problems about solve exponential function

Question

xueqi 2013-2-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/63283-problems-about-solve-exponential-function

评论： TUHAME STEVEN 2015-10-28

Hi,

please have a look at a part of my code in the function maxmin0. The general idea is that I use solve to get the solution for a exponential function funcion, and then assign the solution to a variable called pf5, which is the output for maxmin0. The function runs well itself. But when I call this function in another function, it always show a error message . I am doubting it is the way I write the this function is not smart and so there is problem when it is being called...Could you help to check that?

Here is the code

    if true
      %  p1=1-lb2-lb3;
              p2=lb2;
              p3=lb3;
              if (d2(i,3)==d2(i,1))
                  pf5(2,i)=[0;0];
                  eu(i)=-Inf;
              else
  coef1=d1(i,1)+(d1(i,1)-d1(i,3))*d2(i,1)/(d2(i,3)-d2(i,1));
  coef2=d1(i,2)+(d1(i,1)-d1(i,3))*d2(i,2)/(d2(i,3)-d2(i,1));
  coef3=d1(i,3)+(d1(i,1)-d1(i,3))*d2(i,3)/(d2(i,3)-d2(i,1));
  syms x 
  f5=simplify(p1*coef1*exp(-r*coef1*x)+p2*coef2*exp(-r*coef2*x)+p3*coef3*exp(-r*coef3*x));
  y5(i)=solve(f5,x);
  pf5(1,i)=double(y5(i));
    end

and here is the error message

Warning: Explicit solution could not be found. 
> In solve at 169
  In maxmin0 at 138
  In maxmin at 15
  In beta at 18
  In C:\Program Files\MATLAB\R2012a\toolbox\shared\optimlib\finDiffEvalAndChkErr.p>finDiffEvalAndChkErr at 26
  In C:\Program Files\MATLAB\R2012a\toolbox\shared\optimlib\finitedifferences.p>finitedifferences at 128
  In C:\Program Files\MATLAB\R2012a\toolbox\optim\optim\private\computeFinDiffGradAndJac.p>computeFinDiffGradAndJac at 28
  In C:\Program Files\MATLAB\R2012a\toolbox\optim\optim\barrier.p>barrier at 135
  In fmincon at 841
  In main at 6 
Error using mupadmex
Error in MuPAD command: Subscripted assignment dimension mismatch
Error in sym/subsasgn (line 1638)
                C = mupadmex('symobj::subsasgn',A.s,B.s,inds{:});
Error in maxmin0 (line 138)
y5(i)=solve(f5,x);
Error in maxmin (line 15)
[PF0,eu0]=maxmin0(sub,DD);
Error in beta (line 18)
mu = maxmin(sub,DD);
Error in C:\Program Files\MATLAB\R2012a\toolbox\shared\optimlib\finDiffEvalAndChkErr.p>finDiffEvalAndChkErr (line 26)
Error in C:\Program Files\MATLAB\R2012a\toolbox\shared\optimlib\finitedifferences.p>finitedifferences (line 128)
Error in C:\Program Files\MATLAB\R2012a\toolbox\optim\optim\private\computeFinDiffGradAndJac.p>computeFinDiffGradAndJac (line 28)
Error in C:\Program Files\MATLAB\R2012a\toolbox\optim\optim\barrier.p>barrier (line 135)
Error in fmincon (line 841)
    [X,FVAL,EXITFLAG,OUTPUT,LAMBDA,GRAD,HESSIAN] = barrier(funfcn,X,A,B,Aeq,Beq,l,u,confcn,options.HessFcn, ...
Error in main (line 6)
[x,fval,exitflag,output,lambda,grad] =  fmincon(@beta,[0.2,0.2,0.2,0.01,20,0.1,20,0.1],A,b,[],[],lb,ub,[],options)

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xueqi 2013-2-16

Hi do you mean the explicit expression for f5? The expression depends on the input of the function, and this error shows when the function in called in an optimization routine. So I cant know the exact input either... PS I have made the imrovement that add in if (isempty(y)==1) PF(i,1)=0; else PF(i,1)=double(y); end But this similar error still show!!!And this time it is 2010 shows this message...

TUHAME STEVEN 2015-10-28

it is fine thank you

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Answer 1

Carlos 2013-2-14

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/63283-problems-about-solve-exponential-function#answer_74997

编辑：Walter Roberson 2013-2-14

在 MATLAB Online 中打开

When you execute this code in Matlab

>> syms x;
>> f=-10*exp(-x);
>> solve(f,x)
>> isempty(a)
ans =
       1

Whereas if you execute this code

>> syms x;
>> f=x^2-4
>> solve(f,x)
>> isempty(a)
ans =
       0

You can use the isempty command. If isempty returns true (1), it means your roots array is empty because there are no roots.

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xueqi 2013-2-18

编辑：xueqi 2013-2-18

Hi, I have add in this bit in my code, it still shows me the same message. Can you think of the reason? Also can you go to my question here http://www.mathworks.com/matlabcentral/answers/63963-problems-about-solve-exponential-function

Carlos 2013-2-19

Sorry, I cannot figure out the reason for different behaivours in different versions.

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Answer 2

Carlos 2013-2-13

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/63283-problems-about-solve-exponential-function#answer_74912

编辑：Walter Roberson 2013-2-14

在 MATLAB Online 中打开

In the warning message displayed by Matlab it says no solution has been found(just as it happens if the function has no roots), and exp(-ax) has no roots, this is what happens when I try the following code in Matlab

>> syms x;
>> f=-10*exp(-x);
>> solve(f,x)
Warning: Explicit solution could not be found. 
> In solve at 81
ans =
[ empty sym ]

However if there is no problem with Matlab 2010 this might not be the case, I just wanted you to consider the possibility the function has no roots.

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xueqi 2013-2-13

Yes I begin to think it may be the problems. So I think it would be better if I modify the code so that pf=0 if there is no solution. Could you offer any advice how to say 'if there is no solution' into matlab lauguage?

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