# Why this piece of code gives error?

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Sadiq Akbar on 20 Apr 2022
Commented: Steven Lord on 21 Apr 2022
CostFunction='fun4sn0';
nPop=120;
nVar=4;%nVar=1000;
nPop_Initial=nPop;
nPop_Final=30;
MaxNFE=3e3;
MaxIter0=ceil(MaxNFE/((nPop_Initial+nPop_Final)/2)); % Approximate Maximum Iterations
Cr=0.5;
xmin=[0 0 0 0];
xmax=[10 10 pi pi];
%%%%%%%%%%%
NFE=0;
Gbest.Position=[];
Gbest.Cost=inf;
BestCosts=nan(1,MaxIter0);
nfe=BestCosts;
for i=1:nPop
Velocity(i,:)=zeros(1,nVar); %#ok<*SAGROW>
Position(i,:)=xmin+(xmax-xmin).*rand(1,nVar);
Cost(i)=CostFunction(Position(i,:));
PbestPosition(i,:)=Position(i,:);
PbestCost(i)=Cost(i);
PbestVel(i,:)=Velocity(i,:);
if PbestCost(i)<Gbest.Cost
Gbest.Position=PbestPosition(i,:);
Gbest.Cost=PbestCost(i);
Gbest.Velocity= PbestVel(i,:);
end
end
##### 2 CommentsShowHide 1 older comment
Sadiq Akbar on 20 Apr 2022
Thank you very much dear Jon for your response. It gives the following error:
Subscript indices must either be real positive integers or logicals.
Error in WGA1 (line 37)
Cost(i)=CostFunction(Position(i,:));

Steven Lord on 20 Apr 2022
Either make your CostFunction variable a function handle to the fun4sn0 function (which is the approach I'd prefer):
CostFunction= @fun4sn0;
or use feval to evaluate the function by name.
Cost(i) = feval(CostFunction, Position(i, :));
Both of these assume that your function returns a scalar, since you're assigning its output into one element of the Cost variable.
Steven Lord on 21 Apr 2022
@Image Analyst That is another option, and is the easiest if you want to hard-code the cost function. I think that particular name put me in "this is likely going to be used in some form of optimization" mode and in that case (the "function function" scenario, see ode45, fminsearch, fzero, integral, etc.) to keep the code as a general purpose solver you wouldn't want to hard-code the function.

Image Analyst on 20 Apr 2022
CostFunction is a character array, not an actual function. Did you mean
Cost(i) = fun4sn0(Position(i,:));
Cost(i)=CostFunction(Position(i,:));
Thorough discussion of the error in the FAQ:
Sadiq Akbar on 21 Apr 2022
Also thanks a lot dear Image Analyst.