coversion frm sym to double is not possible?plz help

clc;clear all; g_db=[0:2:20];

g=10.^(g_db/10); alpha=1.25; c=1; M=2; syms y r=0; for j=2:M

p=((-1)^j)*(factorial(M)/factorial(j)*factorial(M-j))*exp(-(g.*y*(1-(1/j)))); r=r+p; t=r*(1/M) end x=((y.^(alpha-1)).*exp(-y./c))/((gamma(alpha))*(c^alpha)); z=t*x; semilogy(g_db,double(subs(z,y)),'o') hold on