Ruby: reference to numeric values

How can I keep references to numeric values in a Hash in Ruby? Say I have

h={"a"=>1, "b"=>2}
c=h["a"]

Wh开发者_开发技巧en I do

c+=1

I get:

irb(main):079:0> c+=1
=> 2

but

irb(main):080:0> h
=> {"a"=>1, "b"=>2}

so h has not changed. Can I somehow retain refernces to numeric values and modify them outside the hash h?

---

The c is its own variable, so changing the value of c doesn't change the value of the item in the hash.

To achieve the effect you want, you can make the hash item an array instead:

h={"a"=>[1], "b"=>2}
c=h["a"] # c is now a reference to the hash item, an array
c[0]+=1 # modifying the array
h # print the hash

(The text after # is a comment; I am not sure whether they will be ignored by the Ruby console.)

---

What you need to understand is that there is only ever one '1' (which is why it's a good idea that it be immutable, obviously). It's not like a String where two variables could share a reference to it, and change it under the feet of the other variable, like:

a="foo"
b=a
b.clear
a => ""

To do what you want, you can use the key:

c="a"
h[c]+=1
h=> {"a"=>2, "b"=>2}

Or otherwise use a value that can be changed in-place (as suggested above)

---

The 'c' should be point to the Key, Numeric is immutable, so you can't change the Numeric value. And ruby prefer to use Symbol for key.

h = {:a => 1, :b => 2}
c = :a
h[c] = 3
puts h # {:a => 3, :b => 2}

If your array value is String, you can:

h = {:a => '1', :b => '2'}
c = h[:a]
c.replace '3'
puts h # {:a => '3', :b => '2'}

or you can create your a class, make the Numeric c can be change.

---

You are already holding a reference to the numeric value, it's just that the operation you do to it generates a new numeric value instead of changing its own value.

For practicality, we can say that c holds a reference to the number 1 (although there's a subtle implementation difference between Fixnum and Bignum). For c += 1, you're effectively doing c = c + 1 and the + operator of the number actually returns a new number. So the variable c is assigned with the reference to the new number.

This will be a different case if there's a method that can change the the value of the object itself. For example, on String, the << operator changes the value of the object:

h = {"a" => "foo"}
b = h["a"]
b << 'bar'
puts b # foobar
puts h # {"a"=>"foobar"}

But in your case, it is an Integer and there is no way to change the numeric value of it.