problem with imag command when evaluating symbolic expressions.

Hi, when I execute the following code in Matlab

syms x y; f=4*i+9*x+y; imag(f)

I get the following result

ans =

(9*i*conj(x))/2 - (i*y)/2 - (9*i*x)/2 + (i*conj(y))/2 + 4

and the result should be 4 instead of what I get. I cannot figure out where have I gone wrong.Any ideas?

Thank you in advance.

Answer 1

[Walter Roberson]

The result is correct. The symbolic toolbox assumes that variables might be complex unless it has been told otherwise. Try

syms x y real

Answer 2

[Carlos]

The answer is right, if you try subs((9*i*conj(x))/2 - (i*y)/2 - (9*i*x)/2 + (i*conj(y))/2 + 4),{x,y},{0,0}) or subs((9*i*conj(x))/2 - (i*y)/2 - (9*i*x)/2 + (i*conj(y))/2 + 4),{x,y},{1,1}) or you give x and y any value, the answer is always 4. It was just that I found the answer a bit awkward. From your answer I grasp that the only problem here was that I should have declared x and y as symbolic real variables.

Than you very much Walter.