Digital High Pass Filter in MATLAB

y[n] = x[n] - a*y[n − 1]
Y[z]=X[z]-a*Y[z]*z^-1
Y[z]+a*Y[z]*z^-1=X[z]
Y[z]*[1+a*z^-1]=X[z]
Y[z]/X[z]=1/[1+a*z^-1]
H[z]=1/[1+a*z^-1]=z/z+a
Pole
[1+a*z^-1]=0
a*z^-1=-1
z^-1=-1/a
z=-a

% Code:-

w=-pi:0.01:pi;

num=[1];

den=[1 0.5];

h=freqz(num,den,w);

subplot(2,1,1);

plot(w,abs(h));

ylabel('Magnitude');

subplot(2,1,2);

plot(w,angle(h));

ylabel('Phase');