integral function gives the wrong answer

Hello!

I meet some trouble when I use the function `integral`.

Such like the following code:

I = integral(@(X) myfun(X,4),0,2) % gives the right answer: 2
I = integral(@(X) myfun(X,4),2,4) % gives the right answer: 2
I = integral(@(X) myfun(X,4),0,4) % gives the wrong answer: 8

function value = myfun(x,L)
    if x <= L/2
        value = 4*x/L;
    else
        value = 4-4*x/L;
    end
end

Answer 1

[Chunru]

To define myfun, you need to ensure that it accept a vector argument x. The original code has problem when x is a vector (if x<L/2 when x is a vector). The following code gives correct answers:

subplot(131); fplot(@(X) myfun(X,4),[0,2])
subplot(132); fplot(@(X) myfun(X,4),[2,4])
subplot(133); fplot(@(X) myfun(X,4),[0,4])

I = integral(@(X) myfun(X,4),0,2) % gives the right answer: 2

I = 2.0000

I = integral(@(X) myfun(X,4),2,4) % gives the right answer: 2

I = 2

I = integral(@(X) myfun(X,4),0,4) % gives the wrong answer: 8

I = 4.0000

function value = myfun(x,L)
    value = zeros(size(x));
    idx = x <= L/2;
    value(idx) = 4*x(idx)/L;
    value(~idx) = 4-4*x(~idx)/L;
    
end

Answer 2

[Bruno Luong]

The function need to be "vectorized", i.e. able to work on an input that is a vector when using with integral

I = integral(@(X) myfun(X,4),0,2) % gives the right answer: 2

I = 2.0000

I = integral(@(X) myfun(X,4),2,4) % gives the right answer: 2

I = 2

I = integral(@(X) myfun(X,4),0,4) % gives the wrong answer: 8

I = 4.0000

% 
function value = myfun(x,L)
value = zeros(size(x));
left = x <= L/2;
value(left) = 4*x(left)/L;
value(~left) = 4-4*x(~left)/L;
end