fimplicit function does not work

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Federico Maglione 2017-9-10

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https://ww2.mathworks.cn/matlabcentral/answers/355923-fimplicit-function-does-not-work

回答： Jim Joy 2017-9-13

Hi everyone I am trying to plot an implicit function with fimplicit but I receive this (orange) warning:

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.

The function (of two variables x and y) is well defined as when I plot the surface z=f(x,y) using ezsurf the surface is produced. (the functions I would like to plot with fimplicit are z and w in the file surfaces.m)

I have attached the code and the data used for the calculations. I suppose that the issue could be related by the use of mvncdf inside the definition of the functions.

Thanks in advance for your help

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Federico Maglione 2017-9-10

编辑：Federico Maglione 2017-9-10

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Same happens if I use

fcontour(w,'LevelList',0)

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Answer 1

Jim Joy 2017-9-13

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https://ww2.mathworks.cn/matlabcentral/answers/355923-fimplicit-function-does-not-work#answer_281446

Hi Federico,

The documentation for "fimplicit" mentions that the input anonymous function must take two matrix inputs, and return a matrix of the same size. See the link below:

https://www.mathworks.com/help/matlab/ref/fimplicit.html#inputarg_f

The anonymous functions "z" and "w" are not defined for matrix inputs. This is due to the fact that the second argument to "mvncdf" must be a row vector with the same number of columns as the first input. However, this condition is not met for inputs with more than one column. One way around this would be to put your function definitions in separate files, and loop over the entries of 'x' and 'y' to evaluate the input at each point in a matrix.

Best Regards,

Jim