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From: Matteo Caprari <matteo.caprari@gmail.com>
Date: Wed, 9 Dec 2009 19:43:43 +0000
Message-ID: <1bca98390912091143n7fa05f0ged53dd9ef2d913c1@mail.gmail.com>
Subject: Re: Help on grouping data by date
To: user@couchdb.apache.org
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Hi Paul.

This is interesting. I assumed that the rereduce may be feeded with
output from the rereduce itself,
while you seem to imply that the output of the rereduce is always a
final value (ie is one row in the view output).

Am I misreading your mail?

Thanks for the pointers.

On Wed, Dec 9, 2009 at 6:53 PM, Paul Davis <paul.joseph.davis@gmail.com> wr=
ote:
> On Wed, Dec 9, 2009 at 12:55 PM, Matteo Caprari
> <matteo.caprari@gmail.com> wrote:
>> Hello.
>>
>> I'm learning couchdb and I'd appreciate if someone could have look and
>> comment on the code and description that follows.
>>
>> Given a set of documents like {date:"2009-10-11", amount:10500}, I'm
>> using the view below
>> and group_level=3D{1,2,3} to group by year, month or day and return the
>> average amount for each group.
>>
>> I don't think it's possible to write a rereduce-friendly averaging
>> function; Instead, I return an object
>> that holds the total amount and =C2=A0the number of rows for each group.=
The
>> client has to calculate the average on his own.
>>
>> _view/by_date?group_level=3D1 returns one row for each year, with the
>> average amount for that year
>> _view/by_date?group_level=3D2 returns one row for each mont, with the
>> average amount for that month
>> _view/by_date?group_level=3D3 returns one row for each day
>>
>> //map.js:
>> function(doc) {
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0emit(doc.date.split('-'), doc.amount);
>> }
>>
>> // reduce.js
>> function(keys, values, rereduce) {
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0if (rereduce) {
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0var result =3D {t=
ot:0, count:0};
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0for (var idx in v=
alues) {
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0result.tot +=3D values[idx].tot;
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0result.count +=3D values[idx].count;
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0}
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return result;
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0}
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0else {
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0var result =3D {t=
ot:sum(values), count:values.length};
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0return result;
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0}
>> }
>>
>> Thanks :)
>>
>> --
>> :Matteo Caprari
>> matteo.caprari@gmail.com
>>
>
> Matteo,
>
> That all looks pretty spot on. Though there's nothing keeping you from
> adding a line in your rereduce code to just do result.average =3D
> result.tot / result.count before returning. You can see a similar
> example in the reduce.js Futon tests of a similar calculation for
> standard deviation.
>
> There's also the view snippets page on the wiki [1].
>
> Paul Davis
>
> [1] http://wiki.apache.org/couchdb/View_Snippets
>


--=20
:Matteo Caprari
matteo.caprari@gmail.com