GA error "your fitness function must return a scalar value"

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Nikolas Spiliopoulos 2018-4-3

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https://ww2.mathworks.cn/matlabcentral/answers/392397-ga-error-your-fitness-function-must-return-a-scalar-value

评论： Geoff Hayes 2018-4-6

采纳的回答： Geoff Hayes

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Hi all,

I am trying to use GA to minimize a function which is like this:

    f=@(x) ((0.0011*(x(2:180)-x(1:179)))'*exp(-0.0078*x(Battery_num*180+1:Battery_num*180+179)))*Battery_cost/20+(0.0011*(x(182:360)-x(181:359)))'*exp(-0.0078*x(Battery_num*180+179+1:Battery_num*180+358))*Battery_cost/20+...
        (0.0011*(x(362:540)-x(361:539)))'*exp(-0.0078*x(Battery_num*180+359:Battery_num*180+537))*Battery_cost/20+(0.0011*(x(542:720)-x(541:719)))'*exp(-0.0078*x(Battery_num*180+538:Battery_num*180+716))*Battery_cost/20+...
        (0.0011*(x(722:900)-x(721:899)))'*exp(-0.0078*x(Battery_num*180+717:Battery_num*180+895))*Battery_cost/20+
      (0.0011*(x(902:1080)-x(901:1079)))'*exp(-0.0078*x(Battery_num*180+896:Battery_num*180+1074))*Battery_cost/20+...
  (0.0011*(x(1082:1260)-x(1081:1259)))'*exp(-0.0078*x(Battery_num*180+1075:Battery_num*180+1253))*Battery_cost/20;

however I get an error (after a long time), "Your fitness function must return a scalar value".

The problem is that the function f(x) gives a scalar value when I call it in the command window, so I don't understand why I get this error

any ideas?

thanks in advance

Nikolas

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Answer 1

Geoff Hayes 2018-4-3

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Nikolas - your objective function must return a scalar value. In the above, f is returning a matrix instead of the scalar. If I evaluate your f with some dummy data, then this function returns a 179x179 matrix. A scalar is needed so that the GA can compare the fitness of one member of the population against all other members...

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Geoff Hayes 2018-4-4

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You may want to define a function to create your initial population. I suspect that you are just setting nvars (see number of variables to 2513 and then MATLAB is generating a 1x2513 array. Rather than using an anonymous function, you could create one that would guard against this problem

 function [fitness] = myGAFunction(x)
     if isrow(x)
         x = x';
     end
     fitness = ((0.0011*(x(2:180)-x(1:179)))'*exp(-0.0078*x(Battery_num*180+1:Battery_num*180+179)))*Battery_cost/20+(0.0011*(x(182:360)-x(181:359)))'*exp(-0.0078*x(Battery_num*180+179+1:Battery_num*180+358))*Battery_cost/20+...
        (0.0011*(x(362:540)-x(361:539)))'*exp(-0.0078*x(Battery_num*180+359:Battery_num*180+537))*Battery_cost/20+(0.0011*(x(542:720)-x(541:719)))'*exp(-0.0078*x(Battery_num*180+538:Battery_num*180+716))*Battery_cost/20+...
        (0.0011*(x(722:900)-x(721:899)))'*exp(-0.0078*x(Battery_num*180+717:Battery_num*180+895))*Battery_cost/20+…
      (0.0011*(x(902:1080)-x(901:1079)))'*exp(-0.0078*x(Battery_num*180+896:Battery_num*180+1074))*Battery_cost/20+...
  (0.0011*(x(1082:1260)-x(1081:1259)))'*exp(-0.0078*x(Battery_num*180+1075:Battery_num*180+1253))*Battery_cost/20;
 end

I'm assuming that the above function is nested within your main function and so has access to the battery_* variables.

Nikolas Spiliopoulos 2018-4-4

编辑：Nikolas Spiliopoulos 2018-4-4

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Hi again, thanks for the reply. Yeah you're right I have set the nvars variable and matlab probably takes an array of 1x2513 instead of 2513x1.

However, being a beginner in matlab I don't really get it how should I write it in the script file. For the moment I have this:

     f=@(x) ((0.0011*(x(2:180)-x(1:179)))'*exp(-0.0078*x(Battery_num*180+1:Battery_num*180+179)))*Battery_cost/20+(0.0011*(x(182:360)-x(181:359)))'*exp(-0.0078*x(Battery_num*180+179+1:Battery_num*180+358))*Battery_cost/20+...
        (0.0011*(x(362:540)-x(361:539)))'*exp(-0.0078*x(Battery_num*180+359:Battery_num*180+537))*Battery_cost/20+(0.0011*(x(542:720)-x(541:719)))'*exp(-0.0078*x(Battery_num*180+538:Battery_num*180+716))*Battery_cost/20+...
        (0.0011*(x(722:900)-x(721:899)))'*exp(-0.0078*x(Battery_num*180+717:Battery_num*180+895))*Battery_cost/20+(0.0011*(x(902:1080)-x(901:1079)))'*exp(-0.0078*x(Battery_num*180+896:Battery_num*180+1074))*Battery_cost/20+...
        (0.0011*(x(1082:1260)-x(1081:1259)))'*exp(-0.0078*x(Battery_num*180+1075:Battery_num*180+1253))*Battery_cost/20;
options = gaoptimset('UseParallel',true,'Display','iter');
nvars=(N*Battery_num)+(N-1)*Battery_num;
[x,fval]=ga(f,nvars,A,b,Aeq,beq,lb,[],[],options);

can you please explain where i put the function? sorry for being a pain many thanks!!

Geoff Hayes 2018-4-4

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myGAOptimization.m

Hi Nikolas - if we nest our objective function within the main function like

function myGAOptimization
      Battery_num = 42;
      Battery_cost= 24;
      % other variables like A, b, etc.
      function [fitness] = myGAFunction(x)
           if isrow(x)
               x = x';
           end
           fitness = ((0.0011*(x(2:180)-x(1:179)))'*exp(-0.0078*x(Battery_num*180+1:Battery_num*180+179)))*Battery_cost/20+(0.0011*(x(182:360)-x(181:359)))'*exp(-0.0078*x(Battery_num*180+179+1:Battery_num*180+358))*Battery_cost/20+...
              (0.0011*(x(362:540)-x(361:539)))'*exp(-0.0078*x(Battery_num*180+359:Battery_num*180+537))*Battery_cost/20+(0.0011*(x(542:720)-x(541:719)))'*exp(-0.0078*x(Battery_num*180+538:Battery_num*180+716))*Battery_cost/20+...
              (0.0011*(x(722:900)-x(721:899)))'*exp(-0.0078*x(Battery_num*180+717:Battery_num*180+895))*Battery_cost/20+...
              (0.0011*(x(902:1080)-x(901:1079)))'*exp(-0.0078*x(Battery_num*180+896:Battery_num*180+1074))*Battery_cost/20+...
              (0.0011*(x(1082:1260)-x(1081:1259)))'*exp(-0.0078*x(Battery_num*180+1075:Battery_num*180+1253))*Battery_cost/20;
      end
      options = gaoptimset('UseParallel',true,'Display','iter');
      nvars=(N*Battery_num)+(N-1)*Battery_num;
      [x,fval]=ga(myGAFunction,nvars,A,b,Aeq,beq,lb,[],[],options);
end

and save it to a file called myGAOptimization.m it should work. (I don't have the Optimization toolkit so can't verify.) See the attached file for an example.

Nikolas Spiliopoulos 2018-4-6

Well I tried something else, I transpose my fitness function so now you can input a row like 1x2513 and you get a scalar (before I was taking a matrix 179x179).

So thank you very much for all these comments!

the problem now is that it takes ages, but I guess tha'ts another question

thanks again!!