converting from sym to double

Hi, how do i overcome this problem:

"The following error occurred converting from sym to double:

Unable to convert expression containing symbolic variables into double array. Apply 'subs' function

first to substitute values for variables."

I integrated cosine function and the matrix are in the form of symbols.

[Ameer Hamza]

Can you show the code?

[Jama Ali]

Answer 1

[Walter Roberson]

There are a few different reasons why that can happen:

• Your formula has at least one unbound symbolic variable. This is likely your problem.
• your formula involves a symbolic integral that does not appear to converge
• your formula involves numbers too large for the symbolic toolbox to represent. I do not recall the exact limit at the moment but it is beyond 10^300000
• your formula involves the root() of an equation that involves software floating point numbers, but to the current number of digits active, it is not possible to find the root because the equation is too nonlinear

[Walter Roberson]

For the first case you need to subs() particular numeric values for each of the variables.

Answer 2

[Bjorn Gustavsson]

Most of the time I want to convert a symbolic expression to something else I use matlabFunction:

syms s x real
f = int(cos(s^2),0,x);
% returns: 
% f =
% 
% (2^(1/2)*pi^(1/2)*fresnelc((2^(1/2)*x)/pi^(1/2)))/2
% 
% Convert that to a matlab-function-handle for use on arbitrary x:
F = matlabFunction(f);
% returns:
% F =
%
%  function_handle with value:
%
%    @(x)sqrt(2.0).*sqrt(pi).*fresnelc(sqrt(2.0).*x.*1.0./sqrt(pi)).*(1.0./2.0)
%
F([1 12])
% Returns:
% ans =
% 
%     0.9045    0.6061

HTH

[Jama Ali]

i want to integrate cos(w*t), with respect to w and from 0.01 to 0.4. giving me (sin((2*t)/5) - sin(t/100))/t. and then later plot that function with a range of t. how can i do that with your function?

thanks for your

[Walter Roberson]

fplot the results of integration

[Bjorn Gustavsson]

Change f to:

f = cos(w*t);

Change the integration to something like this:

I = int(f,w,w0,w1);

Then convert I to F.