Calculation BPSK using MATLAB

clc;
p=0:.1:1;
p1=1-p;
a=0:0.1:1;
a1=1-a;

for j=1:1:length(p)
    for i=1:1:length(a);
   
    z1(j)=a(i)*p(j)+a1(i)*p1(j);
    z2(j)=a(i)*p1(j)+a1(i)*p(j);
    if (z1(j)==0);
        y1(j)=0;
    else
    y1(j)=z1(j)*log2(1/z1(j));
    end
    if (z2(j)==0);
        y2(j)=0;
    else
     y2(j)=z2(j)*log2(1/z2(j));
    end
    if (p(j)==0);
        y3(j)=0;
    else
      y3(j)=p(j)*log2(1/p(j));
    end
    if (p1(j)==0);
        y4(j)=0;
    else
      y4(j)=p1(j)*log2(1/p1(j));
    end
     
      y(j,i)=y1(j)+y2(j)-(y3(j)+y4(j));
   end
end
y
plot(p,y(1,:),'r');