- **Ruby**
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- - **count : array to array of hashes**
(*http://www.velocityreviews.com/forums/t842925-count-array-to-array-of-hashes.html*)

count : array to array of hashesHi,
I'm tryind to count distinct ids from an array (array_of_ids = [1,2,3,1,2]), and want a result array of hashes looking like it : result = [{;id=>1,:count=>2},{;id=>2,:count=>2},{;id=>3,:cou nt=>1}] i tried it, but it always returns errors : result = array_of_ids.inject(Array.new){ |a,x| elt = a.find{|h| h[:id] == x} || {:id => x, :count => 0}; elt[:count] += 1; elt} i took my information from this post : http://groups.google.com/group/comp....a7b10da8195cec If someone could help. regards |

Re: count : array to array of hashesOn 8/1/07, Jean-S=E9bastien <jeansebastien.ney@gmail.com> wrote:
> Hi, > I'm tryind to count distinct ids from an array (array_of_ids =3D > [1,2,3,1,2]), and want a result array of hashes looking like it : > result =3D [{;id=3D>1,:count=3D>2},{;id=3D>2,:count=3D>2},{;id =3D>3,:coun= t=3D>1}] > > i tried it, but it always returns errors : > result =3D array_of_ids.inject(Array.new){ |a,x| elt =3D a.find{|h| h[:id= ] > =3D=3D x} || {:id =3D> x, :count =3D> 0}; elt[:count] +=3D 1; elt} > > i took my information from this post : > http://groups.google.com/group/comp....thread/312221= 2f6db8d7f5/04a7b10da8195cec?lnk=3Dgst&q=3Darray++count&rnum=3 D8#04a7b10da81= 95cec > > If someone could help. > > regards > > > irb(main):005:0> a=3D[1,2,3,42,2,2,3] =3D> [1, 2, 3, 42, 2, 2, 3] irb(main):006:0> a.inject([]){|r,ele| r[ele][:count]+=3D1 rescue r[ele]=3D{:id =3D> ele, :count=3D>1}; r}.compact =3D> [{:count=3D>1, :id=3D>1}, {:count=3D>3, :id=3D>2}, {:count=3D>2, :id= =3D>3}, {:count=3D>1, :id=3D>42}] irb(main):007:0> HTH Robert --=20 [...] as simple as possible, but no simpler. -- Attributed to Albert Einstein |

Re: count : array to array of hashesAlle mercoled=EC 1 agosto 2007, Jean-S=E9bastien ha scritto:
> Hi, > I'm tryind to count distinct ids from an array (array_of_ids =3D > [1,2,3,1,2]), and want a result array of hashes looking like it : > result =3D [{;id=3D>1,:count=3D>2},{;id=3D>2,:count=3D>2},{;id =3D>3,:coun= t=3D>1}] > > i tried it, but it always returns errors : > result =3D array_of_ids.inject(Array.new){ |a,x| elt =3D a.find{|h| h[:id] > =3D=3D x} || {:id =3D> x, :count =3D> 0}; elt[:count] +=3D 1; elt} > > i took my information from this post : > http://groups.google.com/group/comp....thread/312221= 2f >6db8d7f5/04a7b10da8195cec?lnk=3Dgst&q=3Darray++count&rnum=3 D8#04a7b10da819= 5cec > > If someone could help. > > regards There are two problems in your code: 1- in the inject block, you return elt, which is (or should be, if the code= =20 worked) the hash containing the id which is being processed. You should=20 return a, that is the array which contains the hashes. Correcting this shou= ld=20 give a piece of code which executes without errors, but which returns an=20 empty array 2- you never insert the hashes you create inside the inject block into the= =20 array a: you only store them in the local variable elt, which gets destroye= d=20 after each iteration. The inject block should be: result =3D array_of_ids.inject(Array.new){ |a,x|=20 elt =3D ( a.find{|h| h[:id] =3D=3D x} || a[a.size] =3D {:id =3D> x, :co= unt =3D> 0} ) elt[:count] +=3D 1 a } As you can see, after the || operator a new hash is created and inserted at= =20 the end of a (corresponding to the index a.size). Since an assignment alway= s=20 return the value being assigned (this is why I didn't use <<, it returns th= e=20 array, not the inserted element), elt is then set to the new hash. Of cours= e,=20 all this happens only if find returns nil. If you can rely in the id to be positive integers, and don't care if the=20 resulting array contains the hashes in the same order as the id are stored = in=20 the array, here's another approach you can consider: result =3D array_of_ids.inject([]) do |res, i| res[i-1] ||=3D {:id =3D> i, :count =3D> 0} res[i-1][:count] +=3D 1 res end This code stores the data relative to the id i in the i-1 position in the=20 array (the -1 is there to avoid a nil element at the beginning). This shoul= d=20 make it faster, since you don't need to iterate all the array to check=20 whether the data corresponding to an id is already there or not: either is = in=20 the position id - 1 or it itsn't there. A quick benchmark, done by creating= =20 an array of ids of 100_000 elements, with values randomly chosen between 1= =20 and 11 gives: user system total real original approach 5.730000 1.490000 7.220000 ( 7.310224) user system total real alternative approach 1.250000 0.150000 1.400000 ( 1.416871) Changing the range of the ids from 11 to 101 gives: user system total real alternative approach 1.270000 0.190000 1.460000 ( 1.472353) user system total real original approach 37.730000 11.360000 49.090000 ( 51.056527) Increasing it to 1001 gives user system total real alternative approach 1.500000 0.220000 1.720000 ( 1.733568) The original approach takes much more time (I didn't have the patience to w= ait=20 for it to complete). I hope this helps Stefano |

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