MATLAB Answers

Gian Lorenzo

Order of variables in SOLVE

Asked by Gian Lorenzo
on 14 May 2013
clear all
syms x y l1 l2 l3   %%Generate symb var for multipliers
v=[x y l1 l2 l3];
g1=(y-3)/sqrt((x-2)^2+(y-3)^2)+l1+l2-l3;  %% Gradient condition
cs1=l1(x+y);  %% Complementary slackness conditions
[x, y, l1, l2, l3] = solve(g1==0,g2==0,cs1==0,cs2==0,cs3==0, x, y, l1,...
    l2, l3);
sol=[x, y, l1, l2, l3];

I have this KKT system. Other times when I used the solve command I noticed that in the solution matrix the variables are not reported in the specified order.

How do I make sure that in the sol matrix the variable are the right one (without checking manually, that is).

In the example above, having the solutions, I can see that into sol x,y are the two rightmost columns. If I switch like this

[ l1, l2, l3, *x, y,*] = solve(g1==0,g2==0,cs1==0,cs2==0,cs3==0, x, y, l1,...
      l2, l3);

then everything goes ok. I need to now why this happens in order not to make this mistake again when I don't have solutions. Thanks



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1 Answer

Answer by Walter Roberson
on 14 May 2013
 Accepted answer

Use the structure return form of solve and then extract by structure field name into the variables you want.


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