MATLAB: Problem with imag command when evaluating symbolic expressions.

complex numberssymbolic

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.

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

syms x y real

This plots the real part of y against x:

syms x y
ezplot(y-x^2-1i-2)

Of course, it looks just the same as

syms x y
ezplot(y-x^2-2)

EDIT: For a 3D plot, you could do this:

syms x
ezplot3(x,x^2+2,1)
xlabel('x')
ylabel('Real(y)')
zlabel('Imag(y)')

for i=1:Np
    x(: , i+1) = f(x(: , i) , U(:,i));

That needs to access up to the Np'th column of U

    Cost ([1;1;1;1;1;1] , randn(3,1) , [1;1;1] , 2 , 0.01)

but U only has one column.

The Np that you pass in (2) should not exceed the number of columns of your second parameter.