# Integrating a Discrete Function in MATLAB

1. Oct 5, 2011

### tramar

I have a data curve with discrete time points that I imported into MATLAB. The x axis is an array named t:

Code (Text):
t =

1.0e+003 *

0.0319
0.0505
0.0851
0.1037
0.1356
0.1648
0.2021
0.2313
0.3616
0.5823
0.8880
1.1778
1.4996
1.7814
The y axis named Cp:

Code (Text):
Cp =

1.0e+004 *

0.0077
0.7846
3.7077
3.2923
1.8769
1.3769
1.0539
0.7769
0.5462
0.3923
0.2692
0.1846
0.1462
0.1385
The plot of the curve looks like this:

Essentially I want to integrate the function from zero to one of the time points. I tried using the functions quad and trapz but they seem to return red error messages. The function trapz works when I integrate over the entire curve (trapz(Cp)) but it won't let me specify an integration interval.

2. Oct 7, 2011

### MATLABdude

Trapz implements the trapezoid rule--if this is what you're looking for, that's great (but since you're not putting in the t-values, you're just getting the sum of x, which may or may not be what you're looking for):
http://www.mathworks.com/help/techdoc/ref/trapz.html

If you want to integrate only a portion of the data, put in only a portion of the vectors!

For instance, to extract the 1st through 5th values of x, you'd type in:
>> extract=x(1:5)

More on array indexing (including the colon operator used above):
http://www.mathworks.com/help/techdoc/learn_matlab/f2-12841.html#f2-428

As per the examples under trapz above, as long as the two input vectors are the same size, trapz should produce something--not sure what the error message is, but if you'd post it, we may be able to help you.

3. Oct 7, 2011

### tramar

I should have posed the question in a better way... Based on the research I've done I think Simpson's rule is more efficient. I was initially trying to use the quad function:

Code (Text):
??? Error using ==> fcnchk at 108
FUN must be a function, a valid string expression, or an inline function object.

Error in ==> quad at 66
f = fcnchk(funfcn);
I'm guessing that the error is because Cp is a discrete function and not a definite function of x. Essentially I need to know how to integrate this curve with Simpson's rule for a certain interval. Is there a way to do it directly or would I need to use the extract=x(..) function?

4. Oct 8, 2011

### MATLABdude

The Simpson's rule implementation in MATLAB requires a function handle. Since you have a collection of data points, this won't work. And as you don't have a huge number of datapoints, efficiency isn't terribly relevant.
http://www.mathworks.com/help/techdoc/ref/function_handle.html

I'd suggest you stick with the trapezoid rule and use the subset extraction method I linked to earlier.