# Doubling numbers with Object#object\_id **URL:** \ **Category:** Ruby\ **Created:** [August 25, 2009, 8:07am UTC](https://www.ruby-forum.com/t/doubling-numbers-with-object-object-id/174327 "2009-08-25T08:07:09Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![EbFFA](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/bcef8e/32.png) [@EbFFA](https://www.ruby-forum.com/u/EbFFA)\ **Post date:** [August 25, 2009, 8:07am UTC](https://www.ruby-forum.com/t/doubling-numbers-with-object-object-id/174327/1 "2009-08-25T08:07:09Z")
I was playing with Ruby and wrote an interesting method: ``` def double n n.object_id - 1 end irb> double 5 => 10 irb> double 1 => 2 irb> double 0 => 0 irb> double -101 => -202 irb> double 100000001 => 200000002 irb> double 10000000000 # into Bignum territory now => -606548539 ``` Anyone know why this works? Why Ruby was implemented this way? I tried creating an object\_id collision based on this doubling rule, but it seems that all Fixnum object\_id`s are odd, and all other object_id`s are even (in fact they all seem to end in 8). Clever. ---
**Author:** ![EbFFA](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/bcef8e/32.png) [@EbFFA](https://www.ruby-forum.com/u/EbFFA)\ **Post date:** [August 25, 2009, 8:24am UTC](https://www.ruby-forum.com/t/doubling-numbers-with-object-object-id/174327/2 "2009-08-25T08:24:45Z")
AFAIK, FixNums are implemented as direct values and not C structs that represent objects. Small integers - which means integers that can be held in (sizeof(unsigned long int) \* 8 -1 = 31 bits). 1 bit is used for sign and 30 bits for the value. Here is the INT2FIX macro from Ruby source: #define INT2FIX(i) ((VALUE)(((long)(i))\<\<1 | FIXNUM\_FLAG)) Shift 1 bit to the left, and bitwise OR it with 1. In other words, object\_id = 2n + 1. That explains why your double method works. cheers nilesh ---
**Author:** ![EbFFA](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/bcef8e/32.png) [@EbFFA](https://www.ruby-forum.com/u/EbFFA)\ **Post date:** [August 25, 2009, 8:27am UTC](https://www.ruby-forum.com/t/doubling-numbers-with-object-object-id/174327/3 "2009-08-25T08:27:40Z")
All other Ruby objects are allocated on addresses that are multiples of 4 to avoid clash with FixNum. So Object ID is always even for them. cheers nilesh ---
**Author:** ![EbFFA](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/bcef8e/32.png) [@EbFFA](https://www.ruby-forum.com/u/EbFFA)\ **Post date:** [August 25, 2009, 8:59am UTC](https://www.ruby-forum.com/t/doubling-numbers-with-object-object-id/174327/4 "2009-08-25T08:59:56Z")
Aah that explains everything, thankyou. That’s a very beautiful system. Makes me want to design my own interpreter and take part in such things. ---
**Author:** ![EbFFA](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/bcef8e/32.png) [@EbFFA](https://www.ruby-forum.com/u/EbFFA)\ **Post date:** [August 27, 2009, 12:22am UTC](https://www.ruby-forum.com/t/doubling-numbers-with-object-object-id/174327/5 "2009-08-27T00:22:44Z")
EbFFA: > I was playing with Ruby and wrote an interesting method: > ``` > def double n > n.object_id - 1 > end > > ``` > Anyone know why this works? Why Ruby was implemented this way? Note that other Ruby implementations (JRuby, Rubinius, IronRuby…) do not have to keep this contract; it’s best considered as MRI peculiarity. But yes, I do find this design beautiful – as well as Ruby’s method lookup: [http://www.viddler.com/explore/CobyR/videos/1/](http://www.viddler.com/explore/CobyR/videos/1/) — Shot ---
**Author:** ![EbFFA](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/bcef8e/32.png) [@EbFFA](https://www.ruby-forum.com/u/EbFFA)\ **Post date:** [September 3, 2009, 5:55pm UTC](https://www.ruby-forum.com/t/doubling-numbers-with-object-object-id/174327/6 "2009-09-03T17:55:01Z")
On Wed, Aug 26, 2009 at 5:21 PM, Shot (Piotr S.)[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Note that other Ruby implementations (JRuby, Rubinius, IronRuby…) do > not have to keep this contract; it’s best considered as MRI peculiarity. > But yes, I do find this design beautiful – as well as Ruby’s method > lookup: [http://www.viddler.com/explore/CobyR/videos/1/](http://www.viddler.com/explore/CobyR/videos/1/) I don’t know that it’s particularly beautiful; it’s just a side effect of Fixnums not having any internal state and not actually being objects in MRI. Because they are “immediates” they don’t actually have an object\_id. As a result, their object\_id is just based on their actual value, 2n+1. All other objects then get stuffed into the remaining even object\_ids, and fixnum.object\_id - 1 == fixnum \* 2. - Charlie ---
**Author:** ![EbFFA](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/bcef8e/32.png) [@EbFFA](https://www.ruby-forum.com/u/EbFFA)\ **Post date:** [September 3, 2009, 6:38pm UTC](https://www.ruby-forum.com/t/doubling-numbers-with-object-object-id/174327/7 "2009-09-03T18:38:57Z")
On Thu, Sep 3, 2009 at 11:54 AM, Charles Oliver Nutter[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > > > remaining even object\_ids, and fixnum.object\_id - 1 == fixnum \* 2. > And its not that particularly unique to MRI. I know lots of VM > implementations for various languages (Ruby, Smalltalk, JavaScript > …) which use different variations of representing an integer n > smaller than 2\*\*b for some b less than the word length as: ``` n * 2**flag_shift + int_flag ``` where flag\_shift is some small integer (usually but not always 1) and int\_flag is usually 1 or at least odd, since on most machines a pointer to an object will be aligned on some even address boundary. For some reason, Googles V8 JavaScript VM uses an int\_flag of 1 and SETs the low order bit of references which are pointers, at least according to a video I watched about the implementation. – Rick DeNatale Blog: [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/) Twitter: [http://twitter.com/RickDeNatale](http://twitter.com/RickDeNatale) WWR: [http://www.workingwithrails.com/person/9021-rick-denatale](http://www.workingwithrails.com/person/9021-rick-denatale) LinkedIn: [http://www.linkedin.com/in/rickdenatale](http://www.linkedin.com/in/rickdenatale)