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

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Saruultugs Batzorig 2019-8-23

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https://ww2.mathworks.cn/matlabcentral/answers/477351-why-is-this-giving-me-an-error-message-when-trying-to-use-trapz

回答： Allen 2019-8-24

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

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Walter Roberson 2019-8-23

在 MATLAB Online 中打开

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.

Saruultugs Batzorig 2019-8-24

It worked! Thank you so much.

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回答（1 个）

Allen 2019-8-24

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/477351-why-is-this-giving-me-an-error-message-when-trying-to-use-trapz#answer_388909

在 MATLAB Online 中打开

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);