How to extract the expression inside of a trig function?

Specifically, I want to do this so that I can grab the coefficients of the expression. It's a simple conceptual question but I really have no clue how to go about it in code. Right now, I can just look at the symbolic expression in the workspace and get what I am looking for just by inspection, but I'd have to do this every time I change the variables and if I wanted to use that coefficient for anything else I would have to manually assign the value. Here's what I am trying to do:

syms x

Z = 50 - 25cos(3x+3*pi/2); % some expression obtained from other portions of code

% by inspection
Val1 = 3; % the coefficient in front of x
Val2 = 3*pi/2; % the lone coefficient

Val1 and Val2 correspond to values that I need for other calculations, and I can see them from fitting the equation to one of a general form. Is there any way to do this without having to type their values?

Answer 1

[Walter Roberson]

findSymType(expression, 'cos') to extract the cos expression(s). children() to open the cos call to give you the inner expression. coeffs() to split the constant and the term.

Answer 2

[James Tursa]

You could write your own simple parser for this. E.g., code for finding stuff inside the first function in the line could be:

Z = your symbolic expression
cz = char(Z);
cz(cz==' ') = [];
alphanum = false;
p = 0;
plevel = -1;
for k=1:numel(cz)
    if( cz(k)=='(' )
        p = p + 1;
        if( alphanum && plevel == -1 )
            plevel = p;
            m = k;
        end
    end
    if( cz(k) == ')' )
        if( p == plevel )
            cz = cz(m:k);
            break;
        end
        p = p - 1;
    end
    alphanum = ismember(cz(k),['a':'z' 'A':'Z' '0':'9']);
end
c = coeffs(eval(cz));

Answer 3

[Reshma Nerella]

Hi,

If you want to get the coefficients of symbolic expression, you can coeffs function.

For instance,

syms x
c = coeffs(16*x^2 + 19*x + 11) %symbolic expression

c =
[ 11, 19, 16]; % coefficients of x

If you want to reverse the order of coefficients, you can use fliplr function.

Hope this helps!