# Select loop question **URL:** \ **Category:** ruby-talk\ **Created:** [29 June 2002 23:04 UTC](https://rubytalk.org/t/select-loop-question/435 "2002-06-29T23:04:38Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Joseph\_McDonald](https://avatars.discourse-cdn.com/v4/letter/j/34f0e0/32.png) [@Joseph\_McDonald](https://rubytalk.org/u/Joseph_McDonald)\ **Post date:** [29 June 2002 23:04 UTC](https://rubytalk.org/t/select-loop-question/435/1 "2002-06-29T23:04:38Z")
Hi, I’ve got a question on using a select loop. Here is the basic way I am using select: if (answer = select(readfds, writefds, nil, 10)) answer[0].each do |r| # handle reads end answer[1].each do |w| # handle writes end else # timeout end The question is: How do I know which file descriptors were returned in the answer arrays? That is: after I get an answer and I finish reading from the socket, I want to remove that socket from the readsfs array, but I’m not sure how to do that. Do I need to do something like: answer[0].each do |r| readfds.each\_with\_index do |orig,i| if r == orig # nuke this one readfds.slice!(i) end end end That seems like an awful lot of overhead (cartesian product?) which I why I think I’m doing it wrong… Does anyone have some example code using a select loop? I would like to use it to multiplex connections to web servers. That is, I have 10000 url’s that I would like to retrieve the contents from, and would like to do it using 50 parallel connections using select loop. I know you can use threads for such an operation, but I would prefer to use the select loop method. thanks, -joe ---
**Author:** ![Niklas\_Frykholm2](https://avatars.discourse-cdn.com/v4/letter/n/35a633/32.png) [@Niklas\_Frykholm2](https://rubytalk.org/u/Niklas_Frykholm2)\ **Post date:** [30 June 2002 06:48 UTC](https://rubytalk.org/t/select-loop-question/435/2 "2002-06-30T06:48:31Z")
[Joseph McDonald]: > after I get an answer and I finish > reading from the socket, I want to remove that socket from the readsfs > array, but I’m not sure how to do that. Do I need to do something > like: > answer[0].each do |r| > readfds.each\_with\_index do |orig,i| > if r == orig > # nuke this one > readfds.slice!(i) > end > end > end Just do ``` answer[0].each {|r| readfds.delete(r)} ``` Or even ``` readfds = readfds - answer[0] ``` // Niklas ---
**Author:** ![Joseph\_McDonald](https://avatars.discourse-cdn.com/v4/letter/j/34f0e0/32.png) [@Joseph\_McDonald](https://rubytalk.org/u/Joseph_McDonald)\ **Post date:** [30 June 2002 15:50 UTC](https://rubytalk.org/t/select-loop-question/435/3 "2002-06-30T15:50:41Z")
> Just do >         answer[0].each {|r| readfds.delete(r)} Thanks, I do notice that Array.delete seems to be very slow, If I run this: hits = (200..300).to\_a 1.upto(1000) do   arr =   1.upto(500) do |x|     arr.push(x)   end   # hits.each do |h|   # arr.delete(h)   # end end I get:      0.47 real 0.46 user 0.00 sys If I uncomment out the delete portion I get:       8.74 real 8.23 user 0.00 sys        My guess is that it is doing the cartesion product thing, interesting that it is 8 times slower than:   arr.each do |x|     if x \>= 200 and x \<= 300       arr.slice!(x)     end   end which gives: 1.09 real 1.06 user 0.00 sys another surprising thing, if I use: arr = arr - hits gives: 24.36 real 21.91 user 0.03 sys thanks, -joe ---
**Author:** ![Niklas\_Frykholm2](https://avatars.discourse-cdn.com/v4/letter/n/35a633/32.png) [@Niklas\_Frykholm2](https://rubytalk.org/u/Niklas_Frykholm2)\ **Post date:** [30 June 2002 16:48 UTC](https://rubytalk.org/t/select-loop-question/435/4 "2002-06-30T16:48:42Z")
[Joseph McDonald]: > > Just do > > answer[0].each {|r| readfds.delete(r)} > > Thanks, I do notice that Array.delete seems to be very slow, If I run this: > > hits = (200…300).to\_a > 1.upto(1000) do > > arr = > 1.upto(500) do |x| > arr.push(x) > end > > # hits.each do |h| > > # arr.delete(h) > > # end > > end > > I get: > 0.47 real 0.46 user 0.00 sys > If I uncomment out the delete portion I get: > 8.74 real 8.23 user 0.00 sys > > My guess is that it is doing the cartesion product thing, interesting > that it is 8 times slower than: > > arr.each do |x| > if x \>= 200 and x \<= 300 > arr.slice!(x) > end > end > > which gives: 1.09 real 1.06 user 0.00 sys If you do many deletes, it is slow. It has to search the array for the element to delete and then move all the data that follows it. Note that slice! is not the same thing, when you do slice!(x) you remove the element at position x, not the element with content x. So the array does not have to be searched. If you want to do fast deletes based on content, you can use a hash to store the data. Store the data in the key of the hash and a dummy value such as “true” in the value. You get fast lookups and there is no need to move elements. // Niklas ---
**Author:** ![Joseph\_McDonald](https://avatars.discourse-cdn.com/v4/letter/j/34f0e0/32.png) [@Joseph\_McDonald](https://rubytalk.org/u/Joseph_McDonald)\ **Post date:** [30 June 2002 18:13 UTC](https://rubytalk.org/t/select-loop-question/435/5 "2002-06-30T18:13:54Z")
> If you do many deletes, it is slow. It has to search the array for > the element to delete and then move all the data that follows it. > Note that slice! is not the same thing, when you do slice!(x) you remove > the element at position x, not the element with content x. So the > array does not have to be searched. > If you want to do fast deletes based on content, you can use a hash to store > the data. Store the data in the key of the hash and a dummy value such as > "true" in the value. You get fast lookups and there is no need to move > elements. This is 4 times as fast as arr.delete(obj) on a 500 element array, it probably gets faster as the array grows: hits = (200..300).to\_a 1.upto(1000) do   arr =   1.upto(500) do |x|     arr.push(x)   end   hash = Hash.new   arr.each\_with\_index do |a,i|     hash[a] = i   end   hits.each do |hit|     arr.slice!(hash[hit]) if hash[hit]   end end 2.13 real 2.05 user 0.00 sys I wonder if Array#- should do the same thing internally? The above method is 12 times as fast as Array#- But I digress... I'm still struggling with select()... 🙂 I can't find any examples except in "server" code. thanks, -joe