Why is fixed argument of anonymous function not updated, if this variable has changed between calls?

Question

Developing an optimization script, I realized Matlab does not handle secondary arguments of anonymous functions as I expected.

Consider this simple MWE:

%% Define basic parameters
daysTotal       = 3;
hoursTotal      = daysTotal*24;
nhi             = 4;        % Third argument, does not change.
intervalsTotal  = hoursTotal*nhi;
q_in_mean       = 0.82;

% Original definition of var2
var2            = zeros(hoursTotal, 1);
var2(2:2:end)   = 1;        % Second argument, does change!

var1_initial = var2.*q_in_mean/12; % Initial version of first argument

%% Define anonymous function
objFun          = @(var1) objFunModel(var1, var2, nhi);

%% Call objFun for the first time
objFun(var1_initial); % Result of sum(var2) = 36, which is correct.

%% Change var2
var2            = zeros(hoursTotal, 1);
var2(4:4:end)   = 1;

%% Call objFun again
objFun(var1_initial); % Result of sum(var2) is still 36 inside objFunModel
sum(var2) % Actual value of sum(var2) = 18 after change!

%% Separate functions
function varStd = objFunModel(var1, var2, nhi)
    sum(var2)
    varRes  = cumsum(2*var1 - 0.12*var2);
    varStd  = std(varRes);
end

Though var2 is changed between both function calls, it is not updated as you can see in the terminal output. Is this the intended behavior, or a bug? In case of the former, what can I do to force var2 to be updated inside objFunModel? Define the anonymous function again?

It’s intended behavior.

Cris Luengo
– Cris Luengo

2019-08-30 14:13:23 +00:00
Commented Aug 30, 2019 at 14:13 — Cris Luengo
– Cris Luengo, Commented Aug 30, 2019 at 14:13

Adriaan · Accepted Answer · 2019-08-30 14:17:26Z

4

When you create an anonymous function, the variables that are not in the parenthesis following the @ (var2 and nhi in your example), are passed by value, not by reference. Thus, MATLAB has no way of knowing that the variable changed.

edited Aug 30, 2019 at 14:17

Adriaan

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answered Aug 30, 2019 at 14:16

JAC

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1 Comment

winkmal Over a year ago

Thanks for the clarification. So I'll use objFun = @(var1, var2) objFunModel(var1, var2, nhi) to define the anonymous function instead, and call it as objFun(var1_initial, var2). When needed in an optimization, I'll use @(var1) objFun(var1, var2).

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