Error: " Conversion to Logical from sym is impossible"

I have my code here, it is very simple, I just want to solve the equation in "solve" function and then do a "if judgement", but error comes out and shows like this:

Code:

syms t t1 t2 Vds Vds1 Vds2 x y;
t1=0:1e-8:1e-6;
t2=1e-6:1e-8:2e-6;
t=[t1,t2];
Vpeak=1.5;
Vrise=(Vpeak/1e-6)*t1;
Vfall=(-Vpeak/1e-6)*t2+3;

t=[t1,t2];
Vsweep=[Vrise,Vfall];

Ce=2.2e-12;

Vgs=1.86;
Vt=1.742;

xpre=0;
tpre=0;

a=zeros(1,55);

for k=1:55
           solve(0.032*((Vgs-Vt)*x-(0.5)*x^2)+Ce*((x-xpre)/(t(k)-tpre))==7.0063e-5*(Vsweep(k)-x)^2+2.2929e-5*(Vsweep(k)-x));

             if x(1)<(Vgs-Vt)
                 Vds(k)=x(1);
             else
                 Vds(k)=x(2);
             end

                  xpre=a(1,k);
                  tpre=t(k);
  end

Error:

Conversion to logical from sym is not possible.

Error in Codetest (line 32)
           if x(1)<(Vgs-Vt)

Could somebody help me, thx.

Answer 1

[Walter Roberson]

You do the solve() but you throw away the answer.

You then have

if x(1)<(Vgs-Vt)

but x is still a symbol from the "syms", so you are trying to compare a symbol (that has no value other than its own name) to a numeric value. You cannot test that in a MATLAB "if" statement as it is simultaneously true and not true.

If your solve() had been solving for "x" then you would probably still have a problem because x would then at best be a symbolic number rather than a floating point number. You would need to test it with

if double(x(1)) < (Vgs-Vt)