Calculate integral with an external formula

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??

Answer 1

[David Goodmanson]

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

[Orongo]

Thanks that improved the value. I'm checking my calculation in Excel and values for x>97 still don't reconcile. The calculations I'm doing is:

For x=98 I calculate 0.0067828+5.447812E-08*EXP(0.137849*97)+(98-97)*0.001 = 0.0427247 and the same solution in Matlab gives me 0.3853352. I'm still not grasping the full picture what's happening here.

[David Goodmanson]

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]

Ok. Thanks. I still getting the problem and will close this discussion and open a new one because I think the problem is rippling.