问题
I have a symbolic and numeric mixed expression:
(3145495418313256125*sin(11334310783410932061962315977/17437937757178560512000000000)*cos(theta))/85568392920039424
where theta is a symbolic variable. I want to simplify this expression such that all the numeric numbers and their math operation results are changed to double.
回答1:
In terms of data types, you can't mix floating point and symbolic values. However, you can use variable precision arithmetic so that the values are represented in decimal form. Using vpa:
syms theta
y1 = (3145495418313256125*sin(11334310783410932061962315977/17437937757178560512000000000)*cos(theta))/85568392920039424
y2 = vpa(y1)
which returns
y2 =
22.24607614528243677915796931637*cos(theta)
The data type (class) of y2 is still sym. See the digits function to adjust the number of significant digits.
If you want to work in actual floating point you'll need to convert your symbolic expression into a function. You can automate that procedure by using the confusingly-named matlabFunction:
thetafun = matlabFunction(y1)
which returns a function using double precision variables:
thetafun =
@(theta)cos(theta).*2.224607614528244e1
The anonymous function thetafun can then be called just like any function, e.g., thetafun(0.5).
回答2:
You can make use of coeffs command to achieve the desired:
f=2*cos(theta)/3+5*sin(theta)/19
c_f=coeffs(f);
fraction_c_f=double(c_f);
ans = [0.2632 0.6667]
来源：https://stackoverflow.com/questions/22277525/how-to-simplify-a-symbolic-and-numeric-mixed-expression-in-matlab