Checking if a variable is defined?

Solution 1:

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"

Solution 2:

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.

Solution 3:

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.

Solution 4:

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

Solution 5:

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