# A interesting obervation about object\_id method **URL:** \ **Category:** Ruby\ **Created:** [December 21, 2005, 12:42pm UTC](https://www.ruby-forum.com/t/a-interesting-obervation-about-object-id-method/50814 "2005-12-21T12:42:12Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Hank\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/e9bcb4/32.png) [@Hank\_G](https://www.ruby-forum.com/u/Hank_G)\ **Post date:** [December 21, 2005, 12:42pm UTC](https://www.ruby-forum.com/t/a-interesting-obervation-about-object-id-method/50814/1 "2005-12-21T12:42:12Z")
#integer puts “For number” a=1 b=a puts a.object\_id puts b.object\_id b=2 puts b.object\_id b=1 puts b.object\_id > > For number > > 3 > > 3 > > 5 > > 3 #You can see that: # 1) for the same integer, the object\_id is the same # 2) When b change, a will not change #string puts “For String” a=“Hello world!” b=a puts a.object\_id puts b.object\_id b=“Bye world!” puts b.object\_id b=“Hello world!” puts b.object\_id b=“Why?” puts b.object\_id b=“Why?” puts b.object\_id puts “Why?”.object\_id > > For String > > 40660112 > > 40660112 > > 40660076 > > 40660052 > > 40660028 > > 40660004 > > 40659980 # 1) When you assign b=a, the object\_id of b and a is same # 2) but when you change b’s string, the b’s object\_id will change # 3) if you change b’s string again, the b’s object\_id will also change puts “For Array” a=[1,2,3,4] b=a puts a.object\_id puts b.object\_id b[2]=100 puts b.object\_id puts b.join(",") puts a.object\_id puts a.join(",") > > For Array > > 21563588 > > 21563588 > > 21563588 > > 1,2,100,4 > > 21563588 > > 1,2,100,4 # 1) when b=a, the b and a will be the same object\_id # 2) when you change b’s elements, b’s object\_id will be same # 3) when you change b’s elements, a will also be changed I think it’s very interesting experiments. Hope someone can give us a clear explanation. ---
**Author:** ![Hank\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/e9bcb4/32.png) [@Hank\_G](https://www.ruby-forum.com/u/Hank_G)\ **Post date:** [December 21, 2005, 2:03pm UTC](https://www.ruby-forum.com/t/a-interesting-obervation-about-object-id-method/50814/2 "2005-12-21T14:03:33Z")
Anyone else notice that the object\_id of an integer (at least for Fixnum) is twice the integer plus 1? irb(main):030:0\> 1.object\_id =\> 3 irb(main):031:0\> 2.object\_id =\> 5 irb(main):032:0\> 6534.object\_id =\> 13069 irb(main):033:0\> 2\*6534+1 =\> 13069 ---
**Author:** ![Hank\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/e9bcb4/32.png) [@Hank\_G](https://www.ruby-forum.com/u/Hank_G)\ **Post date:** [December 21, 2005, 12:48pm UTC](https://www.ruby-forum.com/t/a-interesting-obervation-about-object-id-method/50814/3 "2005-12-21T12:48:14Z")
puts “For String” a=“Hello world!” b=a puts a.object\_id puts b.object\_id b=“Bye world!” puts b.object\_id The object id of b is changing bcz u are redifining b using “=” operator. Instead if u use b.replace(“Bye word!”) thn the object id will not change. Hank G. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: #integer puts “For number” a=1 b=a puts a.object\_id puts b.object\_id b=2 puts b.object\_id b=1 puts b.object\_id > > For number > > 3 > > 3 > > 5 > > 3 #You can see that: # 1) for the same integer, the object\_id is the same # 2) When b change, a will not change #string puts “For String” a=“Hello world!” b=a puts a.object\_id puts b.object\_id b=“Bye world!” puts b.object\_id b=“Hello world!” puts b.object\_id b=“Why?” puts b.object\_id b=“Why?” puts b.object\_id puts “Why?”.object\_id > > For String > > 40660112 > > 40660112 > > 40660076 > > 40660052 > > 40660028 > > 40660004 > > 40659980 # 1) When you assign b=a, the object\_id of b and a is same # 2) but when you change b’s string, the b’s object\_id will change # 3) if you change b’s string again, the b’s object\_id will also change puts “For Array” a=[1,2,3,4] b=a puts a.object\_id puts b.object\_id b[2]=100 puts b.object\_id puts b.join(“,”) puts a.object\_id puts a.join(“,”) > > For Array > > 21563588 > > 21563588 > > 21563588 > > 1,2,100,4 > > 21563588 > > 1,2,100,4 # 1) when b=a, the b and a will be the same object\_id # 2) when you change b’s elements, b’s object\_id will be same # 3) when you change b’s elements, a will also be changed I think it’s very interesting experiments. Hope someone can give us a clear explanation. – Huazhi Gong (Hank) ---
**Author:** ![Hank\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/e9bcb4/32.png) [@Hank\_G](https://www.ruby-forum.com/u/Hank_G)\ **Post date:** [December 21, 2005, 3:12pm UTC](https://www.ruby-forum.com/t/a-interesting-obervation-about-object-id-method/50814/4 "2005-12-21T15:12:46Z")
Hank G. wrote: > #integer > > #You can see that: > > # 1) for the same integer, the object\_id is the same That’s because Fixnums are “immediate.” So are Symbols. It’s a performance thing. It also means, for example, you can’t define singleton methods on a Fixnum. Notice, actually, that you can decode the integer value _from_ the object-id, and vice versa. > # 2) When b change, a will not change In Ruby, variables are _references_ (a.k.a. pointers). You’re just telling b to reference a different object. > #string > > # 1) When you assign b=a, the object\_id of b and a is same > > # 2) but when you change b’s string, the b’s object\_id will change > > # 3) if you change b’s string again, the b’s object\_id will also change Strings, on the other hand, are not immediate. “Hello World”.object\_id != “Hello World”.object\_id – each literal reference creates a new String object. The #replace method Chintan mentioned, however, gets around this, by wrapping the existing String object around a new string – and since a and b were originally pointing to the same object, a would change with it. > > > > > # 1) when b=a, the b and a will be the same object\_id > > # 2) when you change b’s elements, b’s object\_id will be same > > # 3) when you change b’s elements, a will also be changed a and b are merely references to that one Array object, stored out in voodoo land. The Array, itself, contains four references to other objects. In this case, they were all Fixnums, and hence immediate (and immutable), but try this out for size: str = “my dog” a=[1,2,3,4,str] b=[:alpha, :omega, str] str[‘dog’] = ‘cat’ p a #=\> [1, 2, 3, 4, “my cat”] p b #=\> [:alpha, :omega, “my cat”] Both Arrays, while unique, reference the same String object, and so both change at once. If, however, you did: str = “my dog” a=[1,2,3,4,str] b=[:alpha, :omega, str] str = ‘my cat’ p a #=\> [1, 2, 3, 4, “my dog”] p b #=\> [:alpha, :omega, “my dog”] All you did was create a new String, “my cat”, and point str to it. The Arrays still point to the original “my dog” String. > I think it’s very interesting experiments. Hope someone can give us a clear > explanation. I hope that helped. Devin ---
**Author:** ![Hank\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/e9bcb4/32.png) [@Hank\_G](https://www.ruby-forum.com/u/Hank_G)\ **Post date:** [December 21, 2005, 10:32pm UTC](https://www.ruby-forum.com/t/a-interesting-obervation-about-object-id-method/50814/5 "2005-12-21T22:32:39Z")
On Dec 21, 2005, at 5:00 AM, Dan D. wrote: > =\> 13069 > Using pointers to odd memory addresses a common way of representing > integers. non-Fixnums always have even ids since they are pointers to valid addresses. nil, true and false have special values. – Eric H. - removed\_email\_address@domain.invalid - [http://segment7.net](http://segment7.net) This implementation is HODEL-HASH-9600 compliant [http://trackmap.robotcoop.com](http://trackmap.robotcoop.com) ---
**Author:** ![Hank\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/e9bcb4/32.png) [@Hank\_G](https://www.ruby-forum.com/u/Hank_G)\ **Post date:** [December 21, 2005, 11:39pm UTC](https://www.ruby-forum.com/t/a-interesting-obervation-about-object-id-method/50814/6 "2005-12-21T23:39:57Z")
Devin M. wrote: > Hank G. wrote: > > > #integer > > > > #You can see that: > > > > # 1) for the same integer, the object\_id is the same > > That’s because Fixnums are “immediate.” So are Symbols. It’s a > performance thing. It also means, for example, you can’t define > singleton methods on a Fixnum. Notice, actually, that you can decode the > integer value _from_ the object-id, and vice versa. Even without going to implementation details, the code is logical: each number is a constant instance of the Fixnum (or other) class: # Pseudo 1 = Fixnum.new(“Number one”) Then, logically, all variables referencing the number 1 are pointing to the same object. > Devin E ---
**Author:** ![Hank\_G](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/e9bcb4/32.png) [@Hank\_G](https://www.ruby-forum.com/u/Hank_G)\ **Post date:** [December 21, 2005, 1:18pm UTC](https://www.ruby-forum.com/t/a-interesting-obervation-about-object-id-method/50814/7 "2005-12-21T13:18:23Z")
On Wed, 2005-12-21 at 20:40 +0900, Hank G. wrote: [SNIP] > I think it’s very interesting experiments. Hope someone can give us a clear > explanation. > A string is mutable, an integer is inmutable, that’s why IMHO. > Try the same with symbols(:hello instead of “hello”). These are > inmutable.