# Isolating non-unique items in an array **URL:** \ **Category:** ruby-talk\ **Created:** [12 October 2006 15:24 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829 "2006-10-12T15:24:48Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Jason](https://avatars.discourse-cdn.com/v4/letter/j/dc4da7/32.png) [@Jason](https://rubytalk.org/u/Jason)\ **Post date:** [12 October 2006 15:24 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/1 "2006-10-12T15:24:48Z")
I'm basically trying to the opposite of .uniq Let's say I have an array: ["a", "a", "a", "b", "c", "d", "d"] I would like to boil this down by first tossing the values that only appear once. Which leaves me with: ["a", "a", "a", "d", "d"] Then I need to somehow determine that "a" appears 3 times and "d" appears 2 times. Any help would be great. Thanks. > **···** > > -- > Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). ---
**Author:** ![Tim\_Pease](https://avatars.discourse-cdn.com/v4/letter/t/3e96dc/32.png) [@Tim\_Pease](https://rubytalk.org/u/Tim_Pease)\ **Post date:** [12 October 2006 15:41 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/2 "2006-10-12T15:41:42Z")
class Array   def count( item )     return 0 unless self.include? item     c = 0     self.each {|i| c += 1 if i == item}     return c   end end ary = %w(a a a b c d d) new\_ary = ary.select {|item| ary.count(item) \> 1} counts = Hash.new {|h,k| h[k] = new\_ary.count(k)} It's not going to be fast, but it should work. Blessings, TwP > **···** > > On 10/12/06, Jason Burgett \ wrote: > > > I'm basically trying to the opposite of .uniq Let's say I have an array: > > > > ["a", "a", "a", "b", "c", "d", "d"] > > > > I would like to boil this down by first tossing the values that only > > appear once. Which leaves me with: > > > > ["a", "a", "a", "d", "d"] > > > > Then I need to somehow determine that "a" appears 3 times and "d" > > appears 2 times. Any help would be great. Thanks. > > > > -- > > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Jeff\_Schwab](https://avatars.discourse-cdn.com/v4/letter/j/b5ac83/32.png) [@Jeff\_Schwab](https://rubytalk.org/u/Jeff_Schwab)\ **Post date:** [12 October 2006 15:50 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/3 "2006-10-12T15:50:14Z")
Jason Burgett wrote: > I'm basically trying to the opposite of .uniq Let's say I have an array: > > ["a", "a", "a", "b", "c", "d", "d"] > > I would like to boil this down by first tossing the values that only appear once. Which leaves me with: > > ["a", "a", "a", "d", "d"] > > Then I need to somehow determine that "a" appears 3 times and "d" appears 2 times. Any help would be great. Thanks. # Output: # ["a", "a", "a", "d", "d"] # {"a"=\>3, "d"=\>2} def get\_counts(keys)      counts = Hash.new(0)      keys.each {|k| counts[k] += 1 }      counts end def non\_uniq(elements)      counts = get\_counts(elements)      counts.delete\_if {|k, v| v \< 2 }      elements.select {|e| counts.key?(e) } end elements = ["a", "a", "a", "b", "c", "d", "d"] p non\_uniq(elements) p get\_counts(elements).delete\_if {|k, v| v \< 2 } ---
**Author:** ![Drew\_Olson](https://avatars.discourse-cdn.com/v4/letter/d/c67d28/32.png) [@Drew\_Olson](https://rubytalk.org/u/Drew_Olson)\ **Post date:** [12 October 2006 16:02 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/4 "2006-10-12T16:02:39Z")
Jason Burgett wrote: > I'm basically trying to the opposite of .uniq Let's say I have an array: > > ["a", "a", "a", "b", "c", "d", "d"] > > I would like to boil this down by first tossing the values that only > appear once. Which leaves me with: > > ["a", "a", "a", "d", "d"] > > Then I need to somehow determine that "a" appears 3 times and "d" > appears 2 times. Any help would be great. Thanks This should work: a = ["a", "a", "a", "b", "c", "d", "d"] a.uniq.map{|i| i if (a.map{|j| j if j==i}.length \> 1)} > **···** > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Ross\_Bamford4](https://avatars.discourse-cdn.com/v4/letter/r/ea666f/32.png) [@Ross\_Bamford4](https://rubytalk.org/u/Ross_Bamford4)\ **Post date:** [12 October 2006 16:04 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/5 "2006-10-12T16:04:02Z")
There are bound to be better ways, but you could try something like: a = ["a", "a", "a", "b", "c", "d", "d"] # =\> ["a", "a", "a", "b", "c", "d", "d"] hsh = a.inject(Hash.new{|h,k| h[k] = 0}) {|h,v| h[v] += 1 ; h}.reject! {|k,v| v == 1} # =\> {"a"=\>3, "d"=\>2} hsh['a'] # =\> 3 hsh['d'] # =\> 2 Depending on how you're using the results, You could also run a second inject to reverse the lookup: rhsh = hsh.inject(Hash.new { |h,k| h[k] = }) { |h,(k,v)| h[v] \<\< k ; h } # =\> {2=\>["d"], 3=\>["a"]} (2..rhsh.keys.max).each { |i| puts "count #{i}: #{rhsh[i]}" } # count 2: d # count 3: a If you went with this way, I guess you could forget about getting rid of the 1s with reject!, since you could just ignore the 1 key in the rhsh... > **···** > > On Fri, 2006-10-13 at 00:24 +0900, Jason Burgett wrote: > > > I'm basically trying to the opposite of .uniq Let's say I have an array: > > > > ["a", "a", "a", "b", "c", "d", "d"] > > > > I would like to boil this down by first tossing the values that only > > appear once. Which leaves me with: > > > > ["a", "a", "a", "d", "d"] > > > > Then I need to somehow determine that "a" appears 3 times and "d" > > appears 2 times. Any help would be great. Thanks. > > -- > Ross Bamford - rosco@roscopeco.REMOVE.co.uk ---
**Author:** ![James\_Cribbs](https://avatars.discourse-cdn.com/v4/letter/j/a88e57/32.png) [@James\_Cribbs](https://rubytalk.org/u/James_Cribbs)\ **Post date:** [12 October 2006 16:22 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/6 "2006-10-12T16:22:06Z")
Jason Burgett wrote: > I'm basically trying to the opposite of .uniq Let's say I have an array: > > ["a", "a", "a", "b", "c", "d", "d"] > > I would like to boil this down by first tossing the values that only appear once. Which leaves me with: > > ["a", "a", "a", "d", "d"] > > Then I need to somehow determine that "a" appears 3 times and "d" appears 2 times. Any help would be great. Thanks. Here's what I whipped up: letters = ['a','a','a','b','c','d','d'] counts = {} letters.each do |letter|   count = letters.find\_all { |x| x == letter }.size   counts[letter] = count if count \> 1 end non\_unique\_letters = letters.find\_all { |x| counts.has\_key?(x) } p counts p non\_unique\_letters Jamey Confidentiality Notice: This email message, including any attachments, is for the sole use of the intended recipient(s) and may contain confidential and/or privileged information. If you are not the intended recipient(s), you are hereby notified that any dissemination, unauthorized review, use, disclosure or distribution of this email and any materials contained in any attachments is prohibited. If you receive this message in error, or are not the intended recipient(s), please immediately notify the sender by email and destroy all copies of the original message, including attachments. ---
**Author:** ![WATANABE\_Hirofumi1](https://avatars.discourse-cdn.com/v4/letter/w/c2a13f/32.png) [@WATANABE\_Hirofumi1](https://rubytalk.org/u/WATANABE_Hirofumi1)\ **Post date:** [12 October 2006 16:25 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/7 "2006-10-12T16:25:15Z")
Hi, Jason Burgett \ writes: > I'm basically trying to the opposite of .uniq Let's say I have an array: > > ["a", "a", "a", "b", "c", "d", "d"] > > I would like to boil this down by first tossing the values that only > appear once. Which leaves me with: > > ["a", "a", "a", "d", "d"] > > Then I need to somehow determine that "a" appears 3 times and "d" > appears 2 times. Any help would be great. Thanks. % irb > > a = ["a", "a", "a", "b", "c", "d", "d"] =\> ["a", "a", "a", "b", "c", "d", "d"] > > a.select{|i| a.grep(i).size \> 1} =\> ["a", "a", "a", "d", "d"] > **···** > > -- > eban ---
**Author:** ![Mike\_Fletcher](https://avatars.discourse-cdn.com/v4/letter/m/ba8739/32.png) [@Mike\_Fletcher](https://rubytalk.org/u/Mike_Fletcher)\ **Post date:** [12 October 2006 18:42 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/8 "2006-10-12T18:42:32Z")
Jason Burgett wrote: > I'm basically trying to the opposite of .uniq Let's say I have an array: > > ["a", "a", "a", "b", "c", "d", "d"] > > I would like to boil this down by first tossing the values that only > appear once. Which leaves me with: > > ["a", "a", "a", "d", "d"] > > Then I need to somehow determine that "a" appears 3 times and "d" > appears 2 times. Any help would be great. Thanks. a = %w( a a a b c d d ) want\_and\_counts = a.inject( Hash.new(0) ) { |h,x| h+=1; h }.reject { > **···** > > > k,v| v == 1 } > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Robert\_K1](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/robert_k1/32/1830_2.png) [@Robert\_K1](https://rubytalk.org/u/Robert_K1)\ **Post date:** [13 October 2006 14:05 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/9 "2006-10-13T14:05:35Z")
Did anyone suggest inject + partition yet? If not: irb(main):001:0\> a = ["a", "a", "a", "b", "c", "d", "d"] =\> ["a", "a", "a", "b", "c", "d", "d"] irb(main):002:0\> stat = a.inject(Hash.new(0)) do |h, e| irb(main):003:1\* h[e] += 1 irb(main):004:1\> h irb(main):005:1\> end =\> {"a"=\>3, "b"=\>1, "c"=\>1, "d"=\>2} irb(main):006:0\> one, multi = stat.partition {|e,c| c == 1} =\> [[["b", 1], ["c", 1]], [["a", 3], ["d", 2]]] irb(main):007:0\> one =\> [["b", 1], ["c", 1]] irb(main):008:0\> multi =\> [["a", 3], ["d", 2]] irb(main):009:0\> one.map! {|e,c| e} =\> ["b", "c"] Kind regards   robert > **···** > > On 12.10.2006 17:24, Jason Burgett wrote: > > > I'm basically trying to the opposite of .uniq Let's say I have an array: > > > > ["a", "a", "a", "b", "c", "d", "d"] > > > > I would like to boil this down by first tossing the values that only appear once. Which leaves me with: > > > > ["a", "a", "a", "d", "d"] > > > > Then I need to somehow determine that "a" appears 3 times and "d" appears 2 times. Any help would be great. Thanks. ---
**Author:** ![Alex\_Young](https://avatars.discourse-cdn.com/v4/letter/a/ee7513/32.png) [@Alex\_Young](https://rubytalk.org/u/Alex_Young)\ **Post date:** [12 October 2006 15:52 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/10 "2006-10-12T15:52:19Z")
Tim Pease wrote: > > I'm basically trying to the opposite of .uniq Let's say I have an array: > > > > ["a", "a", "a", "b", "c", "d", "d"] > > > > I would like to boil this down by first tossing the values that only > > appear once. Which leaves me with: > > > > ["a", "a", "a", "d", "d"] > > > > Then I need to somehow determine that "a" appears 3 times and "d" > > appears 2 times. Any help would be great. Thanks. > > > > -- > > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). > > class Array > def count( item ) >    return 0 unless self.include? item > >    c = 0 >    self.each {|i| c += 1 if i == item} >    return c > end > end > > ary = %w(a a a b c d d) > new\_ary = ary.select {|item| ary.count(item) \> 1} Better: class Array    def counts # could be golfed, but clarity is king      result = Hash.new{0}      self.each {|a| result[a] += 1}      result    end end a = ["a", "a", "a", "b", "c", "d", "d"] c = a.counts a.reject{|b| c[b] == 1} # =\> ["a", "a", "a", "d", "d"] > **···** > > > On 10/12/06, Jason Burgett \ wrote: > > -- > Alex ---
**Author:** ![Jeff\_Schwab](https://avatars.discourse-cdn.com/v4/letter/j/b5ac83/32.png) [@Jeff\_Schwab](https://rubytalk.org/u/Jeff_Schwab)\ **Post date:** [12 October 2006 16:00 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/11 "2006-10-12T16:00:14Z")
Jeffrey Schwab wrote: > Jason Burgett wrote: > > > I'm basically trying to the opposite of .uniq Let's say I have an array: > > > > ["a", "a", "a", "b", "c", "d", "d"] > > > > I would like to boil this down by first tossing the values that only appear once. Which leaves me with: > > > > ["a", "a", "a", "d", "d"] > > > > Then I need to somehow determine that "a" appears 3 times and "d" appears 2 times. Any help would be great. Thanks. > > # Output: > # ["a", "a", "a", "d", "d"] > # {"a"=\>3, "d"=\>2} > > def get\_counts(keys) >     counts = Hash.new(0) >     keys.each {|k| counts[k] += 1 } >     counts > end > > def non\_uniq(elements) >     counts = get\_counts(elements) >     counts.delete\_if {|k, v| v \< 2 } >     elements.select {|e| counts.key?(e) } > end # Better: def non\_uniq(elements)      counts = get\_counts(elements)      elements.select {|e| counts[e] \> 1 } end > **···** > > > elements = ["a", "a", "a", "b", "c", "d", "d"] > > > > p non\_uniq(elements) > > p get\_counts(elements).delete\_if {|k, v| v \< 2 } ---
**Author:** ![Drew\_Olson](https://avatars.discourse-cdn.com/v4/letter/d/c67d28/32.png) [@Drew\_Olson](https://rubytalk.org/u/Drew_Olson)\ **Post date:** [12 October 2006 16:05 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/12 "2006-10-12T16:05:09Z")
Drew Olson wrote: > Jason Burgett wrote: > > > I'm basically trying to the opposite of .uniq Let's say I have an array: > > > > ["a", "a", "a", "b", "c", "d", "d"] > > > > I would like to boil this down by first tossing the values that only > > appear once. Which leaves me with: > > > > ["a", "a", "a", "d", "d"] > > > > Then I need to somehow determine that "a" appears 3 times and "d" > > appears 2 times. Any help would be great. Thanks > > This should work: > > a = ["a", "a", "a", "b", "c", "d", "d"] > a.uniq.map{|i| i if (a.map{|j| j if j==i}.length \> 1)} Actually, so that you have them appear more than once you'd need to do: a = ["a", "a", "a", "b", "c", "d", "d"] a.map{|i| i if (a.map{|j| j if j==i}.length \> 1)} > **···** > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![Drew\_Olson](https://avatars.discourse-cdn.com/v4/letter/d/c67d28/32.png) [@Drew\_Olson](https://rubytalk.org/u/Drew_Olson)\ **Post date:** [12 October 2006 16:28 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/13 "2006-10-12T16:28:19Z")
Tested, and it doesn't work, so scratch that... > **···** > > -- > Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). ---
**Author:** ![Gavin\_Kistner2](https://avatars.discourse-cdn.com/v4/letter/g/b9e5f3/32.png) [@Gavin\_Kistner2](https://rubytalk.org/u/Gavin_Kistner2)\ **Post date:** [12 October 2006 19:35 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/14 "2006-10-12T19:35:20Z")
WATANABE Hirofumi wrote: > Jason Burgett \ writes: > \> ["a", "a", "a", "d", "d"] > \> > \> Then I need to somehow determine that "a" appears 3 times and "d" > \> appears 2 times. Any help would be great. Thanks. > > % irb > \>\> a = ["a", "a", "a", "b", "c", "d", "d"] > =\> ["a", "a", "a", "b", "c", "d", "d"] > \>\> a.select{|i| a.grep(i).size \> 1} > =\> ["a", "a", "a", "d", "d"] Taking that one step further to meet his second need: class Array   def duplicates\_count     uniq.map{ |e|       if ( count = grep(e).size ) \> 1         { e =\> count }       end     }.compact   end end a = ["a", "a", "a", "b", "c", "d", "d"] p a.duplicates\_count #=\> [{"a"=\>3}, {"d"=\>2}] ---
**Author:** ![Roseanne\_Zhang](https://avatars.discourse-cdn.com/v4/letter/r/f07891/32.png) [@Roseanne\_Zhang](https://rubytalk.org/u/Roseanne_Zhang)\ **Post date:** [12 October 2006 19:16 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/15 "2006-10-12T19:16:12Z")
a = ["a", "a", "a", "b", "c", "d", "d"] puts a.uniq Try it, it works! > **···** > > -- > Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). ---
**Author:** ![Ara.T.Howard6](https://avatars.discourse-cdn.com/v4/letter/a/958977/32.png) [@Ara.T.Howard6](https://rubytalk.org/u/Ara.T.Howard6)\ **Post date:** [12 October 2006 20:53 UTC](https://rubytalk.org/t/isolating-non-unique-items-in-an-array/31829/16 "2006-10-12T20:53:27Z")
that's O(n^2) though... the others are O(n) -a > **···** > > On Fri, 13 Oct 2006, Phrogz wrote: > > > WATANABE Hirofumi wrote: > > > > > Jason Burgett \ writes: > > > > > > > ["a", "a", "a", "d", "d"] > > > > > > > > Then I need to somehow determine that "a" appears 3 times and "d" > > > > appears 2 times. Any help would be great. Thanks. > > > > > > % irb > > > > > > > > a = ["a", "a", "a", "b", "c", "d", "d"] > > > > > > =\> ["a", "a", "a", "b", "c", "d", "d"] > > > > > > > > a.select{|i| a.grep(i).size \> 1} > > > > > > =\> ["a", "a", "a", "d", "d"] > > > > Taking that one step further to meet his second need: > > > > class Array > > def duplicates\_count > >    uniq.map{ |e| > >      if ( count = grep(e).size ) \> 1 > >        { e =\> count } > >      end > >    }.compact > > end > > end > > > > a = ["a", "a", "a", "b", "c", "d", "d"] > > p a.duplicates\_count > > #=\> [{"a"=\>3}, {"d"=\>2}] > > -- > my religion is very simple. my religion is kindness. -- the dalai lama