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Date: Wed, 18 Aug 2010 13:12:07 +0100
Message-ID: <AANLkTin8VE9kYb5Afdgq+Av_gw4cz=fMpGcKnc=z52QL@mail.gmail.com>
Subject: Re: Struggling with a particular Map / Reduce
From: Ian Wootten <i.wootten@gmail.com>
To: user@couchdb.apache.org
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Thanks Mark.

I've actually ended up using a combination of Martin, Robert's and
your approach in the end.

Seems to be giving me everything I need now.

On 17 August 2010 18:45, Mark J. Reed <markjreed@gmail.com> wrote:
> Bear in mind that this approach doesn't distinguish between multiple
> instances of a given title within a single doc vs multiple docs. =A0From
> your original email I thought you wanted to collapse multiples of the
> same title within one document but count separately multiples if they
> come from different docs.
>
> If that's the case, you'll still want to make the list unique before
> emitting it... but a double loop isn't the way to go there; something
> like this will work:
>
> map: function(doc) {
> =A0var titles=3D{};
> =A0for (var i=3D0; i<doc.titles.length; ++i) {
> =A0 =A0 titles[doc.titles[i]] =3D 1
> =A0}
> =A0for (var title in titles) {
> =A0 =A0 emit(doc.author, title)
> =A0}
> }
>
>
> On Tue, Aug 17, 2010 at 8:17 AM, Ian Wootten <i.wootten@gmail.com> wrote:
>> Thanks guys. I'd been working toward a solution with multiple level
>> keys but had missed this approach for some reason. It's nice to know
>> that at least some part of it has to be implemented in code.
>>
>> Not fully understanding what was being received by the reduce function
>> and how it could be worked upon was the source of my problems.
>>
>> Anyway, I can get what I require from my view now, thanks for the help.
>>
>> On 17 August 2010 11:37, Robert Newson <robert.newson@gmail.com> wrote:
>>> If you emit([doc.docAuthor, doc.titles[title]], 1) instead you could
>>> use the built-in Erlang reduce function "_sum" instead, which is
>>> faster.
>>>
>>> B.
>>>
>>> On Tue, Aug 17, 2010 at 10:24 AM, Martin Higham <martin@ocasta.co.uk> w=
rote:
>>>> I think it would be better to use the View to split the titles and cre=
ate a
>>>> list of Authors and Titles. A Map function such as
>>>>
>>>> function(doc) {
>>>> =A0for (title in doc.titles)
>>>> =A0 =A0 =A0emit([doc.docAuthor, doc.titles[title]], null);
>>>> }
>>>>
>>>> does just this.
>>>>
>>>> You now have a list of keys in the form [Author, title] and they are s=
orted
>>>> by Author.
>>>>
>>>> It's easy to then take these and produce a list of unique Author/title
>>>> combinations and a count of their frequency with the Reduce function.
>>>>
>>>> function(keys, values, rereduce) {
>>>> =A0if (rereduce) {
>>>> =A0 =A0return sum(values);
>>>> =A0}
>>>> =A0else {
>>>> =A0 =A0return values.length;
>>>> =A0}
>>>> }
>>>>
>>>> However it is difficult for reduce to produce a list of the top 3. Any
>>>> processing within the Reduce can only operate on the data passed in. I=
t
>>>> doesn't know what data is yet to come. If you were to output only the =
top 3
>>>> entries passed in to a given invocation of the Reduce you would produc=
e
>>>> inaccurate results as you would potentially throw away rows that might=
 yet
>>>> accumulate into the all time top 3.
>>>>
>>>> Once you have a list of unique Author/title pairs and their frequency =
you
>>>> can either sort and filter them within the client or within a list fun=
ction
>>>>
>>>> Hope this helps
>>>>
>>>> Martin
>>>>
>>>>
>>>> On 17 August 2010 09:26, Ian Wootten <i.wootten@gmail.com> wrote:
>>>>
>>>>> Hi Everyone,
>>>>>
>>>>> I was hoping somebody might be able to solve a problem I'm having
>>>>> attempting to implement a view at the moment.
>>>>>
>>>>> Essentially, what it does is to take a collection of documents which
>>>>> each have a single author and a list of names (which a possibly
>>>>> repeated). There may be multiple documents by the same author, with
>>>>> the same names within. Here's an example doc.
>>>>>
>>>>> doc.author
>>>>> doc.titles =3D ['sometitle', 'someothertitle', 'sometitle, 'anotherti=
tle']
>>>>>
>>>>> I would like to return a list of the top 3 titles across for each
>>>>> author across all documents. I have tried and failed for several days
>>>>> to get this working correctly.
>>>>>
>>>>> So far, my map is as follows, giving the unique titles for a document=
,
>>>>> not ordered at all:
>>>>>
>>>>> function(doc) {
>>>>>
>>>>> =A0var unique_titles =3D [];
>>>>>
>>>>> =A0for(var i in doc.titles)
>>>>> =A0{
>>>>> =A0 =A0 var count=3D0;
>>>>>
>>>>> =A0 =A0 =A0 for(var j in unique_titles)
>>>>> =A0 =A0 =A0 {
>>>>> =A0 =A0 =A0 =A0 if(doc.titles[i]=3D=3Dunique_titles[j])
>>>>> =A0 =A0 =A0 =A0 {
>>>>> =A0 =A0 =A0 =A0 =A0 =A0count++;
>>>>> =A0 =A0 =A0 =A0 }
>>>>> =A0 =A0 =A0 }
>>>>>
>>>>> =A0 =A0 =A0 if(count=3D=3D0)
>>>>> =A0 =A0 =A0 {
>>>>> =A0 =A0 =A0 =A0 unique_titles.push(doc.titles[i]);
>>>>> =A0 =A0 =A0 }
>>>>> =A0}
>>>>>
>>>>> =A0for(var k=3D0; k<unique_titles.length;k++)
>>>>> =A0{
>>>>> =A0 =A0emit(doc.author, unique_titles[k]);
>>>>> =A0}
>>>>> }
>>>>>
>>>>> My map is as follows, this returns two unique titles from a single
>>>>> document when only a single document exists for an author(I think):
>>>>>
>>>>> function(keys, values, rereduce) {
>>>>> =A0return values.splice(0,2);
>>>>> }
>>>>>
>>>>> My problem is that:
>>>>>
>>>>> a) I can't return more than 2 items from the values array (if I set
>>>>> the splice length to 3, futon spits back a non-reducing error at me).
>>>>> b) Where multiple documents exist for the same author, in some
>>>>> instances I see a weird multi-dimensional array returned (rather than
>>>>> just two values). For example:
>>>>> [['sometitle','someothertitle'],['anothertitle'],['afurthertitle']]
>>>>>
>>>>> Presumably b) is the result of multiple documents for a single author
>>>>> interfering with one another, though I'm confused as to how I
>>>>> configure my map/reduce in order to get the information I'm after (I
>>>>> also wonder if its even possible).
>>>>>
>>>>> I've attempted to understand the documentation on reduce functions,
>>>>> taking a look at the many examples that exist too, but I'm unable to
>>>>> understand them well enough to solve my problem.
>>>>>
>>>>> I'd appreciate any help on this!
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Ian
>>>>>
>>>>
>>>
>>
>
>
>
> --
> Mark J. Reed <markjreed@gmail.com>
>