It seems the problem is that al is a string and therefore counts as one object for MATLAB. You can remedy this by switching to a char array (using single quotes instead of double quotes) or by splitting the string into an array of strings.
single_string = "abcdefghijklmnopqrstuvwxyz ";
string_array = ["a" "b" "c" "d" "e" ...
char_array = char("abcdefghijklmnopqrstuvwxyz ");
char_array = 'abcdefghijklmnopqrstuvwxyz ';
I bet your error was something like "Index exceeds array bounds". Please make sure you include any errors when you make your post so we can help you as much as possible!