How to use define_method inside initialize()

I suspect that you're looking for define_singleton_method:

define_singleton_method(symbol, method) → new_method
define_singleton_method(symbol) { block } → proc

Defines a singleton method in the receiver. The method parameter can be a Proc, a Method or an UnboundMethod object. If a block is specified, it is used as the method body.

If you use define_method on self.class, you'll create the new method as an instance method on the whole class so it will be available as a method on all instances of the class.

You'd use define_singleton_method like this:

class C
  def initialize(s)    
    define_singleton_method(s) { puts "some method #{s}" }    
  end
end

And then:

a = C.new('a')
b = C.new('b')
a.a # puts 'some method a'
a.b # NoMethodError
b.a # NoMethodError
b.b # puts 'some method b'

If your initialize did:

self.class.send(:define_method,n) { puts "some method #{n}" }    

then you'd get:

a.a # puts 'some method a'
a.b # puts 'some method b'
b.a # puts 'some method a'
b.b # puts 'some method b'

and that's probably not what you're looking for. Creating a new instance and having the entire class change as a result is rather odd.

Do as below :

class C
  def initialize(n)    
    self.class.send(:define_method,n) { puts "some method #{n}" }    
  end
end

ob = C.new("abc")
ob.abc
# >> some method abc

Module#define_method is a private method and also a class method.Your one didn't work,as you tried to call it on the instance of C.You have to call it on C,using #send in your case.