Function or constraints have too few arguments on the right to satisfy the left

I'm trying to optimize this equation:

f=@(x) 0.25*7850*pi^2*x(1).^2.*x(2).*x(3)

with this set of constraints:

g=@(x) [8*75./x(5)-7; 3.5-2*pi*sqrt(75^2/(4*75.*x(4)-x(5).^2)); 100*exp(-pi.*x(5)./sqrt(4*75.*x(4)-x(5).^2))-15; x(2)-0.75; 75*9.81.*(1.566.*x(1).^2-0.9295.*x(1).*x(2)-2.546.*x(2).^2)./(x(1).^3.*(x(1)-x(2)))-517.1*10^6; 78*10^9.*x(1).^4./(8.*x(2).^3.*x(3))-x(4)];

I'm using the fmincon function in the Optimization toolbox, but no matter what algorithm I select, I see the error "The right hand side of this assignment has too few values to satisfy the left hand side" with no further details. I have been trying to figure out the cause of this error, but I'm having difficulty figuring out what, specifically, the error is referring to.

[Richard Brown]

could you provide the full call to fmincon, with any other constraints that you are passing in

[Scott Bacvinskas]

I was literally entering those two functions into the Optimization toolbox, but otherwise I use:

>> f=@(x) 0.25*7850*pi^2*x(1).^2.*x(2).*x(3);

>> g=@(x) [8*75./x(5)-7; 3.5-2*pi*sqrt(75^2/(4*75.*x(4)-x(5).^2)); 100*exp(-pi.*x(5)./sqrt(4*75.*x(4)-x(5).^2))-15; x(2)-0.75; 75*9.81.*(1.566.*x(1).^2-0.9295.*x(1).*x(2)-2.546.*x(2).^2)./(x(1).^3.*(x(1)-x(2)))-517.1*10^6; 78*10^9.*x(1).^4./(8.*x(2).^3.*x(3))-x(4)];;

>> x0=[1;1;1;1;1];

>> [X,FVAL,EXITFLAG,OUTPUT,LAMBDA] = fmincon(f,x0,[],[],[],[],[],[],g)

It highlights some kind of issue with my constraint function (g), but I cannot see what the issue is.

Answer 1

[Richard Brown]

Your nonlinear constraints need to be in a function of the form

function [c, ceq] = g(x)
  c = [8*75./x(5) .......
  ceq = [];
end

MATLAB was complaining because there was no equality constraint information being passed out (even there are no equality constraints).

[Scott Bacvinskas]

Okay, well now I have:

>> f=@(x) 0.25*7850*pi^2*x(1).^2.*x(2).*x(3);

>> c=@(x) [(8*75./x(5)-7) (3.5-2*pi*sqrt(75^2/(4*75.*x(4)-x(5).^2))) (100*exp(-pi.*x(5)./sqrt(4*75.*x(4)-x(5).^2))-15) (x(2)-0.75) (75*9.81.*(1.566.*x(1).^2-0.9295.*x(1).*x(2)-2.546.*x(2).^2)./(x(1).^3.*(x(1)-x(2)))-517.1*10^6)];

>> ceq=@(x) [(78*10^9.*x(1).^4./(8.*x(2).^3.*x(3))-x(4))];

>> g=@(x) [c(x),ceq(x)]

>> x0=[1;1;1;1;1];

>> [X,FVAL,EXITFLAG,OUTPUT,LAMBDA] = fmincon(f,x0,[],[],[],[],[],[],g)

Which eliminated my original error, but now I'm getting the error

"Error using feval

Output argument "varargout{2}" (and maybe others) not assigned during call to "@(x)[c(x),ceq(x)]"."

I'm really not sure what I'm supposed to be doing with this one.

[Richard Brown]

You can't use an anonymous function, it still returns a single output (in this case an array with c and ceq in it) - save a function file that has the structure of the one I posted above.

[Scott Bacvinskas]

Okay, well I've defined a Matlab function as you've specified, but now it's telling me I don't have enough input arguments for the constraints function. I don't know why; I have no problem evaluating it with any x-vector.

[Richard Brown]

Odd ... I had it running before when I tried it:

try this function

http://dl.dropbox.com/u/10495173/foo.m

and use the same f and the same call to fmincon as before (but replace g with @foo)

[Scott Bacvinskas]

You know what it was? I wasn't calling the function with the @ sign. It's running now, albeit very slowly on my terrible laptop.

[Richard Brown]

woohoo!

[Teja Muppirala]

There's a neat trick you can do with anonymous functions to make them return more than one value.

F = @(x) deal( x(1) + x(2), x(1) - x(2) )

[a,b] = F([5 2])

I sometimes use this when trying simple things out in scripts and I don't want to have to save a file.