Comparing values of 2 vectors and extracting rows from a matrix corresponding to matching value locations.

I'm doing a curve fitting problem in Matlab and so far I've set up some orthonormal polynomials along a specified range of x-values with x = (0:0.0001:40);

The polynomials themselves are each a manipulation of that x vector and are stored as a row in a matrix. I also have some have data entries in the form of two vectors - one for the data x-coords and one for the actual values. I need a way to use the x-coords of my data points to find the same values in my continuous x-vector and then take the corresponding columns from my polynomial matrix and add them to a new matrix.

EDIT: To be more clear. I have, for example, a range of x values in a vector as [0 1 2 3 4 5] from which I've made a vector with the values of y = x^2 -1 which is [-1 0 3 8 15 24]. I also have some actual data values with x-coords in a vector [1 3 4] with corresponding values of [5 3 8]. In order to calculate the difference between my y = x^2 - 1 line and the measured data I need just the columns where the x-values match up. So I want to change the [-1 0 3 8 15 24] to [-1 3 8] but without knowing which columns I'll need beforehand.