[Udo]

There is nothing wrong with asking a person to describe binary search. However it's not a relevant interview question. I assert that the number of times a programmer must single-handedly implement binary search without access to an editor, a parser, and the internet, from scratch, is very very low. You could make some argument about how incompetence in binary search algorithms betrays general incompetence, but I'd rather test for that general incompetence directly by looking at a person's projects.

[oconnore]

I think that it's highly likely that a person who can't work out a binary search algorithm probably cannot solve interesting problems in pet projects, unless by interesting, you mean simple web applications or something of that nature.