问题
I have a multivariable symbolic expression say
c = x^2 +y^2 + z^2
using matlabFunction(c) returns
ans = @(x,y,z)x.^2+y.^2+z.^2
I can't input this into fminsearch (because it has multiple scalar inputs right?). How can I change the format of the output so it takes something that fminsearch actually allows, something like
@(x)x(1)^2+x(2)^2+x(3)^2
It is feasible to do this manually for 3 variables but not for hundreds.
The errors look something like if it helps:
Error using symengine?makeFhandle/@(......) Not enough input arguments.
Error in fminsearch (line 190) fv(:,1) = funfcn(x,varargin{:}):
回答1:
A quick workaround that comes to mind is to create another anonymous function as a go-between:
fun = @(x,y,z)x.^2+y.^2+z.^2;
funMiddleMan = @(x) fun(x(1),x(2),x(3));
For a large number of arguments, the solution becomes a little more complicated. My first instinct is to use str2func in the following manner
nVar = 3;
funMiddleMan = str2func(['@(x)fun(',sprintf('x(%d),',1:nVar-1),'x(',num2str(nVar),'))']);
However, this will not work since str2func
cannot (currently) embed the definition of fun
within a local workspace attached to funMiddleMan
; meaning that calling funMiddleMan
in this manner will generate an "Undefined function 'fun'
" error. The problem can be circumvented by using eval:
funMiddleMan = eval(['@(x)fun(',sprintf('x(%d),',1:nVar-1),'x(',num2str(nVar),'))']);
which will work since the string is actually evaluated; however, the use of eval
is typically discouraged for a number of reasons and is only presented for completeness (and a quick and dirty way to get the ball rolling).
Another option is to use convert the vector x
into a cell array and use comma-separated list expansion in the following manner:
splay = @(x) fun(x{:});
funMiddleMan = @(x) splay(mat2cell(x(:),ones(1,numel(x)),1));
which is not necessarily optimal but works.
来源:https://stackoverflow.com/questions/32364660/matlab-how-do-i-convert-multivariable-symbolic-expression-to-a-function-that-fm