# How can I calculate the area between two curves in a specific section?

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daniel ordnung on 27 Jan 2020
Commented: daniel ordnung on 2 Feb 2020
I have two curves which have several intersection points (see attachment). I want to calculate the green filled area in between two intersection points. It works to calculate and display the intersection points. However I am struggling to calculate just this specific (green filled) area with the trapz function since there always pops up following error message:
"Index in position 1 is invalid. Array indices must be positive integers or logical values."
[xj,yj] = polyxpoly(x,y1,x,y2); %calculate intersection points %y1 straight line %y2 mean
mapshow(xj,yj,'DisplayType','point','Marker','o');%show intersection points
A=[xj,yj];
for n=length(xj):-1:1
k=A(n,:);
k(:,2)=[];
l=A(n-1,:);
l(:,2)=[];
isec1= find(y2==l);
isec2= find(y2==k);
a = trapz(isec1:isec2,y1(isec1:isec2,1))-trapz(isec1:isec2 ,y2(isec1:isec2,1));
end

Priyanshu Mishra on 31 Jan 2020
Hi Daniel,
When n=1, n-1 goes to zero for which A(0,:) is an invalid index. The error you are getting is becuse of this only.
daniel ordnung on 2 Feb 2020
perfect thank you!

R2019b

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