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Laze
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Integration producd NaN as answer

Asked by Laze
on 8 Sep 2017
Latest activity Commented on by Star Strider
on 8 Sep 2017
I tried to write a function file to implement a simple integration where 't' as input must be positive.
My code:
function y = Integration(t)
%Input a value t larger than 0, it will return an intergral
if t <= 0
disp('Input must be positive');
y = NaN;
else
syms x;
y = int(-(t-x)*exp(-(t-x))*(t-x>=0)*x^2*exp(-x)*sin(x)*(x>=0));
end
end %end of Integration2
But this code keep giving me NaN when I input postive numbers. Could someone kindly explain? Thanks!

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2 Answers

Answer by Star Strider
on 8 Sep 2017

First, ‘t’ needs to be defined as a sym object;
Second, use the convenient heaviside function to define the unit step function:
syms x t
t = sym(5); % Example
y = int(-(t-x)*exp(-(t-x))*heaviside(t-x)*x^2*exp(-x)*sin(x)*heaviside(x));
This produces an expression for ‘y’ incorporating heaviside terms, since ‘x’ is not defined numerically.
I am not certain what result you want. At the very least, this solves the NaN problem.

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Laze
on 8 Sep 2017
Thanks for your response! What I wanted to do is to a function 'Integration' that can calculate the above formula. I checked the definition of heaveside, it's kinda different from a unit step function because at x=0, heavside(x)=1/2 while u(x) should equal to 1.
Besides, by trying your revision, the problem of NaN has been solved but I still could not get a numerical result rather than the above formular with t substituted by my input. Could you tell me why this is happening? Thanks a lot!
My pleasure.
The ‘unit step function’ is defined as the Heaviside function.

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Answer by Pal Szabo on 8 Sep 2017

I think you should delete the >=0 parts from line 8. Like this:
y = Integration(2)
function y = Integration(t)
%Input a value t larger than 0, it will return an intergral
if t <= 0
disp('Input must be positive');
y = NaN;
else
syms x
y = int(-(t-x)*exp(-(t-x))*(t-x)*x^2*exp(-x)*sin(x)*(x));
end
end %end of Integration2
If you want numerical result, I think you should specify the value of x.

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