  false   Why it doesn t usually pay to coerce    gt  gt     defined  a     var is defined  colorize  green     var is not defined  colorize  red       defined  a     var is defined  colorize  green     var is not defined  colorize  red     gt  true   Here s an example where it matters because it relies on the implicit coercion of the boolean value to its string representation    gt  gt  puts  var is defined      defined  a   vs   defined  a    var is defined  true vs local-variable   gt  nil

User · Answer

Try  unless  instead of  if   a    apple    Note that b is not declared c   nil  unless defined  a     puts  a is not defined  end  unless defined  b     puts  b is not defined  end  unless defined  c     puts  c is not defined  end

User · Answer

The correct syntax for the above statement is   if  defined  var   nil    will now return true or false  print  var is not defined n  color  red  else  print  var is defined n  color  green  end   substituting  var  with your variable  This syntax will return a true false value for evaluation in the if statement

User · Answer

Here is some code  nothing rocket science but it works well enough  require  rubygems  require  rainbow  if defined  var  nil      nil  is optional but might make for clearer intent   print  var is not defined n  color  red  else  print  car is defined n  color  green  end   Clearly  the colouring code is not necessary  just a nice visualation in this toy example

User · Answer

defined  your var  will work  Depending on what you re doing you can also do something like your var nil

User · Answer

Please note the distinction between  defined  and  assigned      ruby -e  def f  if 1 gt 2  x 99  end p x  defined  x  end f  nil  local-variable    x is defined even though it is never assigned

[ruby] Checking if a variable is defined?

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