CODEWITHSUNDEEP
matlabplot
mvcorrea
mvcorrea

Reputation: 101

2 different outputs when trying to fplot function

trying to understand how matlab plot functions (fplot/ezplot) works.

dont understand why different outputs.

in the original example I see a dashed line on the x=2 (none here) where there is no value.

any help on printing the "right" version via fplot?

I noticed in the matlab help they point you not to use ezplot but fplot

none of the references helped a lot :(

syms x
fx = (x+2)/(x^2 - 4);
ht = matlabFunction(fx);

figure(1), clf

fplot(ht,[0 10])     % wrong plot
title('using fplot');

%ezplot(ht,[0 10])   % prints ok
%title('using ezplot');

using ezplot

enter image description here

references: https://www.mathworks.com/matlabcentral/answers/32820-trouble-substituting-a-value-into-a-symbolic-expression-for-use-with-fplot heaviside function procudes different outputs in ezplot and fplot

Upvotes: 0

Views: 231

Answers (1)

Cris Luengo
Cris Luengo

Reputation: 60494

I'm using MATLAB R2017a.

When I run this code:

f = @(x) (x+2)./(x.^2 - 4);
fplot(f,[0 10])

I see the following plot:

correct plot

I'm not using symbolic math above. I directly define an anonymous function that does numeric computation. I'm using ./ and .^, the element-wise operators, because fplot evaluates the function at many locations at the same time, by passing in x as an array, as described in the documentation:

The function must accept a vector input argument and return a vector output argument of the same size.

Note that OP's code with matlabFunction should produce an equivalent function, I don't know what might be wrong there. But in general, avoiding symbolic math when it is not necessary is always beneficial. (By "necessary" I mean if, for example, you are trying to find an analytical solution to an equation.)

PS: In MATLAB R2021a I ran this code and OP's code, and obtained the exact same result. The ht from OP's code, and the f from my code are identical. It is possible that there was a bug in a specific version of MATLAB that has been fixed in a later version.

Upvotes: 2

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