# Running multiple closures per iteration **URL:** \ **Category:** Ruby\ **Created:** [August 3, 2007, 7:38pm UTC](https://www.ruby-forum.com/t/running-multiple-closures-per-iteration/111200 "2007-08-03T19:38:52Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![syncopated](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/85f322/32.png) [@syncopated](https://www.ruby-forum.com/u/syncopated)\ **Post date:** [August 3, 2007, 7:38pm UTC](https://www.ruby-forum.com/t/running-multiple-closures-per-iteration/111200/1 "2007-08-03T19:38:52Z")
Is there a way to run more than one closure per iteration through a collection? For example, if I want to do the following two things, can I combine them so that I only end up with one pass over the collection, without doing a manual for loop? o = a.select {|x| x \>= 0} p = a.inject(0) {|s, x| s += x} Such a combination is useful when iterating through a collection is expensive, e.g. on a large file on a slow disk. ---
**Author:** ![syncopated](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/85f322/32.png) [@syncopated](https://www.ruby-forum.com/u/syncopated)\ **Post date:** [August 3, 2007, 7:53pm UTC](https://www.ruby-forum.com/t/running-multiple-closures-per-iteration/111200/2 "2007-08-03T19:53:42Z")
Joshua C. wrote: > Is there a way to run more than one closure per iteration through a > collection? For example, if I want to do the following two things, can > I combine them so that I only end up with one pass over the collection, > without doing a manual for loop? > > o = a.select {|x| x \>= 0} > p = a.inject(0) {|s, x| s += x} > > Such a combination is useful when iterating through a collection is > expensive, e.g. on a large file on a slow disk. Did you mean o = a.select {|x| x \>= 0} p = o.inject(0) {|s, x| s += x} ^^^ ? Then you can do this: a = [1,-1,2,-2,3,-3] sp = a.inject(0) {|s, x| x \>= 0 ? s + x : s} p sp # ==\> 6 (note that += is not needed, + is good enough) But there is no way in general to combine two iterations. ---
**Author:** ![syncopated](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/85f322/32.png) [@syncopated](https://www.ruby-forum.com/u/syncopated)\ **Post date:** [August 3, 2007, 8:19pm UTC](https://www.ruby-forum.com/t/running-multiple-closures-per-iteration/111200/3 "2007-08-03T20:19:40Z")
Joel VanderWerf wrote: > Did you mean > > o = a.select {|x| x \>= 0} > p = o.inject(0) {|s, x| s += x} > ^^^ > ? No, I meant to compute two different things from a. ---
**Author:** ![syncopated](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/85f322/32.png) [@syncopated](https://www.ruby-forum.com/u/syncopated)\ **Post date:** [August 3, 2007, 8:34pm UTC](https://www.ruby-forum.com/t/running-multiple-closures-per-iteration/111200/4 "2007-08-03T20:34:49Z")
On 8/3/07, Joshua C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Joel VanderWerf wrote: > > > Did you mean > > > > o = a.select {|x| x \>= 0} > > p = o.inject(0) {|s, x| s += x} > > ^^^ > > ? > > No, I meant to compute two different things from a. Well, it’s not pretty, but here’s my best shot at it o, p = a.inject( [[], 0] ) do |ary,x| ary[0] \<\< x if x \>= 0 ary[1] += x ary end I don’t think Ruby natively supports such a creature, though. Please correct me if I’m wrong. Blessings, TwP ---
**Author:** ![syncopated](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/85f322/32.png) [@syncopated](https://www.ruby-forum.com/u/syncopated)\ **Post date:** [August 3, 2007, 10:54pm UTC](https://www.ruby-forum.com/t/running-multiple-closures-per-iteration/111200/5 "2007-08-03T22:54:08Z")
Hi – On Sat, 4 Aug 2007, Stefano C. wrote: > a.inject(0) {|s, x| s + x} > =\> 6 > > > Such a combination is useful when iterating through a collection is > > expensive, e.g. on a large file on a slow disk. > > o = 0; p = [] > a.each { |x| o \<\< x if x \>= 0; p += x } > a.each { |x| p \<\< x if x \>= 0; o += x } 🙂 David ---
**Author:** ![syncopated](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/85f322/32.png) [@syncopated](https://www.ruby-forum.com/u/syncopated)\ **Post date:** [August 3, 2007, 10:51pm UTC](https://www.ruby-forum.com/t/running-multiple-closures-per-iteration/111200/6 "2007-08-03T22:51:06Z")
On Aug 3, 7:38 pm, Joshua C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Is there a way to run more than one closure per iteration through a > collection? For example, if I want to do the following two things, can > I combine them so that I only end up with one pass over the collection, > without doing a manual for loop? > > o = a.select {|x| x \>= 0} > p = a.inject(0) {|s, x| s += x} btw, you don’t need the += operator here: a.inject(0) {|s, x| s + x} =\> 6 > Such a combination is useful when iterating through a collection is > expensive, e.g. on a large file on a slow disk. o = 0; p = [] a.each { |x| o \<\< x if x \>= 0; p += x } ---
**Author:** ![syncopated](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/85f322/32.png) [@syncopated](https://www.ruby-forum.com/u/syncopated)\ **Post date:** [August 4, 2007, 1:21pm UTC](https://www.ruby-forum.com/t/running-multiple-closures-per-iteration/111200/7 "2007-08-04T13:21:15Z")
Tim P. schrieb: > Well, it’s not pretty, but here’s my best shot at it > > o, p = a.inject( [[], 0] ) do |ary,x| > ary[0] \<\< x if x \>= 0 > ary[1] += x > ary > end you can beautify this to o, p = a.inject( [[], 0] ) do |(l, s), x| [x \>= 0 ? l \<\< x : l, s + x] end > I don’t think Ruby natively supports such a creature, though. Please > correct me if I’m wrong. Well, depends on what ‘natively’ realy means… > Blessings, > TwP cheers Simon