How to calculate partial area under a graph?

Hello,

I am trying to find the area as a part of a graph( Red area in the picture below). I am asking here a simplified question of my complext problem. The area is definitely 0.5. However, when I am using "trapz" function, It is giving me 4.5.

x=[0 1 2 3 4 5 10 15 20];
y=[0 -1 -2 -3 -4 -5 0 5 0];
plot(x,y)
area=trapz(4:5)

Thanks, Frite

[KSSV]

You have asked the same question....and it got answered too.... http://www.mathworks.com/matlabcentral/answers/314470-how-can-i-calculate-the-area-under-a-graph-shaded-area

[friet]

I ask another simplified version of the previous question.

Answer 1

[Star Strider]

You are not coding the integration correctly to integrate the area you want.

Explore this and experiment with it:

x=[0 1 2 3 4 5 10 15 20];
y=[0 -1 -2 -3 -4 -5 0 5 0];

yi = interp1(x, y, [4:5]);
shaded_area = trapz([4:5],yi+5)

plot(x,y)
hold on
patch([4 5 5 4], [yi -5 -5], 'r')
hold off

shaded_area =
   500.0000e-003

[Namira]

I was looking your code. Why did you add 5 with yi in line 4? I can't understand it. Sorry I am not good at Matlab coding.

[Star Strider]

I added 5 because the area to be integrated goes from -5 to the line. The trapz function would otherwise integrate from 0 to the line. The offset ‘resets’ the 0 reference to -5.