MATLAB: Count number of one values in vector in specific positions

Question

I have a vector, x, and am trying to count the number of one values at each position every 3 rows.

x = [1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 ]'

In this case it should be 5 for the first row.

x = [1 0 0 1 0 0 0 1 0 1 0 0 0 1 0]'

In this case it should be 3 for the first row and 2 for the second row.

How can it get these answers out of it?

Answer 1

sum(reshape(x, 3, []), 2)

Example

x1 = [1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 ]';
y1 = sum(reshape(x1, 3, []), 2)
x2  = [1 0 0 1 0 0 0 1 0 1 0 0 0 1 0]';
y2 = sum(reshape(x2, 3, []), 2)

Result

>> y1
y1 =
     5
     0
     0
>> y2
y2 =
     3
     2
     0