[13 November 2008]

Dimitre Novatchev has called attention to a recent question on the
stackoverflow programming Q and A web site:

I have an XPath expression which provides me a sequence of values like the one below:

1 2 2 3 4 5 5 6 7

It is easy to convert this to a set of unique values “1 2 3 4 5 6 7” using the distinct-values function. However, what I want to extract is the list of duplicate values = “2 5”. I can’t think of an easy way to do this. Can anyone help?

Dimitre’s solution is beautiful and concise: 21 characters long (longer if you use variables with names longer than singler characters), and well worth the five or ten minutes it took me to work out why it works. I won’t explain it to you, dear reader; you deserve the brief puzzlement and Aha-moment of understanding it.

Despite being terse, it’s not the kind of thing you’d enter in an Obfuscated XPath contest, it just uses an idiom I haven’t seen before. I’ll see it again, though, because I’ll use it myself; as I say, it’s beautiful. (I do confess to a certain curiosity about how he would modify it if, as he says, efficiency needed to be addressed.

Dmitre gets my vote for this month’s best programming-language application of Strunk and White’s rule: “Omit needless words!”