how to calculate the value from program

ef = 1.5^2; ec = -2^2; es = -1.8^2; 
k0 = 1; a = 0.63; pc = ef/ec; ps = ef/es; binit = 0.5*sqrt(ef); 
kf = sqrt(ef-binit^2); 
as = sqrt(binit^2-es); 
ac = sqrt(binit^2-ec);
tol = 1e-12; n = 1;
while 1 
    kf_new = (atan(-kf/ac/pc)+atan(-kf/as/ps))/2/a;
    if abs(kf_new-kf) < tol, break; end 
    kf = kf_new; 
    be = sqrt(ef - kf^2); 
    as = sqrt(be^2 - es); %c % edit Rik: assuming this is a typo
    ac = sqrt(be^2 - ec);
    n=n+1;
end
E = abs(tan(2*k0*a*kf) - kf*(pc*ac+ps*as)/(kf^2 - pc*ac*ps*as)) ;
phi = (atan(pc*ac/kf) - atan(ps*as/kf) + pi)/2;

how to find the value of be from this program

[AndresVar]

The value of be will be displayed in the Worskspace sidebar.

or use the command window, typing be shows the value of be. Or you can add disp(be) at the end of the code.

For this questions it's best to google it because it has been answered many times.

[Rik]

As @AndresVar implies: this is a fairly basic Matlab question. You may consider doing the Onramp tutorial (which is provided for free by Mathworks).

If you format your question as code, you can even run your code in the forum interface. Have a read here and here.

Since this is mostly a duplicate from you new question I'm going to close this one.