Why trapz is giving me negative area?

Hey all,

I have two vectors, frequency and power. I'm trying to get the area under the curve of the power plot but it ends up giving me a negative value. When I do trapz(power) alone, it's a positive value. diff(freq) is also positive. I'm attaching the data for your reference. Thanks!

[Matt J]

You should get into the habit of demonstrating the issue you are posting about with code, as I have done below. As you can see, if I just guess the code you used, I don't reproduce what you say you are seeing.

P=load('power').power;
F=load('frequency').freq;

plot(F,P)

trapz(F,P)

ans = 3.3292e-10

Answer 1

[Walter Roberson]

The syntax of trapz() is trapz(X, Y) -- so independent variable followed by dependent variable.

If you trapz(freq, power) then you get a positive value.

[Azarang]

Oh god, my bad! Thanks!

Answer 2

[Les Beckham]

I get a positive value for the area.

load power.mat
load frequency.mat
plot(freq, power)
grid on

area = trapz(freq, power)

area = 3.3292e-10

Maybe you switched the X and Y arguments to trapz. See the doc for trapz. When there are two arguments X comes first.

trapz(power, freq)

ans = -3.3291e-10