Question

For example:

<a>
  <b>
    <c1>c1 text</b1>
    <c2 c2attr="10">c2 text</b2>
    <c3>c3 text</b3>
  </b>
  <b>
    <c1>c1 text</b1>
    <c2 c2attr="5">c2 text</b2>
    <c3>c3 text</b3>
  </b>
</a>

I want to select the <b> element which has a <c2> child with the maximum c2attr attribute; in this example, the first one (10 > 5).

I have tried

/a/b[not(c2@c2attr <= preceding-sibling::c2@c2attr) and not(c2@c2attr <=following-sibling::c2@c2attr)]

But PHP says that it’s invalid…

Answer 1

You’re missing slashes before the @ signs, and you need to look for preceding-sibling elements named b, not c2:

/a/b[not(c2/@c2attr <= preceding-sibling::b/c2/@c2attr) and not(c2/@c2attr <=following-sibling::b/c2/@c2attr)]

However this kind of check will be quadratic in the number of b elements, as it’s comparing every b with every other b. It would be more efficient to simply extract all the b elements, then iterate over them (once) using PHP code, keeping track of the highest c2attr value you’ve seen so far and which b that belongs to. This would be linear rather than quadratic.