IIR FIlter code using for loop

In relation to this problem, I have made a code but was rejected because the answer my code gave was very far from it. Can you guys correct what's wrong with the code.

function y=IIR_Filter(b,a,x)
c=[4 8 1 2 3];
b0=0.05*c(4);
b1=0.03*c(3);
b2=0.02*c(2);
b=[b0 b1 b2];
a=[0.5 0.5];
x=[1 0 0 0 0 0 0];

N=length(a)-1;
M=length(b)-1;
y=zeros(1,length(x));

for n=1:length(x)
    y1=0;
    for k=1:N
        if n-k>=1
            y1=y1+a(k+1)*y(n-k);
        end
    end
    y2=0;
    for k=0:M
        if n-k>=1
        y2=y2+b(k+1)*x(n-k);
        end
    end
    y(n)=-y1+y2;
end