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< ?php
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/*
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Goal:
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* Take a GPX (which is XML) file
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* Extract interesting records from it
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* Calculate the arithmetic average of latitudes and longitutes
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I won't tell you why exactly I needed this, just that it is part of a game
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called geocaching
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—
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Class is called collector, because I am _collecting_ only the records
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I'm interested in
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Example record:
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<wpt lat="59.4658317566" lon="24.8649997711">
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<name>< ![CDATA[Gtehnokratt]]></name>
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<url>< ![CDATA[http://www.geopeitus.ee/?p=350&c=2]]></url>
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Actually there is more data, but I'm only interested in lat and lon attributes
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and name and url elements.
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Records do not have any id-s, only the url has the unique identifier at the end.
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*/
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class Collector extends FilterIterator
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{
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/**
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* I only care about records with id's that are listed here
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*/
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private $ids = array(187, 513, 537, 542, 544, 563, 605, 715, 717, 748);
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/**
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* Set up the FilterIterator by passing our SimpleXMLIterator to it
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*/
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public function __construct(Iterator $it) {
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parent::__construct($it);
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}
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/**
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* Part of FilterIterator implementation, called for each record,
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* responsible for the decision whether the current element is needed
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* or not
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*/
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public function accept() {
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$id = $this->get_id_from_url($this->current()->url);
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if (in_array($id,$this->ids)) {
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return true;
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}
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return false;
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}
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/**
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* Extracts record ID from string
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*/
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private function get_id_from_url($url)
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{
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// http://www.geopeitus.ee/?p=350&c=514
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preg_match('/&c=(\d+)$/',$url,$matches);
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return $matches[1];
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}
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}
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/*
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SimpleXMLIterator is one of the standard SPL iterators, which makes it possible
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to use other standard iterators when parsing XML
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Second argument of simplexml_load_file is a class name and as result the XML
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in-memory representation becomes instance of that class. Seems awkward, but
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really useful
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*/
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$it = simplexml_load_file('geopeitusee.gpx','SimpleXMLIterator');
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$lat_sum = $lon_sum = $count = 0.0;
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// so, all the data goes in, but only the elements I'm interested in, come out
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foreach(new Collector($it) as $element) {
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// access subnodes with $element->name
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// access attributes with array notation
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printf("taking %s, lat=%f, lon = %f\n", $element->name,$element['lat'],$element['lon']);
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// typecasting is needed to get correct results from SimpleXML, otherwise you would
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// get integers
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$lat_sum += (float)$element['lat'];
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$lon_sum += (float)$element['lon'];
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$count++;
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}
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// finally do the math
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printf("Average: cnt=%d lat=%s, lon=%s\n", $count, ($lat_sum / $count), $lon_sum / $count);
