Calculate integral with an external formula

Question

Orongo 2017-10-11

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https://ww2.mathworks.cn/matlabcentral/answers/360838-calculate-integral-with-an-external-formula

评论： Orongo 2017-10-15

I calculating an integral over a function that is defined in seperate m-file. My program looks like:

% main.m
mu_x=@(t) lx(t);
l_x = exp(-integral(@(t) mu_x(t),0,98));
% lx.m
function res=lx(x)
a=0.006782872;
b=5.44781E-08;
c=0.137849468;
if x>97
    res = a+b*exp(c*97)+(x-97)*0.001;
else
    res=a+b*exp(c*x);
end

I expect lx(x)=lx(98) => res=a+b*exp(c*97)+(x-97)*0.001; to be calculated but instead res=a+b*exp(c*x) is done. I want to keep this if-function in lx.m, is there anyway I can do this? Also, what is going wrong??

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Answer 1

David Goodmanson 2017-10-11

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https://ww2.mathworks.cn/matlabcentral/answers/360838-calculate-integral-with-an-external-formula#answer_285302

编辑：David Goodmanson 2017-10-11

在 MATLAB Online 中打开

Hi Lenovo,

Your 'if' check is not working correctly, because the res function takes vector input for x. One way to do the task is to create an index that shows whether or not x > 97:

x = 0:.01:120;
plot(x,lx(x))    % demo
function res=lx(x)
a=0.006782872;
b=5.44781E-08;
c=0.137849468;
res = zeros(size(x));
ind = x>97;
res(ind) = a+b*exp(c*97)+(x(ind)-97)*0.001;
res(~ind) =a+b*exp(c*x(~ind));
end

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David Goodmanson 2017-10-12

I just plugged your expression

0.0067828+5.447812E-08*EXP(0.137849*97)+(98-97)*0.001

verbatim into Matlab, changed EXP to exp and got 0.0427, just like the demo plot in the posted answer gives and Excel gives. So I do not know where the .385 might be coming from.

Orongo 2017-10-15