Question

I have a vector a = 1111000011100001110000100100 and I have to compute two values based on it: p00 and p11.

p00 is the number of times 00 is occurring in the vector, divided by the total number of zeros. For example, in the above code then number of times 00 is occurring is 8/16 (total number of zeros).

Similarly, p11 is the number of occurrences of 11 divided by the total number of ones.

How can this be implemented in Matlab?

Answer 1

The safest and most generic way to do it is using regular expressions, because of the way they match runs.

a = [1 1 1 1 0 0 0 0 1 1 1 0 0 0 0 1 1 1 0 0 0 0 1 0 0 1 0 0]
s = char(a + '0');
p00 = numel(regexp(s, '00')) / sum(a == 0)
p11 = numel(regexp(s, '11')) / sum(a == 1)

NOTE:
I was tempted to do something along the following lines:

a = [1 1 1 1 0 0 0 0 1 1 1 0 0 0 0 1 1 1 0 0 0 0 1 0 0 1 0 0]
n = numel(a);
p00 = sum(a(1:n-1) == 0 & a(2:n) == 0) / sum(a == 0)
p11 = sum(a(1:n-1) == 1 & a(2:n) == 1) / sum(a == 1)

But this won’t give the correct result, because it counts the sequence 0 0 0 0 as 3, rather than 2.