Double command does not work when converting 1*1 sym variable to a numerical value. What will work?

I am having trouble converting my unknown variables, solved using the symbolic toolbox(a_1,a_2,a_3), into numerical values. the error i get is

Error using symengine (line 59)
DOUBLE cannot convert the input expression into a double array.
If the input expression contains a symbolic variable, use VPA.
Error in sym/double (line 583)
        Xstr = mupadmex('symobj::double', S.s, 0);

Heres my code:

Th_final= sym('Th_start+a_1*T*f^5+a_2*T*f^4+a_3*T*f^3')
Vel_final=sym('0== 5*a_1*T*f^4+4*a_2*T*f^3+3*a_3*T*f^2')
Acc_final=sym('0==20*a_1*T*f^3+12*a_2*T*f^2+6*a_3*T*f')
answer=solve(Th_final,Vel_final,Acc_final,'a_1','a_2','a_3');
Th_final=double(answer.a_1)
Th_final1=double(answer.a_2)
Th_final3=double(answer.a_3)

This is probably a very simple question but what command can i use to return numerical values of a_1,a_2,a_3?

Thanks

Answer 1

[Walter Roberson]

You cannot convert those to double. They contain the symbols Th_start, f, and T . You would need to subs() in numeric values for those symbols in order to be able to convert to double.

Note: you should remove the "0==" parts of your expressions. When you are using sym() of a string, the proper equation would be with '=' instead of '==', but you can avoid the whole issue by removing the '0==' as equality to 0 is the default condition.