Why does SOLVE return an incorrect result?

Why does SOLVE return an incorrect result?

Specifically,

 > syms I01 I1 V R ts tau I2 I02 V1 V2 di1 di2 L

 > f1=R*I1+L*di1-V1;

 > f2=R*I2+L*di2-V2;

 > [R,L]=solve(f1,f2,'R,L');

 > Rsym=simple(R);

 > Lsym=simple(L);

After checked by hand, the solution for R and L is exchanged, and thus incorrect although the answer is correct had the it not been exchanged.

Is this a bug?

Answer 1

[MathWorks Support Team]

This is not a bug.

This is an expected answer from SOLVE.

From the help entry of SOLVE:

 ...solves the system of equations implied by

 eq1,eq2,...,eqn in the n variables determined by applying findsym to the system.

and in the help entry of FINDSYM:

     FINDSYM(S), where S is a scalar or matrix sym, returns a string 

     containing all of the symbolic variables appearing in S. The 

     variables are returned in alphabetical order and are separated by

     commas. If no symbolic variables are found, FINDSYM returns the

     empty string.

Thus,

 >> findsym(f1)

 ans =

 I1, L, R, V1, di1

 >> findsym(f2)

 ans =

 I2, L, R, V2, di2

The output will follow the same sequence determined by FINDSYM and thus should be alphabetical, although you can name the output anything you like.

The best solution however is to have only one variable for the output of SOLVE. This variable will be of type structure, and each field will have the same name as the variable being solved:

Using the above example:

Solution=solve(f1,f2,'R,L');

and

Solution.L is solution of L and Solution.R is the solution of R.