how to convert "+=" to operator in ruby

Question

I can convert "+" , "-" or "/" to operator use 2.send("-",3)

but it does not work with "+="

a = 2
a += 2 #=> 4

a = 2
a.send("+=", 4) #=> NoMethodError: undefined method `+=' for 2:Fixnum

I have tried to convert symbol first ; but not works too;

how to fix this?

`+=`, `-=` are not operators. When ruby sees `x += 1` she converts that to `x = x + 1` and then evaluates that. `+=` (and the others) are provided merely for the convenience of the coder. — Cary Swoveland, Sep 21 '16 at 06:01
I closed it since in the linked answer’s comments there is a perfect answer. — Aleksei Matiushkin, Sep 21 '16 at 06:05
There is no such method called `+=`, so you can't `send` it. — Sagar Pandya, Sep 21 '16 at 06:18
@mudasobwa I've reopened the question. Although there is an answer in a comment, I think this question deserves an actual answer. — Stefan, Sep 21 '16 at 06:36
@Stefan sure, I doubted whether it should be closed or not and you made the right choice for me :) — Aleksei Matiushkin, Sep 21 '16 at 06:38
The `send` method already automatically converts strings to symbols. — Sagar Pandya, Sep 21 '16 at 06:51

Stefan · Accepted Answer · 2016-09-21T07:27:31.970

2.send("-", 3) works, because - is a method and 2 responds to that method:

2.respond_to?('-') #=> true

= and += on the other hand are not methods:

2.respond_to?('=')  #=> false
2.respond_to?('+=') #=> false

And even if = was a valid method¹, then

a = 2
a.send("=", 4)

would be equivalent to:

2.send("=", 4)

or simply:

2 = 4

In other words: it would redefine 2 as 4, something Ruby doesn't allow you to do.

This is because variables (like a above) are not objects. a.send doesn't send a message to the variable a, but to the object, a is referring to (2 in the example).

The same applies to abbreviated assignment like +=:

a = 2
a += 2

is equivalent to:

a = 2
a = a + 2

You can just rewrite it as:

a = 2
a = a.send("+", 2)

The assignment is not part of the method invocation.

¹ you can actually define a method = via define_method:

class Foo
  define_method('=') { |other| puts "`=' called with #{other}" }
end

But it is just an ordinary method. In particular, it does not and can not alter the variable the object was assigned-to or the object's identity:

 f = Foo.new
 #=> #<Foo:0x007ff20c0eeda8>
 f.send('=', 123)
 # `=' called with 123
 #=> nil
 f
 #=> #<Foo:0x007ff20c0eeda8>

how to convert "+=" to operator in ruby

1 Answers1