Unable to understand the error

Question

Mehul kumar 2021-11-19

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1590774-unable-to-understand-the-error

回答： Akash Singh 2021-12-7

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)

2 个评论
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KSSV 2021-11-19

What inputs have you given? It seems D is syms class, so you cannot use like that.

Mehul kumar 2021-11-19

在 MATLAB Online 中打开

inputs are
[1,0,omega*omega]
Fo*sin(gamma*x)
[0,0]

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Answer 1

Akash Singh 2021-12-7

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1590774-unable-to-understand-the-error#answer_849440

在 MATLAB Online 中打开

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.

https://www.mathworks.com/help/symbolic/subs.html

You can do something like -

if(double(subs(D, yourvalue))>0)

Please refer a similar answered question below -

https://www.mathworks.com/matlabcentral/answers/69143-conversion-to-logical-from-sym-is-not-possible-error-shows-up-when-i-try-to-execute-the-following