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Difference between revisions of "Sum and Count"

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Line 42: Line 42:
 
<div class=q data-lang="py3">Array.sum()
 
<div class=q data-lang="py3">Array.sum()
 
<div class="def">temp = db.world.map_reduce(
 
<div class="def">temp = db.world.map_reduce(
     map=Code("function(){emit(this.name, this.population)}"),
+
     map=Code("function(){emit(this.continent, this.population)}"),
 
     reduce=Code("""function(k,v){
 
     reduce=Code("""function(k,v){
 
                       return Array.sum(v)
 
                       return Array.sum(v)

Revision as of 09:23, 28 July 2015

#ENCODING
import io
import sys
sys.stdout = io.TextIOWrapper(sys.stdout.buffer, encoding='utf-16')
#MONGO
from pymongo import MongoClient
client = MongoClient()
client.progzoo.authenticate('scott','tiger')
db = client['progzoo']
#PRETTY
import pprint
pp = pprint.PrettyPrinter(indent=4)
#CODE
from bson.code import Code

There are many ways to do this in MongoDB.

count() is a cursor method that takes a query and returns a number equal to the amount of documents that matched the query.
$sum is an aggregation operator availible in the $group stage, that can be used to both sum values and count the number of documents.
mapReduce can produce a sum or a count during the results stage by using JavaScript.

.count()
print(db.world.count({"continent":"Africa"}))
$sum
pp.pprint(list(
   db.world.aggregate([
       {"$group":{
           "_id":"$continent",
           "sum of populations":{"$sum":"$population"},
           "count of countries":{"$sum":1}
       }}
   ])

))

Array.sum()
temp = db.world.map_reduce(
   map=Code("function(){emit(this.continent, this.population)}"),
   reduce=Code("""function(k,v){
                      return Array.sum(v)
                  }
               """),
   out={"inline":1}

)

pp.pprint(temp['results'])