Myriam,
If A, B and C were variables with the values 1, 2 and 3, then in your example:
array = [1, 1, 2, 1, 3, 1, 2, 2, 2, 3, 3, 1, 1, 3]
The diff() function returns the difference between each value and the next value, so
diff(array) = [0, 1, -2, 2, -2, 1, 0, 0, 1, 0, -2, 0, 2]
Every time an A is followed by a C, the difference is 2. Every time a B is followed by a C, the difference is 1. So, I was suggesting that you count the number of times A is followed by C by counting the number of times that the value 2 appears in diff(array) with a statement like c = sum(diff(array) == 2). Unfortunately, I see now that this does not work correctly for the number of times B is followed by C, because this results in a value of 1 in diff(array), and a value of 1 is also produced when an A is followed by a B.
Since you have said that A, B and C are characters, I assume that you mean that:
array = 'A A B A C A B B B C C A A C';
In this case, maybe a better solution would be:
y = strfind(array, 'A C');
N_AC = length(y);
y = strfind(array, 'B C');
N_BC = length(y);
