# MAPREDUCE Tutorial

## MapReduce - The Basics

This tutorial introduces the MapReduce command.

MapReduce examples are available.

The MapReduce function allows to functions to be applied over a collection. The two functions map and reduce are important because:
• Together they can solve a huge range of common database queries.
• They can be distributed effectively; you can share the work load among a number of processors and get your answer faster.

It is recommended to have some familiarity with JavaScript before attempting the following questions: JavaScript: Fundamentals
JavaScript | MDN

## Map

The map function can be used to filter and transform the data:

Use a map function to get the names of countries in Europe.

db.world.mapReduce(
function () {
if (this.continent === "Europe"){
emit(this.name, null);
}
},
function (k, v) {
return v;
},
{out: {inline: 1}}
);
db.world.mapReduce(function(){if(this.continent=="Europe"){emit(this.name,null)}},function(_,v){return v},{out:{inline:1}})

Use the previous answer to find the population of the world to the nearest million.

Use the JavaScript round function : Math.round(population/1000000)*1000000
db.world.mapReduce(
function () {
emit(null, null);
},
function (k, v) {
return null;
},
{out: {inline: 1}}
);
db.world.mapReduce(function(){emit('World Population in Millions',this.population)},function(_,values){return Math.round(Array.sum(values)/1000000)*1000000},{out:{inline:1}});

For each letter, determine how many country names begin with that letter.

db.world.mapReduce(
function () {
emit(null, null);
},
function (k, v) {
return null;
},
{out: {inline: 1}}
);
db.world.mapReduce(function(){emit((this.name).substring(0,1),1)},function(_,values){return Array.sum(values)},{out:{inline:1}});

Show the number of countries in each continent.

db.world.mapReduce(
function () {
emit(null, null);
},
function (k, v) {
return null;
},
{out: {inline: 1}}
);
db.world.mapReduce(function(){emit(this.continent,1)},function(_,v){return Array.sum(v)},{out:{inline:1}})

Show the name and area of the smallest 3 countries by area. Ignore records where the area is 0 or null.

db.world.mapReduce(
function () {
emit(null, null);
},
function (k, v) {
return null;
},
{out: {inline: 1}}
);
db.world.mapReduce(function(){emit(this.name,this.area)},function(_,v){return v},{limit:3,out:{inline:1},query:{"\$and":[{"area":{"\$ne":null}},{"area":{"\$ne":0}}]},sort:{"area":1}});

For each continent, find the first and last country alphabetically.

db.world.mapReduce(
function () {
emit(null, null);
},
function (k, v) {
return null;
},
{out: {inline: 1}}
);
db.world.mapReduce(function(){emit(this.continent,{first:this.name,last:this.name})},function(_,v){let r={first:'ZZZ',last:'AAA'};for(let i=0;i<v.length;i++){if(r.first>v[i].first){r.first=v[i].first}
if(r.last<v[i].last){r.last=v[i].last}}
return r},{out:{inline:1}});

Return country names or capital cities that start with a letter 'M' as keys, use null as the value.

db.world.mapReduce(
function () {
emit(null, null);
},
function (k, v) {
return null;
},
{out: {inline: 1}}
);
db.world.mapReduce(function(){if((this.name).startsWith('M')){emit(this.name,null)}
if((this.capital).startsWith('M')){emit(this.capital,null)}},function(_,v){return v},{out:{inline:1}});

For each letter in the alphabet find the amount of cities that start with that letter.
Additionally, find the two cities that come first and last alphabetically for that letter. e.g.

{
"_id" : "B",
"value" : {
"last" : "Bujumbura",
"count" : 25
}
},
db.world.mapReduce(
function () {
emit(null, null);
},
function (k, v) {
return null;
},
{out: {inline: 1}}
);
db.world.mapReduce(function(){this.capital&&emit(this.capital[0],{first:this.capital,last:this.capital,count:1})},function(t,i){return i.sort(function(t,i){return t.first<i.first?-1:1}),{first:i[0].first,last:i[i.length-1].first,count:i.length}},{out:{inline:1}});

Show country count for countries in the following ranges:

0 to 1000000
1000000 to 2000000
2000000 to 3000000
3000000 to 5000000
5000000 to 10000000
10000000 to 15000000
More than 15000000
db.world.mapReduce(
function () {
emit(null, null);
},
function (k, v) {
return null;
},
{out: {inline: 1}}
);
db.world.mapReduce(function(){var p=this.population;switch(!0){case p<1000000:emit("0 TO 1000000",1);break;case p<2000000:emit("1000000 TO 2000000",1);break;case p<3000000:emit("2000000 TO 3000000",1);break;case p<5000000:emit("3000000 TO 5000000",1);break;case p<10000000:emit("5000000 TO 10000000",1);break;case p<15000000:emit("10000000 TO 15000000",1);break
case p>15000000:emit("MORE THAN 15000000",1);break}},function(_,v){return Array.sum(v)},{out:{inline:1}});

Survey