Imagine I have a set of discrete points equally spaced out and indexed from 1 to n (a 1D grid). On a cartesian grid if the spacing, dx, is constant the index can be obtained simply by: i = floor(x/dx) That was pretty simple, now if the cartesian grid is stretched (i.e. dx is not constant), it is not clear to me how to go about finding the index analytically. I am guessing that since we know the grid analytically we should be able to find the index analytically regardless if the grid is stretched or not. Any thoughts? Thanks in advance.