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From |
"Ashim Kapoor" <ashimkapoor@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: RE: A query about sorting. |

Date |
Wed, 27 Aug 2008 11:33:55 +0530 |

Sergey, Thank you for your reply. My OWN simple code takes 40 minutes to run on historical date ( around 10 years ) which sortrows takes minutes. Is'nt that amazing ? Thank you, Ashim. On Wed, Aug 27, 2008 at 8:33 AM, Sergiy Radyakin <serjradyakin@gmail.com> wrote: > Hello All, > > I have probably missed the first part of this thread, but is this what > needs to be done? > > From this: > x1 x2 x3 > 2 3 1 > 3 5 4 > 9 7 9 > > Get this: > x1 x2 x3 > 1 2 3 > 3 4 5 > 7 9 9 > > For all I know, there is no linear complexity O(n) sorting > algorithm(http://en.wikipedia.org/wiki/Sorting_algorithm) > > Here you can see how different algorithms actually perform: > http://linux.wku.edu/~lamonml/algor/sort/sort.html > > For N observations and K variables, Nick suggested reshaping data to > get NxK observations, sort them (perhaps on a complex key) and > reshape it back. I would doubt that this strategy improves > performance, because of the above considerations of nonlinearity of > sorting with respect to N: > > "sorting K values N times" is much simplier than "sorting NxK values". > Just how much easier - it depends. When K is rather small, the gain > can be quite big. Consider e.g. K=2 :) -- a simple conditional swap -- > linear! (number of swaps in the worst case = N, number of comparisons: > also N). > > Reshape itself works quite slow. Like really slow. Because it is > implemented as an .ado program. A commentary by the author of rowsort > notes that "It may be faster to reshape, sort within blocks, and > reshape again". So I take that as a sign of even worse performance. > > Another issue to consider is memory requirements. How much memory does > reshape require? (we are all so concerned about fitting our data into > 2GB/1GB/700MB/... limit, but often forget that once it is there, > working with it requires memory too!) In my experience reshape > requires at least two times as much: > > . memory > bytes > -------------------------------------------------------------------- > Details of set memory usage > overhead (pointers) 14,800 14.45% > data 37,000 36.14% > ---------------------------- > data + overhead 51,800 50.59% > free 50,592 49.41% > ---------------------------- > Total allocated 102,392 100.00% > -------------------------------------------------------------------- > Other memory usage > set maxvar usage 2,041,738 > set matsize usage 1,315,200 > programs, saved results, etc. 45,523 > --------------- > Total 3,402,461 > ------------------------------------------------------- > Grand total 3,504,853 > > . reshape long r p, i(id) > (note: j = 1 2 3 4 5 6) > no room to add more observations > r(901) > > (sorry for mentioning this taboo topic on public, have you ever > estimated how much memory does an XYZ command require? Even Stata > people guesstimate here). > > So Ashim, don't get discouraged and write your own code. It might work > better for your particular case. > > One last advice is that sometimes the problem can be resolved by a > radically different solution. E.g. if your variables are, say age of > children for a particular mother in a [e.g. DHS] survey, you might > find a dataset already in the long-format and start working with it. > The dataset description will let you know what is the ordering there. > In particular, data may be ordered with extra (not available to you) > information which may be lost during sorting. > > E.g.: the codebook may say that children are ordered by age: 6, 8, 8, > 12. You might not know whether 2nd and 3rd children are twins, or were > born in the same year, but the data providers might very well do! > Changing their places (remember that Stata does not guarantee sort > stability for observations, unless special option is used, and I am > not sure how this works for sorting rows) will change their birth > ranks - quite an important characteristic, if we believe a large body > of literature on the topic. > > Hope this helps. > > Anyways, I don't immediately see how -sortrows- or -rowssort- will > deal with this situatuion: > > motherid child_id1 child_id2 child_id3 age1 age2 age3 gender1 gender2 gender3 > ..... > and I might want to sort children by age keeping their ordering stable > and moving their (string) ids and (numeric) gender dummies in sync. > > Regards, Sergiy Radyakin > > > > > > > > On 8/26/08, Nick Cox <n.j.cox@durham.ac.uk> wrote: >> This generated various suggestions. >> >> My main thought is that you should never have to write your own sort >> programs, bubble sort or other, in Stata. >> >> Even sorting rows can make use of Stata's own sort functionality -- as >> in a combination of -reshape- and -sort-, as mentioned by Maarten Buis >> -- or as in -rowsort or -sortrows-, as mentioned in part by Martin >> Weiss. >> >> -rowsort- is on SSC and was written for Stata 8. >> >> -sortrows- is also on SSC and was written for Stata 9. It makes use of >> Mata and in large part supersedes -rowsort-, so long as you have Stata 9 >> at least. >> >> Nick >> n.j.cox@durham.ac.uk >> >> Ashim Kapoor >> >> I have a simple code to write. >> >> I have 6 variables v1, v2 , v3, v4 ,v5 , v6. >> >> I want to sort "according to each observation". That is , in the end >> for each i I want , v1[i] < = v2[i] <=... v6[i]. >> >> So I wrote a simple bubble for each row and then loop from 1 to _N. >> >> Now the problem is that this works but it's VERY VERY slow. Any >> comments anyone ? >> >> >> * >> * For searches and help try: >> * http://www.stata.com/help.cgi?search >> * http://www.stata.com/support/statalist/faq >> * http://www.ats.ucla.edu/stat/stata/ >> > * > * For searches and help try: > * http://www.stata.com/help.cgi?search > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ > * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: A query about sorting.***From:*"Ashim Kapoor" <ashimkapoor@gmail.com>

**st: RE: A query about sorting.***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

**Re: st: RE: A query about sorting.***From:*"Sergiy Radyakin" <serjradyakin@gmail.com>

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