how to use the sum-function in xpath?

Question

There's the sum-function in xpath:

<list>
    <a>1</a>
    <a>3</a>
    <a>4</a>
</list>

Now with SimpleXml...

var_dump($xml->xpath("sum(/list/a)"));

delivers NULL instead of 8

What's wrong here?

see it not working: https://eval.in/135558

EDIT: I've used this workaround for SimpleXml to avoid iterating:

$sum = $xml->xpath("/list/a"); 
$sum = array_sum(array_walk("intval", $sum));

score 3 · Accepted Answer · answered Apr 11 '14 at 17:46

3

If you're using SimpleXML, you're going to have to do it manually. However, you can use DOMDocument to achieve this. Just use evaluate() method to evaluate the XPath expression:

$dom = new DOMDocument;
$dom->loadXML($xml); // $xml is the XML string

$xpath = new DOMXPath($dom);
$sum = (int) $xpath->evaluate('sum(/list/a)'); // => 8

Demo

SimpleXML solution:

$xml = simplexml_load_string($xml);
$sum = 0;

foreach ($xml->a as $node) {
    $sum += (int) $node;
}

echo $sum; // => 8

Demo

answered Apr 11 '14 at 17:46

Amal Murali

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I didn't know before that there is really no way to achieve this with simplexml (because I never tried that). +1 for stating that and another reason to use the `DOM` extension consequently :) – hek2mgl Apr 11 '14 at 17:55
@AmalMurali for `SimpleXml`, I found `$sum = $xml->xpath("/list/a"); $sum = array_sum(array_walk("intval", $sum));` – michi Apr 11 '14 at 21:58
@AmalMurali This is definitely a limit of `SimpleXml`. Usually, I prefer `SimpleXml` a lot over `DOM` for its, well, simplicity. – michi Apr 11 '14 at 22:01

score 1 · Answer 2 · answered Apr 11 '14 at 17:45

Use DOMXPath::evaluate() here:

$str = <<<XML
<list>
    <a>1</a>
    <a>3</a>
    <a>4</a>
</list>
XML;

$output = new SimpleXMLElement($str);
$doc = new DOMDocument();
$doc->loadXML($str);

$selector = new DOMXPath($doc);

var_dump($selector->evaluate('sum(//list/a/text())'));
// double(8)

Side-note: It will return a double not an integer. This might be surprising in the first place.

how to use the sum-function in xpath?

2 Answers2