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29

How do you check whether a variable is defined in Ruby? Is there an "isset"-type method available?

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8 Answers

up vote 246 down vote accepted

Use the defined? keyword (documentation). It will return a String with the kind of the item, or nil if it doesn’t exist.

>> a = 1
 => 1
>> defined? a
 => "local-variable"
>> defined? b
 => nil
>> defined? nil
 => "nil"
>> defined? String
 => "constant"
>> defined? 1
 => "expression"

As skalee commented: "It is worth noting that variable which is set to nil is initialized."

>> n = nil  
>> defined? n
 => "local-variable"
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28  
It is worth noting that variable which is set to nil is initialized. – skalee Feb 16 '11 at 13:18
3  
If you want to set a variable if it doesn't exist and leave it alone if it does, see @danmayer's answer (involving the ||= operator) below. – jrdioko Jul 12 '11 at 21:29
up vote 37 down vote

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 class once after that just keeps returning the var

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4  
This is very idiomatic Ruby too and very typical, by the way. – jrdioko Jul 12 '11 at 21:28
Thanks for this input ! I'm a ruby/rails beginner and came accross this syntax yesterday evening, had a hard time figuring out what it meant. Looks very handy ! – LePad Nov 7 '11 at 10:35
this won't work for defining constants – Ivailo Bardarov Nov 24 '11 at 14:52
9  
Just don't use ||= with boolean values, lest you feel the pain of confusion. – Andrew Marshall May 18 '12 at 6:27
2  
along with what @AndrewMarshall said, avoid this idiom with anything that might return nil as well unless you really want to evaluate the expression every time it's called when it does return nil – nzifnab Nov 19 '12 at 23:58
up vote 21 down vote

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.

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1  
That's not necessary as nil evaluates to false when used in a test – Jerome Apr 5 '12 at 14:23
Why not defined?(var) == nil ? – vol7ron Apr 23 at 22:17
up vote 15 down vote

defined?(your_var) will work. Depending on what you're doing you can also do something like your_var.nil?

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up vote 5 down vote

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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What does this answer add that hasn't been said by the other answers? – Andrew Grimm May 31 '12 at 22:54
2  
It very nicely answers the question in one more useful way, doesn't it? – look Aug 9 '12 at 22:54
up vote 1 down vote

Here is some code, nothing rocket science but it works well enough

require 'rubygems'
require 'rainbow'
if defined?(var).nil?
 print "var is not defined\n".color(:red)
else
 print "car is defined\n".color(:green)
end
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Why the down vote?... – Sardathrion Apr 17 '12 at 11:59
Presumably because the nil? is optional. – James Jun 18 '12 at 16:15
@James: really? Downvote instead of edit and change?.... meh, never mind. ^_~ – Sardathrion Jun 20 '12 at 8:12
Blame whoever downvoted it back in April; I just commented. ;-) – James Jun 20 '12 at 13:47
@James: I do blame them, not you! ;) – Sardathrion Jun 20 '12 at 14:35
up vote 1 down vote

You can try:

unless defined?(var)
  #ruby code goes here
end
=> true

Because it returns a boolean.

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SyntaxError: compile error (irb):2: syntax error, unexpected $end, expecting kEND – Andrew Grimm Aug 2 '12 at 23:11
to use an unless statement seems overly complicated – johannes Oct 29 '12 at 8:38
up vote 0 down vote

Use defined? YourVariable
Keep it simple silly .. ;)

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