How can I integrate a set of data that has been fit with a cubic spline interpolation?

I have a few sets of data and I am trying to apply a fitted curve to, which would then allow me to integrate the functions. I have learned how to do this with nearly all the fits (such as polynomial, Fourier, Gauss, etc.). However I've recently found out about the cubic spline interpolation, which produces a piece wise function. However I am running into issues as I do not have any idea how to integrate this. I've been trying for the past few hours to figure this out and I cant seem to make any progress:

Here is a sample of the code I'm working on:

A = csvread('C:\Users\Chris\Desktop\SORA Science\Test Files\Grating #H1.csv');
    x = A(:,1);
        xf = x(end);
    y = A(:,2);
        yf = y(end);
xx = 0:xf
pp = spline(x,y,xx);
plot(x,y,'.',xx,pp);
    axis([0,xf,0,100]);

This code successfully plots my data and applies the cubic spline interpolation for a particular file.

How might I be able to integrate the function that is produced?

Answer 1

[Walter Roberson]

ppint

see also http://www.mathworks.com/matlabcentral/fileexchange/38859-spline-and-piecewise-polynomial-tool