in reply to Re^2: Lookup closest hash key
in thread Lookup closest hash key

The OP is only interested in nearest in a one-dimensional space.
Actually, I don't think he is. This looks like an XY problem to me, and I think Corion thought the same thing...

Distances between cities is intrinsically a 2-dimensional problem.

We don't know what the rest of the OP's code does, but I assume it's something that could be better integrated with a solution to the question that was asked.