# Unable to understand the error

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Mehul kumar 2021년 11월 19일
답변: Akash Singh 2021년 12월 7일
close all
clc
syms c1 c2 x m omega gamma Fo
F=input('enter the coefficients [a,b,c]:');
f=input('enter the RHS function f(x):');
a=F(1);b=F(2);c=F(3);
AE=a*m^2+b*m+c;
m=solve(AE);
m1=m(1);m2=m(2);
D=b^2-4*a*c;
if(D>0)
y1=exp(m1*x);
y2=exp(m2*x);
elseif (D==0)
y1=exp(m1*x);y2=x*exp(m1*x);
else
alfa=real(m1);beta=imag(m1);
y1=exp(alfa*x)*cos(beta*x);
y2=exp(alfa*x)*sin(beta*x);
end
yc=c1*y1+c2*y2;
fx=f/a;
W=y1*diff(y2,x)-y2*diff(y1,x);
u=int(-y2*fx/W,x);
v=int(y1*fx/W,x);
yp=y1*u+y2*v;
y_gen=yc+yp;
check=input('If the problem has initial conditions then enter 1 else enter 2:');
if(check==1)
cn=input('Enter the initial conditions[x0,y(x0),Dyx(0)]:');
dy_gen=diff(y_gen);
eq1=(subs(y_gen,x,cn(1))-cn(2));
eq2=(subs(dy_gen,x,cn(1))-cn(3));
[c1,c2]=solve(eq1,eq2);
y=simplify(subs(y_gen));
disp('The complete solution is');
disp(y);
fplot(y,[cn(1),cn(1)+2]);
else
y=simplify(y_gen);
disp('The general solution is');
disp(y);
end
ERROR
Conversion to logical from sym is not possible.
Error in onetwothree (line 12)
if(D>0)
##### 댓글 수: 2없음 표시없음 숨기기
KSSV 2021년 11월 19일
What inputs have you given? It seems D is syms class, so you cannot use like that.
Mehul kumar 2021년 11월 19일
inputs are
[1,0,omega*omega]
Fo*sin(gamma*x)
[0,0]

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### 답변 (1개)

Akash Singh 2021년 12월 7일
As you mentioned your input is [1,0,omega*omega] which makes value of a, b, c as 1, 0 and omega*omega respectively.
Omega is a sym, which makes c a sym.
In this line of code
D=b^2-4*a*c;
D is also sym
Please note, sym is not a function. It will not evaluate and assign a result to D.
if(D>0)
So, when you compare D(sym) with 0 (double), system throws error because sym can’t be compared to a numeric value.
If you really want to compare D with 0, you will have to first substitute the value of 'omega' in D. The way to do that is by using subs, which will substitute the value.
You can do something like -
if(double(subs(D, yourvalue))>0)

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