about if statement and solution

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Dr. Siva Malla
Dr. Siva Malla 2012-5-31
Dear sir,
when I run this bellow code, it shows this error,
code:
syms x
a=solve(x^2+4*x+4);
b=a(1)
if b>2
s=2
else
s=3
end
error:
Undefined function or method 'gt' for input arguments of type 'sym'.
Error in ==> solution at 4
if b>2
my target is, I have to compare the value of 'x' in the equation with constant. but I got this error.
please help me the solution for my problem..

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采纳的回答

Walter Roberson
Walter Roberson 2012-5-31
If you want numeric answers, work numerically:
a = roots([1, 4, 4]);
solve() does not return numbers: it returns formula, some of which happen to print out like numbers. If you happen to get a solve() result that involves only known functions of constants, then you can use double() on the result to calculate the double-precision approximation of the result.
By the way, you should not assume that the results of solve() are returned in any particular order.
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Walter Roberson
Walter Roberson 2012-5-31
In any case, test double(f) not f itself.
Dr. Siva Malla
Dr. Siva Malla 2012-6-1
Sir,
I have to extract first f1, f2....fn in the vector f, means in all values of f, and I have to test any value of f. like if f2>2; because, first I have to take only real values of f and only I should test these real values of f to 2,( any real value of f). this way I want.
here I am giving complete my program please test. in this program, I cant work with if statement. so please rectify this problem and please help me.
program:
L=1.0*1e-4;
a=0.15*1e-4;
W=10*1e-4;
q=1.6*1e-19;
Es=85.845*1e-14;
Nd=5*1e17;
Nc=1.83*1e19;
Nv=4.07*1e19;
Nref=2*1e17;
KT=0.0259;
q1=1;
h1=0.98;
Umin=40;
Umax=950;
r=0.76
x1=1;
vsat=2.07*1e7;
k1=q*Nd*W*a*x1*vsat;
Vbi=0.8-((KT*q1)*log(Nc/Nd));
U0= Umin + ((Umax - Umin)/(1+ ((Nd/Nref)^r)));
G0=(q*Nd*U0*W*a)/L;
Vp=(q*Nd*a^2)/(2*Es);
Ip=G0*Vp/3;
Z=(q*Nd*a^2*U0)/(2*Es*L*vsat);
Ec=vsat*(h1/(1-h1))/U0;
VG=-8
for k=1:8
Vd=1:20;
a1=((Vbi-VG)/Vp)^0.5;
syms x
for i=1:20
b=x^2-a1^2;
c=h1*(1-x);
d=2*(x^3-a1^3)/3;
e=L*Z*((b-d)/c-b);
y=Vp*b+2*Ec*a*x/pi*sinh(pi*(L-e)/(2*a*x))-Vd(i);
f=solve(y);
f
VL1=Vp*(f^2-a1^2);
end
VG=VG+1;
if Vd(i)>VL1
w=2;
else
w=3;
end
end

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