Why do I get "Index exceeds matrix dimensions"?

I'm trying to create a matrix of two letter combinations from a string x. A(1,1) = # of aa's, A(1,2) = # of ab's etc. to A(26,26) = # of zz's. The text is all lower case and I thought I solved it, but I keep getting this error message. Ideas as to why? How would I fix / avoid this in the future?

alphabet = 'a':'z';
A = zeros(26);
for j = alphabet
    for i = alphabet
        y = strcat(alphabet(i),alphabet(j));
        A(i,j) = length(strfind(x,y));
    end
end

Answer 1

[Cedric]

Numeric arrays cannot store arrays of two characters. Try with a cell array. Also, don't index arrays with characters but with numeric indices

 alphabet = 'a' : 'z' ;
 A = cell( 26, 26 ) ;
 for j = 1 : length( alphabet )
    for i = 1 : length( alphabet )
        A{i,j} = [alphabet(i),alphabet(j)] ;      % Concatenation of two letters.
    end
 end