Find the input values of a function within an output range.

Question

Edward Simpson Songel 2021-6-9

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评论： Nigel Brauser 2022-11-4

Let us say we have a funtion like y= @(n) f(n).

How would you find the range of n values for the y range y[ ymin,ymax] is there some sort of inverse function command in MATLAB with which you could do this? I.E. n=@(y) f^(-1)(y) or something similar.

A practical example would be:

Rend = @(n) Pu(n)/Pa(n);   % Where Pu(n) and Pa(n) are other functions which also depend of n.
% Find value range of n for Rend==[R1,R2] where R1 is the min of the range
% and R2 the max of the range.

Thanks in advance.

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Answer 1

Joel Lynch 2021-6-9

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编辑：Joel Lynch 2021-6-9

在 MATLAB Online 中打开

If you cannot solve algebraically (no closed form solution), then fzero is the best bet. Something like this:

% example y range
y = linspace(ymin,ymax); 
% Create empty array of n values of same size
nsol = zeros(size(y));
% Loop y values
for i=1:numel(y)
    
    % grab local y
    ylocal = y(i); 
    
    % Setup fzero function function 
    myfunc = @(n) Pu(n)/Pa(n) - ylocal;
    
    % Solve for myfunc=0
    nsol(i) = fzero( myfunc, nguess );
    
end

Note that "fzero" is being adapted from solving for 0, to solving for ylocal by subtracting that term. There are probably faster ways to get "myfunc" defined outside the loop.