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Why is this giving me an error message when trying to use trapz.

Asked by Saruultugs Batzorig on 23 Aug 2019
Latest activity Answered by Allen
on 24 Aug 2019
Hello, I'm trying to integrate for each time step the following. But it keeps giving me an error message.
B=18.6;
t0=0;
step=0.5;
tmax=90;
npas=tmax/step+1;
U=60;
for i= 1:npas
t0=t0+step;
s = (U.*t0)./B;
fun3 = @(s) (0.0081*(exp(-0.0058.*s))-0.0392.*exp(-0.0833.*s));
y3 = fun3(s);
q3(i) = trapz(s,y3);
end
B=18.6;
t0=0;
step=0.5;
tmax=90;
npas=tmax/step+1;
U=60;
for i= 1:npas
t0=t0+step;
s = (U.*t0)./B;
fun3 = @(s) (0.0081*(exp(-0.0058.*s))-0.0392.*exp(-0.0833.*s));
y3 = fun3(s);
q3(i) = trapz(s,y3);
end

  6 Comments

Geoff's hint is correct:
trapz() with two parameters can be either
trapz(X, Y)
or
trapz(X, dimension)
The way it tells the two apart is that if the second parameter is a scalar, it assumes that it is a dimension.
You should not be using trapz step by step: you should be recording all of those s and y3 values and doing a trapz() on the final vector of them.
fun3 = @(s) (0.0081*(exp(-0.0058.*s))-0.0392.*exp(-0.0833.*s)); %does not change with i
for i= 1:npas
t0=t0+step;
s(i) = (U.*t0)./B;
y3(i) = fun3(s(i));
end
q3 = trapz(s, y3);
Though I suspect you would prefer to use cumtrapz() instead of trapz() here.

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1 Answer

Allen 님의 답변 24 Aug 2019

A more concise aproach.
B = 18.6;
step = 0.5;
tmax = 90;
npas = tmax/step+1;
U = 60;
fun3 = @(s) (0.0081*(exp(-0.0058.*s))-0.0392.*exp(-0.0833.*s));
s = U/B*step*(1:npas/2/step);
y3 = fun3(s);
q3 = trapz(s,y3);

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