Taking an Average in CouchDB

toby ho — Wed, 07 Oct 2009 18:31:02 -0500

More fun with CouchDB, this time taking an average of something:

// map
function(doc){
    emit(null, doc.info.size) // in place of doc.info.size, you'd put whatever

                               // value you want averaged here
}
// reduce
function(keys, values, rereduce) {
    if (!rereduce){
        var length = values.length
        return [sum(values) / length, length]
    }else{
        var length = sum(values.map(function(v){return v[1]}))
        var avg = sum(values.map(function(v){
            return v[0] * (v[1] / length)
            }))
        return [avg, length]
    }
}

The end result will be 2 values, the first is the average, the second is the total number of values that we took an average of. Phew! Who woulda thought taking an average would be so involved!

Retrieve The Top N Tags in CouchDB

toby ho — Wed, 07 Oct 2009 00:33:29 -0500

I am reading up on how map/reduce works in CouchDB, think I am getting the hang of it now, thanks to this great tool. I tried the "Retrieve the top N tags" example on this page. It didn't work for me at all. So I wrote my own as an exercise. Here's my code:

// the map function
function(doc){
    for(var i in doc.tags)
    {
        emit(null, doc.tags[i]);
    }
}

// the reduce function

function(key, values, rereduce){
    var hash = {}
    if (!rereduce){
        for (var i in values){
            var tag = values[i]
            hash[tag] = (hash[tag] || 0) + 1
        }
    }else{
        for (var i in values){
            var topN = values[i]
            for (var i in topN){
                var pair = topN[i]
                var tag = pair[0]
                hash[tag] = (hash[tag] || 0) + pair[1]
            }
        }
    }
    var all = []
    for (var key in hash)
        all.push([key, hash[key]])
    return all.sort(function(one, other){
        return other[1] - one[1]
    }).slice(0, 3)

The approach I took was different one from the one from the example page, but I believe it to be the more correct one: rather than returning the results keyed by the tag in the map step, I would emit every occurrence of every tag instead. Then in the reduce step, I would calculate the aggregation values grouped by tag using a hash, transform it into an array, sort it, and choose the top 3. For the rereduce case, I would combine a set of top 3 choices and then again pick the top 3 among them.

Posts about Couchdb

Taking an Average in CouchDB

Retrieve The Top N Tags in CouchDB