Issue with PopulationSize in Genetic Algorithm toolbox

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Victor
Victor 2026-1-26，17:35
评论： Star Strider 2026-1-27，4:18
There is a project I wanted to use the genetic algorithm tools on and attempted to rig a basic script to test it out. However, I could not run the code because it kept giving me the following error.
Error using gaoptimset>checkfield (line 436)
Invalid value for OPTIONS parameter PopulationSize: must be a positive numeric (not a character vector).
I didn't understand where I had specified PopulationSize as a character vector, so I used optimoptions to change PopulationSize to 50 manually. However it kept throwing me the same error.
When stepped through the program, I entered ga() and found that the default value for PopulationSize is the character vector '50 when numberOfVariables <= 5, else 200' which obviously isn't a numeric value. In any case, this value is supposed to be overridden when optimoptions is used to specify a value other than the default, but it didn't.
Am I doing something wrong or is the genetic algorithm solver broken? Code is as follows.
Further details is that I am doing this on my laptop with a copy of MATLAB R2024b downloaded on it. I have the Global Optimization toolbox installed.
% Testing Genetic Algorithm
clc, clear, close all
rng default % For reproducibility
numberOfVariables = 2;
lb = [-3,-3];
ub = [3,3];
a = 0.1;
b = 0.05; % define constant values
FitnessFunction = @(x) test_fitness(x,a,b);
options = optimoptions("ga",PopulationSize=50);
[x,fval] = ga(FitnessFunction,2,[],[],[],[],lb,ub,[],options);
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Torsten
Torsten 2026-1-26，18:21
编辑：Torsten 2026-1-26，18:36
Ran in:
According to this page
https://uk.mathworks.com/help/gads/ga.html
"PopulationSize" is to be prescribed as a positive integer, not as a character.
But if the above code doesn't work on your computer, I cannot tell you what the problem is.
Maybe you have a second function "ga.m" on your computer ? What do you get if you type
which -all ga.m
/MATLAB/toolbox/globaloptim/globaloptim/ga.m
Maybe there is confusion between MATLAB's "ga" and the Genetic Algorithm Toolbox under
https://uk.mathworks.com/matlabcentral/fileexchange/37998-open-genetic-algorithm-toolbox
?
Victor
Victor 2026-1-27，1:42
Only the one included with MATLAB appears. This issue mystifies me.

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Star Strider
Star Strider 2026-1-26，17:59
编辑：Star Strider 2026-1-26，18:01
Ran in:
The problem is that the arguments to ga are defined by thier positions, and there have to be 10 arguments before the 'options' argument at the end. Adding one more '[],' before hte 'options' argument solves that problem.
All it needs now is the 'test_fitness' function to be defined, and it should work.
Try something like this --
% Testing Genetic Algorithm
clc, clear, close all
rng default % For reproducibility
numberOfVariables = 2;
lb = [-3,-3];
ub = [3,3];
a = 0.1;
b = 0.05; % define constant values
FitnessFunction = @(x) test_fitness(x,a,b);
options = optimoptions("ga",PopulationSize=50);
[x,fval] = ga(FitnessFunction,2,[],[],[],[],lb,ub,[],[],options);
Unrecognized function or variable 'test_fitness'.

Error in solution>@(x)test_fitness(x,a,b) (line 10)
FitnessFunction = @(x) test_fitness(x,a,b);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error in createAnonymousFcn>@(x)fcn(x,FcnArgs{:}) (line 11)
fcn_handle = @(x) fcn(x,FcnArgs{:});
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error in makeState (line 58)
firstMemberScore = FitnessFcn(state.Population(initScoreProvided+1,:));
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error in galincon (line 24)
state = makeState(GenomeLength,FitnessFcn,Iterate,output.problemtype,options);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error in ga (line 420)
[x,fval,exitFlag,output,population,scores] = galincon(FitnessFcn,nvars, ...
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.
EDIT -- (26 Jan 2026 at 18:01)
Minor text tweaks.
.
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Victor
Victor 2026-1-27，2:00
I've ran my code in MATLAB online and it works there. I still have not determined the cause of the initial problem but this is the best answer this far. i'll be doing this project in MATLAB Online for the foreseeable future.

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