fplot - vectorisation with inequalities

Question

Vect13 2019-7-2

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/469913-fplot-vectorisation-with-inequalities

回答： Star Strider 2019-7-2

Hi,

I am using fplot for the first time, and I am getting the message: Warning: Function behaves unexpectedly on array inputs. To improve performance, properly vectorize your function to return an output with the same size and shape as the input arguments.

After some researches, I found out that fplot requires that I vectorise my code. So I switched / by ./ . However I have some if, elseif, else with inequalities as conditions.

function [phio] = phi_i(x,xi,i)
N=size(x,1)-1;
if i==1
    if (xi(1)<=x && x<=xi(2))
        phio=(xi(2)-x)./(xi(2)-xi(1));
    else
        phio=0;
    end
elseif i==N+1
    if (xi(N)<=x && x<=xi(N+1))
        phio=(x-xi(N))./(xi(N+1)-xi(N));
    else
        phio=0;
    end
else
    if (xi(i-1)<=x && x<xi(i))
        phio=(x-xi(i-1))./(xi(i)-xi(i-1));
    elseif (xi(i)<=x && x<=xi(i+1))
        phio=(xi(i+1)-x)./(xi(i+1)-xi(i));
    else
        phio=0;
    end
end
end

I am using it such that

xi=0:0.5:10;
phii=@(x)phi_i(x,xi,6);
fplot(@(x)phii(x),[0 10],'b')

I guess that all the inequalities such as xi(i-1)<=x are a problem, if xi(i-1) is a scalar and x a vector, isn't it?

How could I remedy that? Thanks

Alex

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Answer 1

Star Strider 2019-7-2

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/469913-fplot-vectorisation-with-inequalities#answer_381667

The fplot function may be a bit sensitive with respect ot the functions you give it. Your code runs without error (only the warnings), so just ignore them.