# Running area under the curve at each time step.

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Jozef on 10 Feb 2013
Hello,
In the problem that I am working on, I am given an x and y set of data. The x being the time and the y being ft/s^2. I need to find the running area under the curve because that will be the velocity, needless to say I am having some troubles. The code is below
My code:
clear;
x=A(:,1)
y=A(:,2)
for i=1:length(A) %run the length of the array
v(i)=trapz(x(i),y(i)) %integral at each point
end
plot(x,v) %plot integration over same time step
I get an error stating "ORDER contains an invalid permutation index."
Am I at all on the right track for trying to solve this?
Thank you.

ChristianW on 10 Feb 2013
Your input to trapz is a single point, you want to input a line. And for n = length(x) acceleration points you can only get n-1 velocity points.
clear;
x = 0:10;
y = sin(x);
for i=2:length(x) %run the length of the array
v(i-1) = trapz(x(1:i),y(1:i)); % integral at each point
end
subplot(211),plot(x,y)
subplot(212),plot(x(2:end),v)
Jozef on 10 Feb 2013
A Thousand thank yous! hours were wasted, the solution was to simple...
Walter Roberson on 10 Feb 2013
I would suggest using cumtrapz instead of this loop.