subs command is not working on differentiation in MATLAB

Question

SOUVIK BASAK 2022-6-4

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https://ww2.mathworks.cn/matlabcentral/answers/1733955-subs-command-is-not-working-on-differentiation-in-matlab

回答： Walter Roberson 2022-6-5

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);

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Walter Roberson 2022-6-5

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 2022-6-5

编辑：SOUVIK BASAK 2022-6-5

@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

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