integral function gives the wrong answer

Question

Li Ea 2022-7-25

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1766825-integral-function-gives-the-wrong-answer

编辑： Bruno Luong 2022-7-25

采纳的回答： Chunru

在 MATLAB Online 中打开

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

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Answer 1

Chunru 2022-7-25

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https://ww2.mathworks.cn/matlabcentral/answers/1766825-integral-function-gives-the-wrong-answer#answer_1014155

编辑：Chunru 2022-7-25

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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

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Answer 2

Bruno Luong 2022-7-25

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https://ww2.mathworks.cn/matlabcentral/answers/1766825-integral-function-gives-the-wrong-answer#answer_1014160

编辑：Bruno Luong 2022-7-25

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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