# Why does Array#first not use rb\_ary\_subseq? **URL:** \ **Category:** Ruby\ **Created:** [May 18, 2007, 2:36pm UTC](https://www.ruby-forum.com/t/why-does-array-first-not-use-rb-ary-subseq/104476 "2007-05-18T14:36:17Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![apeiros](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/838e76/32.png) [@apeiros](https://www.ruby-forum.com/u/apeiros)\ **Post date:** [May 18, 2007, 2:36pm UTC](https://www.ruby-forum.com/t/why-does-array-first-not-use-rb-ary-subseq/104476/1 "2007-05-18T14:36:17Z")
Hi there When I run a small bench I noticed that Array#first(n) with an argument is vastly slower than Array#[0,n]. Poking a bit in the source revealed that Array#[] uses rb\_ary\_subseq, which seems to create a COW Array while Array#first creates a new Array on its own and fills it with values. Is there any rationale as to why Array#first doesn’t just use rb\_ary\_subseq too? I searched the ML for an answer to this and found none. Forgive me please if I overlooked the answer. Regards Stefan R. ---
**Author:** ![apeiros](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/838e76/32.png) [@apeiros](https://www.ruby-forum.com/u/apeiros)\ **Post date:** [May 18, 2007, 2:47pm UTC](https://www.ruby-forum.com/t/why-does-array-first-not-use-rb-ary-subseq/104476/2 "2007-05-18T14:47:16Z")
Hi, In message “Re: Why does Array#first not use rb\_ary\_subseq?” on Fri, 18 May 2007 21:36:19 +0900, Stefan R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: |When I run a small bench I noticed that Array#first(n) with an argument |is vastly slower than Array#[0,n]. The difference is due to Array#first is optimized for memory consumption, I guess. We’d like profile on the use-case, so could you show us “vastly slower” example code? ``` matz. ``` ---
**Author:** ![apeiros](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/838e76/32.png) [@apeiros](https://www.ruby-forum.com/u/apeiros)\ **Post date:** [May 18, 2007, 3:22pm UTC](https://www.ruby-forum.com/t/why-does-array-first-not-use-rb-ary-subseq/104476/3 "2007-05-18T15:22:30Z")
Yukihiro M. wrote: > Hi, > > In message “Re: Why does Array#first not use rb\_ary\_subseq?” > on Fri, 18 May 2007 21:36:19 +0900, Stefan R. > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: > > |When I run a small bench I noticed that Array#first(n) with an argument > |is vastly slower than Array#[0,n]. > > The difference is due to Array#first is optimized for memory > consumption, I guess. We’d like profile on the use-case, so could you > show us “vastly slower” example code? > > ``` > matz. > > ``` Thank you for your reply. Memory consumption: could you please elaborate that a bit for me? I somehow expected the COW Array created by Array#[] to be more memory efficient as I understood that to be the reason why that is used at all. I’m new to C, so please forgive me if I’m a bit slow ☹ I posted a benchmark with results and code on [Parked at Loopia](http://pastie.caboo.se/62638). I don’t have a specific use-case but I’d say a lot of array operations base on .each (all Enumerable methods) which is why I added a bench running an each loop over the elements. It’s intriguing that the difference only seems to be huge for the creation. In my initial bench I only run the [] and first method, I didn’t add an each-loop. I didn’t expect this as the returned array is not modified. I think I have to delve a bit more into the whys and whats with that COW array. But what I forgot to ask in my initial question: if those two implementations have different characteristics, why isn’t that documented? (from the docs I’d just expect first to have nicer semantics for its use-case and nothing else) regards Stefan R. ---
**Author:** ![apeiros](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/838e76/32.png) [@apeiros](https://www.ruby-forum.com/u/apeiros)\ **Post date:** [May 18, 2007, 3:36pm UTC](https://www.ruby-forum.com/t/why-does-array-first-not-use-rb-ary-subseq/104476/4 "2007-05-18T15:36:25Z")
Hi, In message “Re: Why does Array#first not use rb\_ary\_subseq?” on Fri, 18 May 2007 22:22:31 +0900, Stefan R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: |But what I forgot to ask in my initial question: if those two |implementations have different characteristics, why isn’t that |documented? (from the docs I’d just expect first to have nicer semantics |for its use-case and nothing else) Just because it is just an implementation matter. Anyway, I will examine your code example, and check what we can do. ``` matz. ``` ---
**Author:** ![apeiros](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/838e76/32.png) [@apeiros](https://www.ruby-forum.com/u/apeiros)\ **Post date:** [May 18, 2007, 4:11pm UTC](https://www.ruby-forum.com/t/why-does-array-first-not-use-rb-ary-subseq/104476/5 "2007-05-18T16:11:29Z")
Hi, In message “Re: Why does Array#first not use rb\_ary\_subseq?” on Fri, 18 May 2007 22:35:08 +0900, Yukihiro M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: |Just because it is just an implementation matter. Anyway, I will |examine your code example, and check what we can do. I am sorry, I was bit confused with 1.9 (trunk) and 1.8 which you are talking about. You are right. Array#first should utilize copy-on-write as much as it can. I will fix. ``` matz. ``` ---
**Author:** ![apeiros](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/838e76/32.png) [@apeiros](https://www.ruby-forum.com/u/apeiros)\ **Post date:** [May 18, 2007, 4:22pm UTC](https://www.ruby-forum.com/t/why-does-array-first-not-use-rb-ary-subseq/104476/6 "2007-05-18T16:22:09Z")
Yukihiro M. wrote: > Hi, > > In message “Re: Why does Array#first not use rb\_ary\_subseq?” > on Fri, 18 May 2007 22:35:08 +0900, Yukihiro M. > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: > > |Just because it is just an implementation matter. Anyway, I will > |examine your code example, and check what we can do. > > I am sorry, I was bit confused with 1.9 (trunk) and 1.8 which you are > talking about. You are right. Array#first should utilize > copy-on-write as much as it can. I will fix. > > ``` > matz. > > ``` Oh, I should have supplied the version. Sorry about that. Heh, actually I’m surprised that my curiosity results in a change in code, but happy to be of help 🙂 Regards Stefan