Obtain function value

You could either plug your values directly in the function f:

w0 = -1.9;
w1 = -0.9;
f = (0.5+phi)*(w0.^2 + w1.^2) + w0.*w1*cos(2*pi/M) + 2*w1*sin(2*pi/M) + 2;

or use indices on x and y:

x = -2:0.1:8;
y = -10:0.1:0;
[w0,w1] = meshgrid(x, y);
f = (0.5+phi)*(w0.^2 + w1.^2) + w0.*w1*cos(2*pi/M) + 2*w1*sin(2*pi/M) + 2;
f(find(y==-0.9), find(x==-1.9));

Both methods will return:

ans = 5.1452

Assuming f(-1.9,-0.9) = f(w0, w1)

[row0, col0] = find(w0==-1.9);
[row1, col1] = find(w1==-0.9);
ans = f(row1(1), col0(1));

I think the best way should be using annonymous function:

f = @(x,y) (0.5+phi)*(x.^2 + y.^2) + x.*y*cos(2*pi/M) + 2*y*sin(2*pi/M) + 2

f(-1.9,0.9)

x)