suhovius · December 31, 2012 16:05 · Feb 22, 2012 · Feb 22, 2012
diff --git a/functions.js b/functions.js
@@ -27,7 +27,6 @@ function reduce(key, values) {
         a.min = Math.min(a.min, b.min);
         a.max = Math.max(a.max, b.max);
     }
-    printjson(a);
 
     return a;
 }

diff --git a/functions.js b/functions.js
@@ -0,0 +1,40 @@
+// derived from http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Parallel_algorithm
+
+function map() {
+    emit(1, // Or put a GROUP BY key here
+         {sum: this.value, // the field you want stats for
+          min: this.value,
+          max: this.value,
+          count:1,
+          diff: 0, // M2,n:  sum((val-mean)^2)
+    });
+}
+
+function reduce(key, values) {
+    var a = values[0]; // will reduce into here
+    for (var i=1/*!*/; i < values.length; i++){
+        var b = values[i]; // will merge 'b' into 'a'
+
+
+        // temp helpers
+        var delta = a.sum/a.count - b.sum/b.count; // a.mean - b.mean
+        var weight = (a.count * b.count)/(a.count + b.count);
+
+        // do the reducing
+        a.diff += b.diff + delta*delta*weight;
+        a.sum += b.sum;
+        a.count += b.count;
+        a.min = Math.min(a.min, b.min);
+        a.max = Math.max(a.max, b.max);
+    }
+    printjson(a);
+
+    return a;
+}
+
+function finalize(key, value){ 
+    value.avg = value.sum / value.count;
+    value.variance = value.diff / value.count;
+    value.stddev = Math.sqrt(value.variance);
+    return value;
+}
diff --git a/gistfile1.js b/gistfile1.js
@@ -0,0 +1,22 @@
+> load('functions.js')
+> db.stuff.drop()
+false
+> db.stuff.insert({value:1})
+> db.stuff.insert({value:2})
+> db.stuff.insert({value:2})
+> db.stuff.insert({value:2})
+> db.stuff.insert({value:3})
+> db.stuff.mapReduce(map, reduce, {finalize:finalize, out:{inline:1}}).results[0]
+{
+	"_id" : 1,
+	"value" : {
+		"sum" : 10,
+		"min" : 1,
+		"max" : 3,
+		"count" : 5,
+		"diff" : 2,
+		"avg" : 2,
+		"variance" : 0.4,
+		"stddev" : 0.6324555320336759
+	}
+}
No results found