error in fourie script

hi I use MatLab 2016a and write this script but I receive this error. how should I fix it:

g=@(x)1.*(x>0)+(-1).*(x<0)
a_0=(1/pi)*quad(g,-pi,pi);
for n=1:10
    a_n(n)=(1/pi)*quad(g*cos(n*x),-pi,pi);
    b_n(n)=(1/pi)*quad(g*sin(n*x),-pi,pi);
end
a_n
b_n
f=a_0;
for n=1:10
    f=f+a_n(n)*cos(n*x)+b_n(n)*sin(n*x);
end
ezplot(x,f)