MapReduce - Revision history

40166222: /* Count only some countries */

2018-06-26T08:47:06Z

Count only some countries

← Older revision		Revision as of 08:47, 26 June 2018
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	}		}
	},		},
	function(k, v){ return Array.sum(v); },		function (k, v) { return Array.sum(v); },
	{out: {"inline": 1}}		{out: {"inline": 1}}
	);</nowiki></pre>		);</nowiki></pre>

40166222: /* Number of countries in each continent */

2018-06-26T08:46:51Z

Number of countries in each continent

← Older revision		Revision as of 08:46, 26 June 2018
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	function () {emit(this.continent, 1);},		function () {emit(this.continent, 1);},
	function (k, v) { return Array.sum(v); },		function (k, v) { return Array.sum(v); },
	{out:{inline:1}}		{out: {inline: 1}}
	);</nowiki></pre>		);</nowiki></pre>
	</div>		</div>

40166222: /* Population of each continent */

2018-06-26T08:46:36Z

Population of each continent

← Older revision		Revision as of 08:46, 26 June 2018
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	db.world.mapReduce(		db.world.mapReduce(
	function () {emit(this.continent, this.population);},		function () {emit(this.continent, this.population);},
	function (k, v){ return Array.sum(v); },		function (k, v) { return Array.sum(v); },
	{out: {inline: 1}}		{out: {inline: 1}}
	);</nowiki></pre>		);</nowiki></pre>

40166222: /* Examine the reduce function */

2018-06-26T08:45:50Z

Examine the reduce function

← Older revision		Revision as of 08:45, 26 June 2018
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	==Examine the reduce function==		==Examine the reduce function==
	<div class=q data-lang="mongo">		<div class=q data-lang="mongo">
	Examine the reduce function.		<p class="strong">Examine the reduce function.</p>

	Here we emit the continent and the name, in the reduce function we return v.join(',') to see a comma separated list of the values in the list.		Here we emit the continent and the name, and in the reduce function we <code>return v.join(',')</code> to see a comma separated list of the values in the list.
	<pre class="def"><nowiki>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(

40166222: /* Count only some countries */

2018-06-26T08:45:11Z

Count only some countries

← Older revision		Revision as of 08:45, 26 June 2018
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	==Count only some countries==		==Count only some countries==
	<div class=q data-lang="mongo">		<div class=q data-lang="mongo">
	The map function ~~may~~ emit ~~only sometimes~~.		The map function does not need to emit once for every entry.

	In ~~the~~ example we are only counting the countries that have a large population		In this example we are only counting the countries that have a large population.
	<pre class="def"><nowiki>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(

40166222: /* Number of countries in each continent */

2018-06-26T08:44:26Z

Number of countries in each continent

← Older revision		Revision as of 08:44, 26 June 2018
Line 29:		Line 29:
	Instead of sending populations you can send a list one 1s to the reduce function.		Instead of sending populations you can send a list one 1s to the reduce function.

	~~That was the~~ reduce function will count the number of countries in each continent.		The reduce function will now create a count of the number of countries in each continent.
	<pre class="def"><nowiki>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(

40166222: /* Population of each continent */

2018-06-26T08:43:44Z

Population of each continent

← Older revision		Revision as of 08:43, 26 June 2018
Line 16:		Line 16:
	Here the map function emits the continent and the population for each country.		Here the map function emits the continent and the population for each country.

	The reduce function uses the JavaScript function Array.sum to add the populations.		The reduce function uses the JavaScript function <code>Array.sum</code> to add the populations.
	<pre class="def"><nowiki>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(

40166222: /* Population of each continent */

2018-06-21T19:53:33Z

Population of each continent

← Older revision		Revision as of 19:53, 21 June 2018
Line 21:		Line 21:
	function () {emit(this.continent, this.population);},		function () {emit(this.continent, this.population);},
	function (k, v){ return Array.sum(v); },		function (k, v){ return Array.sum(v); },
	{out:{inline:1}}		{out: {inline: 1}}
	);</nowiki></pre>		);</nowiki></pre>
	</div>		</div>


	==Number of countries in each continent==		==Number of countries in each continent==

40166222 at 19:53, 21 June 2018

2018-06-21T19:53:24Z

← Older revision		Revision as of 19:53, 21 June 2018
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	<pre class="def"><nowiki>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function(){emit(this.continent, this.population);},		function () {emit(this.continent, this.population);},
	function(k, v){ return Array.sum(v); },		function (k, v){ return Array.sum(v); },
	{out:{inline:1}}		{out:{inline:1}}
	)		);</nowiki></pre>
	</nowiki></pre>
	</div>		</div>

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	<pre class="def"><nowiki>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function(){emit(this.continent, 1);},		function () {emit(this.continent, 1);},
	function(k, v){ return Array.sum(v); },		function (k, v) { return Array.sum(v); },
	{out:{inline:1}}		{out:{inline:1}}
	)		);</nowiki></pre>
	</nowiki></pre>
	</div>		</div>

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	<pre class="def"><nowiki>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function()		function () {
	{		if (this.population > 100000000)
	if (this.population>100000000)		{
	emit(this.continent, 1);		emit(this.continent, 1);
			}
	},		},
	function(k, v){ return Array.sum(v); },		function(k, v){ return Array.sum(v); },
	{out:{"inline":1}}		{out: {"inline": 1}}
	)		);</nowiki></pre>
	</nowiki></pre>
	</div>		</div>

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	<pre class="def"><nowiki>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function(){		function () {
	if (this.population>100000000)		if (this.population > 100000000) {
	emit(this.continent, this.name);		emit(this.continent, this.name);
			}
	},		},
	function(k, v){ return v.join(','); },		function (k, v) { return v.join(','); },
	{out:{"inline":1}}		{out: {"inline": 1}}
	)		);</nowiki></pre>
	</nowiki></pre>
	</div>		</div>

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	<pre class="def"><nowiki>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function(){		function () {
	emit(1,1);		emit(1, 1);
	},		},
	function(k, v){ return Array.sum(v); },		function (k, v) { return Array.sum(v); },
	{out:{"inline":1}}		{out: {"inline": 1}}
	)		);</nowiki></pre>
	</nowiki></pre>
	</div>		</div>

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	<pre class="def"><nowiki>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function(){		function () {
	emit(this.continent,this.name);		emit(this.continent, this.name);
	},		},
	function(k, v){ return v[0]; },		function (k, v) { return v[0]; },
	{out:{"inline":1}}		{out: {"inline": 1}}
	)		);</nowiki></pre>
	</nowiki></pre>
	</div>		</div>

40166222 at 13:34, 15 June 2018

2018-06-15T13:34:07Z

← Older revision		Revision as of 13:34, 15 June 2018
Line 17:		Line 17:

	The reduce function uses the JavaScript function Array.sum to add the populations.		The reduce function uses the JavaScript function Array.sum to add the populations.
	<pre class=def>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function(){emit(this.continent, this.population);},		function(){emit(this.continent, this.population);},
Line 23:		Line 23:
	{out:{inline:1}}		{out:{inline:1}}
	)		)
	</pre>		</nowiki></pre>
	</div>		</div>

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	That was the reduce function will count the number of countries in each continent.		That was the reduce function will count the number of countries in each continent.
	<pre class=def>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function(){emit(this.continent, 1);},		function(){emit(this.continent, 1);},
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	{out:{inline:1}}		{out:{inline:1}}
	)		)
	</pre>		</nowiki></pre>
	</div>		</div>

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	In the example we are only counting the countries that have a large population		In the example we are only counting the countries that have a large population
	<pre class=def>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function()		function()
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	{out:{"inline":1}}		{out:{"inline":1}}
	)		)
	</pre>		</nowiki></pre>
	</div>		</div>

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	Here we emit the continent and the name, in the reduce function we return v.join(',') to see a comma separated list of the values in the list.		Here we emit the continent and the name, in the reduce function we return v.join(',') to see a comma separated list of the values in the list.
	<pre class=def>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function(){		function(){
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	{out:{"inline":1}}		{out:{"inline":1}}
	)		)
	</pre>		</nowiki></pre>
	</div>		</div>

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	Here we emit the value 1 as the key and 1 as the value. The reduce function sums those 1s to get a count of the total number of countries.		Here we emit the value 1 as the key and 1 as the value. The reduce function sums those 1s to get a count of the total number of countries.
	<pre class=def>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function(){		function(){
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	{out:{"inline":1}}		{out:{"inline":1}}
	)		)
	</pre>		</nowiki></pre>
	</div>		</div>

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	Here we emit the key '''this.continent''' and the value '''this.name'''.		Here we emit the key '''this.continent''' and the value '''this.name'''.
	The reduce function returns the first element of the collected list.		The reduce function returns the first element of the collected list.
	<pre class=def>		<pre class="def"><nowiki>
	db.world.mapReduce(		db.world.mapReduce(
	function(){		function(){
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	{out:{"inline":1}}		{out:{"inline":1}}
	)		)
	</pre>		</nowiki></pre>
	</div>		</div>