Grouping the output of a CouchDB View - mapreduce

Question

I have a map reduce view:
.....
emit( diffYears, doc.xyz );
reduced with _sum.
xyz is then a number which is summed per integer(diffYears).
The output looks roughly like this:
4 1204.9
5 796.19
6 1124.8
7 1112.6
8 1993.62
9 159.26
10 395.41
11 456.05
12 457.97
13 39.80
14 483.68
15 269.469
etc..
What I would like to do is group the results as follows:
Grouping Total per group
0-4 1959.2 i.e add up the xyz's for years 0,1,2,3,4
5-9 3998.5 same for 5,6,7,8,9 ...etc.
10-14 3566.3
I saw a suggestion where a list was used on a view output here: Using a CouchDB view, can I count groups and filter by key range at the same time?
but have been unable to adapt it to get any kind of result.
The code given is:
{
_id: "_design/authors",
views: {
authors_by_date: {
map: function(doc) {
emit(doc.date, doc.author);
}
}
},
lists: {
count_occurrences: function(head, req) {
start({ headers: { "Content-Type": "application/json" }});
var result = {};
var row;
while(row = getRow()) {
var val = row.value;
if(result[val]) result[val]++;
else result[val] = 1;
}
return result;
}
}
}
I substituted var val = row.key in this section:
while(row = getRow()) {
var val = row.value;
if(result[val]) result[val]++;
else result[val] = 1;
}
(although in this case the result is a count.)
This seems to be the way to do it.
(It is like having a startkey and endkey for each grouping which I can do manually, naturally, but not inside a process. Or is there a way of entering multiple start- and endkeys into one GET command???? )
This must be a fairly normal thing to do especially for researchers using statistical analysis.
I assume therefore that it does get done but I cannot locate examples
as far as CouchDB is concerned.
I would appreciate some help with this please or a pointer in the right direction.
Many thanks.
EDIT:
Perhaps the answer lies in a process in 'reduce' to group the output??

Answer 1

You can accomplish what you want using a complex key. The limitation is that the group size is static and needs to be defined in the view.
You'll need a simple step function to create your groups within map like:
var size = 5;
var group = ( doc.diffYears - (doc.diffYears % size)) / size;
emit( [group, doc.diffYears], doc.xyz);
The reduce function can remain _sum.
Now when you query the view use group_level to control the grouping. At group_level=0, everything will be summed and one value will be returned. At group_level=1 you'll receive your desired sums of 0-4, 5-9 etc. At group_level=2 you'll get your original output.