subs command is not working on differentiation in MATLAB

I have defined theta1 and theta2 as my random functions of time. I have defined vec_a1 and vec_v1 by differentiaing them. Afterwards when I am defining theta1 and theta2 and trying to evaluate vec_a1 and vec_v1 it is not converting to numerical values.

g = -9.81;
I1=1;I2=1.5;l1=2;l2=1;rho1=1.1;rho2=0.4;
syms theta1(t) theta2(t)

vec_rho1 = [rho1*cos(theta1), rho1*sin(theta1)];
vec_l1 = [l1*cos(theta1), l1*sin(theta1)];
vec_rho2 = vec_l1 + [rho2*cos(theta2), rho2*sin(theta2)];
vec_l2 = vec_l1 + [l2*cos(theta2), l2*sin(theta2)];
vec_v1 = diff(vec_rho1, t);
vec_a1 = diff(vec_v1, t);

vec_v2 = diff(vec_rho2, t);
vec_a2 = diff(vec_v2, t);
theta1=0.02*t;
theta2=0.3*t;
xx=1

xx = 1

vec_a1=subs(vec_a1)

vec_a1(t) = 

x=vec_a1(xx);
x=double(x)

Error using symengine
Unable to convert expression containing remaining symbolic function calls into double array. Argument must be expression that evaluates to number.

Error in sym/double (line 868)
Xstr = mupadmex('symobj::double', S.s, 0);

[Torsten]

Show the complete code (including declaration of variables).

The lines

vec_a1=subs(vec_a1);
x=subs(vec_a1(xx));

don't make sense.

You don't tell MATLAB which variable in vec_a1 shall be substituted and by what.

Further, vec_a1 is not a function - so what does vec_a1(xx) mean (where xx itself is also not defined anywhere) ?

[Walter Roberson]

subs(expression) is well defined. It means that matlab should look through all of the symvar() and replace them by their values in scope.

[SOUVIK BASAK]

@Walter Roberson and @Torsten I have updated the question according to your comments. The error I am facing is that subs is replacing theta 1 and theta2 with their respective functions, but could replace the derivative part. As you can see in vec_a1 instead of replacing t it is getting replaced with t(t). Kindly guide me

Thanks,

Souvik

Answer 1

[Dyuman Joshi]

g = -9.81;
I1=1;I2=1.5;l1=2;l2=1;rho1=1.1;rho2=0.4;
syms theta1(t) theta2(t)
theta1=0.02*t;
theta2=0.3*t;
vec_rho1 = [rho1*cos(theta1), rho1*sin(theta1)];
vec_l1 = [l1*cos(theta1), l1*sin(theta1)];
vec_rho2 = vec_l1 + [rho2*cos(theta2), rho2*sin(theta2)];
vec_l2 = vec_l1 + [l2*cos(theta2), l2*sin(theta2)];
vec_v1 = diff(vec_rho1, t);
vec_a1 = diff(vec_v1, t);
vec_v2 = diff(vec_rho2, t);
vec_a2 = diff(vec_v2, t);

xx=1;
x=subs(vec_a1,t,xx)

x = 

double(x)

ans = 1×2

1.0e-03 * -0.4399 -0.0088

Answer 2

[Walter Roberson]

theta1(t)=0.02*t;
theta2(t)=0.3*t;

That is, you had replaced the function theta1 (a function of t) with the symbolic variable theta1 that was not a function.