"Subscript indices must either be real positive integers or logicals" error while using trapz function

Hello, I am using following code to calculate area under the peak.

            D = amplitude(:);
            l = 59;
            chg(1) = 1;
            chg(2) = l;         

            for k1 = 1:numel(chg)-1
                segment_area_1k1(a,j) = trapz(chg(k1:k1+1), D(chg(k1:k1+1)));
            end  
        end

The code upon run shows error "subscript indices must either be real positive integers or logicals". I have attached amplitude data for reference. Can anyone solve this problem?

Answer 1

[Ameer Hamza]

What are the values of a and j? They must be a positive integer. Also if you just want to calculate the area under the curve, then pass an entire vector to trapz()

area = trapz(D);

If you want to get segmented area, you can avoid for loop altogether. A better approach is to do it like this

segmentedArea = diff(cumtrapz(D))

[ishita agrawal]

a and j are positive integers.

[Ameer Hamza]

In that case, it is quite strange, because your code works fine with positive values of a and j when I run it. Have you tried the functions I wrote in my answer.

[ishita agrawal]

Actually, this code worked perfectly for other datasets. This time, it is working for some values in the dataset and then this error occurred.

There is something I want to ask. When I tried your code (trapz(D)), the value of calculated area was different than what I calculated using my code. I couldn't understand the reason.

[Image Analyst]

Give your complete code, including the code where you read in the data file, and your complete error message, including line numbers and ALL the red text.

Answer 2

[Image Analyst]

See the FAQ about your error: http://matlab.wikia.com/wiki/FAQ#How_do_I_fix_the_error_.22Subscript_indices_must_either_be_real_positive_integers_or_logicals..22.3F

And see this very short tutorial on how to format your code: format your code