how to convert co-ordinates of a plot as a single vector ?

I have two vectors that contain a series of nos with a lot of repetitions ranging from 1:100 and from 1:83. but they are of the same length (1x1685)

I want to obtain the co-ordinates as a single vector with length of 83 ranging between the value 1:100 basically i can plot my data as

plot(a(1:1685),b(1:1685));

But I need to get the area under the curve of this plot - its a graph with xlimit 83 and ylimit 100 with 1685 data points

Answer 1

[Star Strider]

To get the area-under-the-curve, use the trapz or cumtrapz function.

AUC_Total = trapz(a(1:1685),b(1:1685));
AUC_Stepwise = cumtrapz(a(1:1685),b(1:1685));

Note that:

AUC_Total = AUC_Stepwise(end);

[Aravind Krishna]

i tried doing it but it didnt work... See this is how my graph looks (attachment). the diagonal got drawn by its own :/

and the area by trapz code u suggested gave me a value of -834 ? (how can area be negative)

[Star Strider]

Without your data I cannot determine what the problem may be. If ‘the diagonal got drawn by its own’, it is part of your data and could be the reason for the negative area.

See if the first and last data are the same. That could be the reason for the diagonal line. If so, eliminate the last element in your data vectors and give it another go.

[Aravind Krishna]

I have attached the file could you tell me now ?

[Star Strider]

It is necessary to eliminate all values where (a==0)&(b==0), since (1) they do not contribute anything to the integral and may cause problems with the calculation, and (2) create the diagonal line. There are 1253 non-zero data points. The AUC is 4984.

This works:

D = load('sample.mat');
a = D.a(1:1685);                                        % Limit Data to Desired Range
b = D.b(1:1685);                                        % Limit Data to Desired Range

idx = (a == 0) & (b == 0);                              % Logical Vector: (a==0 & b==0)
a1 = a(~idx);                                           % Eliminate (0,0) Entries
b1 = b(~idx);                                           % Eliminate (0,0) Entries

AUC_Stepwise = cumtrapz(a1,b1);
AUC_Total = AUC_Stepwise(end);

figure(1)
plot(a1, b1, '-')
text(30, 70, sprintf('AUC = %4d', AUC_Total))

[Aravind Krishna]

THANKS.... this works.... sorry for the late response

[Star Strider]

My pleasure.

No worries!

Answer 2

[Aravind Krishna]

Thanks it worked.... sorry for the late response