why do i receive the error "Subscript indices must either be real positive integers or logicals" ?

Hi

im trying to write down a code that calculate MMSE equalization coefficients , in this code K is the number of coefficients which gives 15 coefficients. all the time i run my m-file it replies "Subscript indices must either be real positive integers or logicals" and i dont no what to do about it . it appears that it need to add some part to it of course C matrix would be calculated later one , as i checked j loop stuck in -1 but still dont get it , could anyone suggest any idea ??? , here is my code :

 clc 
 clear all
 close all
 a=[0.227 0.46 0.688 0.46 0.227];
 aa=fft(a,100);
 f=abs(aa);
 f1=f/max(f);
 f2=20*log10(f1);
 k=7;
 n0=0;
 l=length(a);
 x=conv(f,conj(f));
    for i=-k:k
    for j=-k:k
           Gamma(i+k+1,j+k+1) = x(i-j+l+1);
        end
    end

[Walter Roberson]

Why? Because your l = 5, and when i = -7 and j = -1, i-j+l+1 is -7-(-1)+5+1 = 0

Note: you should also be checking whether you should be using

x = conv(f, conj(f), 'same');

which you would use if you expect x to be the same size as f

[HamidReza Saleh]

 x = conv(f, conj(f), 'same');

it didnt change anything , it appears that i need to define an "if" BUT where and how dont know i guess i consider -14 as 1