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Order of variables in SOLVE

Asked by Gian Lorenzo

Gian Lorenzo

on 14 May 2013
Accepted Answer by Walter Roberson

Walter Roberson

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
g2=(x-2)/sqrt((x-2)^2+(y-3)^2)+l1;
cs1=l1(x+y);  %% Complementary slackness conditions
cs2=l2*(x-2);
cs3=l3*(x+2);
[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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Gian Lorenzo

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1 Answer

Answer by Walter Roberson

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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Walter Roberson

Walter Roberson

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