Using string to declare function variables

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Josh McCaffrey
Josh McCaffrey 2015-3-30
编辑： Josh McCaffrey 2015-3-30
I have a string containing the names of the variables in the function: varlst = 'x1,x2'
I have function fx= 10*(x1 - 3)^2 + (x2 + 5)^2
I want fx(x1,x2) = 10*(x1 - 3)^2 + (x2 + 5)^2
But when I type fx(varlst) I get fx = [ 10*(x1 - 3)^2 + (x2 + 5)^2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 10*(x1 - 3)^2 + (x2 + 5)^2, 0, 0, 0, 0, 10*(x1 - 3)^2 + (x2 + 5)^2, 10*(x1 - 3)^2 + (x2 + 5)^2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 10*(x1 - 3)^2 + (x2 + 5)^2]
Does anyone have any idea why this is happening and what I can do to get my desired result?
  2 个评论
Josh McCaffrey
Josh McCaffrey 2015-3-30
编辑：Josh McCaffrey 2015-3-30
Here is my code. This is my first time coding with matlab so it might look weird but the idea is to be able to pick out how many variables there are in the user input (x1,x2,...,xn), declare these variables as sym and then perform operations on the function.
c=0; k=[1]; varlst='';
prompt1 = 'Enter function to minimize: '; fxs = '10*(x1-3)^2+(x2+5)^2';
while k ~= 0
c=c+1; num = strcat('x', int2str(c)); k = strfind(fxs, num); if k>0; syms(num); varlst = strcat(varlst, num , ','); end end varlst = varlst(1:end-1); c=c-1;
fx=sym(fxs); fx(varlst)=fx;
fx0d(varlst) = gradient(fx);

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回答（1 个）

Stephen23
Stephen23 2015-3-30
编辑：Stephen23 2015-3-30
Why not simply use an anonymous function to define this:
>> fx = @(x1,x2) 10*(x1 - 3)^2 + (x2 + 5)^2;
>> fx(1,2)
ans = 89
  1 个评论
Josh McCaffrey
Josh McCaffrey 2015-3-30
The thing is I have no idea how many variables the user will input. I need my code to work for (x1,x2,...,xn). I posted my code into a comment above so you can see it.

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