Error conversion to double from sym is not possible

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Erfan Hamdi 2016-10-7

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/306223-error-conversion-to-double-from-sym-is-not-possible

评论： Walter Roberson 2016-10-9

i am encountering this error trying to plot these functions :

syms t A OA omg si
r=A*t+OA;
theta=omg*t;
phi=si*t;
rd=diff(r,t);
thetad=diff(theta,t);
phid=diff(phi,t);
omg=10*(pi/180);
si=7*(pi/180);
A=0.5;
tn=90/si;
a_r=-(r*phid^2)-(r*thetad^2*(cos(phi))^2)
a_theta=(cos(phi)/r)*diff(((r^2)*thetad),t)-(2*r*thetad*phid*sin(phi))
a_phi=(1/r)*diff((r^2*phid),t)+r*thetad^2*sin(phi)*cos(phi)
td=0:1:tn;
a_r=-(r*phid^2)-(r*thetad^2*(cos(phi))^2)
a_theta=(cos(phi)/r)*diff(((r^2)*thetad),t)-(2*r*thetad*phid*sin(phi))
a_phi=(1/r)*diff((r^2*phid),t)+r*thetad^2*sin(phi)*cos(phi)
a_r1=subs(a_r,t,td);
plot(td,a_r1)

this is the error :

Error using plot
Conversion to double from sym
is not possible.
Error in p6 (line 23)
plot(t,a_r1)

what should i have done ?

回答（1 个）

Marc Jakobi 2016-10-7

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/306223-error-conversion-to-double-from-sym-is-not-possible#answer_237948

在 MATLAB Online 中打开

The line

td = 0:1:tn

creates a double. So the line

a_r1=subs(a_r,t,td);

also creates a double. With

plot(t,a_r1)

you are using a double and a sym together, which Matlab can't handle.

Matlab recognizes any sym you declare with "syms" as a variable and not as a constant. So if you want to be able to plot it, you will have not choice but to declare omg, si and A as constants with fixed values if you want to plot them. You can still change the values at other times.