Call an anonymous function within a custom function MATLAB

Question

I want to call an anonymous function in a custom MATLAB function. From the script that calls the custom function, I want to minimize the custom MATLAB function. I am having a problem with the anonymous function not being passed correctly to the function.

As a MWE, I have a script, in which I define an anonymous function, afunction,

% directory for minimization algorithm
addpath '/somedirectory/FMINSEARCHBND'

% anonymous function
afunction = @(x) x.^2 + 2.*x - 71;

% 1D minimization guesses
xguess = 20;
xmin = -1000;
xmax = 1000;

% 1D minimization call
minx = fminsearchbnd(@(x) MWEtestfuntominimize(x), xguess, xmin, xmax);

I then have a custom function written in another file MWEtestfuntominimize,

function g = MWEtestfuntominimize(x)
    g = abs(afunction(x));
end

I expect my main script to minimize MWEtestfuntominimize, but it seems that MWEtestfuntominimize cannot call afunction. The error message is

Undefined function or variable 'afunction'

I have tried passing afunction through MWEtestfuntominimize as an argument, which was unsuccessful. This was through modifying minx in the minimization call as

minx = fminsearchbnd(@(afunction,x) MWEtestfuntominimize(afunction,x), xguess, xmin, xmax);

and modifying the custom function to

function g = MWEtestfuntominimize(afunction,x)
    g = abs(afunction(x));
end

The resulting error was

"afunction" was previously used as a variable, conflicting with its use here as the name of a function or command.

I know a solution would be to define the anonymous function in MWEtestfuntominimize itself, but for the specific program I am writing, I don't want to do this.

Wolfie · Accepted Answer · 2019-08-26 13:07:53Z

2

You said that passing afunction "was unsuccessful" but didn't show why... that's exactly how I'd solve this issue

% anonymous function
afunction = @(x) x.^2 + 2.*x - 71;
% 1D minimization call
minx = fminsearchbnd(@(x) MWEtestfuntominimize(x, afunction), xguess, xmin, xmax);

Then inside your minimization function...

function g = MWEtestfuntominimize(x, fcn)
    g = abs( fcn(x) );
end

To expand on why this works, fminsearchbnd expects a function with a single input. This is a function with only one input (x)

@(x) MWEtestfuntominimize( x, afunction )

The function handle (or other variable) afunction is stored within the anonymous function, with the same value within the workspace at that point. Note that if afunction changes afterwards, it will not change within your anonymous function.

A simple example would be

a = 2;
f = @(x) x + a;

f(5); % = 7

a = 4; % change 'a' after the definition of 'f'
f(5); % = 7, does not change as f = @(x) x + 2 still

edited Aug 26, 2019 at 13:07

answered Aug 26, 2019 at 11:06

Wolfie

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1 Comment

Physics314 Over a year ago

This works, but why is the argument of fminsearchbnd only @(x), and not @(afunction,x)?

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