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Asked by upinderjeet on 29 Mar 2013

X=.25;U=1; >> eps1=.05;eps2=.05; >> k=1; >> while (abs(C-gamma))>=eps1 X=X-inv(Cx)*(C-gamma); if abs(C-gamma)<eps1 if abs(Lbar_u)<eps2 break; else k=k+1; U=U-K*(Lbar_u); end end end

Conversion to logical from sym is not possible.

I ALSO TRIED TO USE A ALTERNATE WHILE LOOP CONDITION:

while double(subs(abs(C-gamma))) > double(eps1)

BUT THE ERROR PERSISTS

how can i fix it?

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Answer by Walter Roberson on 30 Mar 2013

Accepted answer

You have

syms L C gamma lambda k K X U C=(X+U)

because X and U are symbolic at that point, C will be symbolic. You never change the value of C, so it is going to remain symbolic.

You then have the test

while (abs(C-gamma))>=eps1

where gamma and eps1 have numeric values. But C does not have a numeric value, it has a symbolic value, so C-gamma will be symbolic, abs(C-gamma) will be symbolic, and you will be trying to use ">=" to compare a symbolic value to a numeric value.

It appears that up until the beginning of the "while", you want C to be symbolic -- for example it needs to be symbolic in order to use gradient(C,X) . But once you start the while loop, it appears that each time you write C, that you want the symbolic formula for C to be examined, and each place the formula refers to a variable that used to be symbolic but now has a numeric value, you want the numeric value to be substituted for the symbol, giving you a numeric value for C.

**If** that is what you want, then each place you reference C and want the substitution of numeric values to take place, you need to instead code

double(subs(C))

such as

while (abs(double(subs(C))-gamma))>=eps1

Symbolic formulas are *not* functions! Once they have symbolic form, any symbolic variables in them are *not* substituted with numeric values **unless** you use subs() to tell it to do the substitutions.

upinderjeet on 30 Mar 2013

thanks alot walter.....i think now I am able to see my mistake....couple of other related doubts....

1. when do we use subs() and when do we use double(subs())?

2.In my code the reason I wrote Lxu=diff(Lx,U); and Lux=diff(Lu,X); is the fact that when I tried using :

Lxu=gradient(Lx,U);

Lux=gradient(Lu,X);

it gave me error:"first argument must be of type::Arithmetic"

so I switched to use diff() instead of gradient() as it ran error free

and my parameters are scalars for now and it wont make any difference.

What you think the reason might be?

upinderjeet on 30 Mar 2013

AND..........

is there any better way to call the value of C,L,Lx,Lxu....etc...when I need them in the while loop or outside the while loop for that matter. I am asking because it wud be better to have something alternate instead of writing double(subs()) in front of every symbolic variable whenever I want their value.??

## 2 Comments

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/69143#comment_139891

Where does the error show up? Which variables are symbols? What is the value of C and gamma and Cx and Lbar_u ?

This appears to be very similar to your previous question, which I do not think is resolved as yet. Are you sure it is not a duplicate ?

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/69143#comment_139913

this is not definitely a duplicate although you can call it a similar problem..i just removed the logical OR operator in the while condition to simplify things.

I declared all the variables as syms. I might be wrong in doing that as I am still a new user to matlab. I assigned gamma=1 at the beginnning of the code and C and Cx are just the function I declared in the command line as follows:(I am including the entire code below)

Conversion to logical from sym is not possible.