How to sum over each n-th element in an array ?

I have a 1 x 108108 array (named output) and I am trying to calculate the sum of elements in this fashion : a1 = sum (1st element + 1002th element + 2003th element +....+ (108108-1001)th element), for each 1008th element after the beginning one. The next sum would be a2 = sum (2nd element + 1003th element + 2004th element....+(108108-1001)th element) and so on till, a1001 = sum (1001th element + 2002th element + 3003th element +....108108th element). Afterwards, I want to collect a1,a2,a3...,a1001 into an array.

Could somebody point me to some documentation that will help me to do this ?

Thanks

Answer 1

[Stephen23]

Just use reshape:

>> b = sum(reshape(a,1001,[]),2);

and checking the output:

>> b(1)
ans = -6343.8
>> sum(a(1:1001:end))
ans = -6343.8
>> b(2)
ans = -6345.5
>> sum(a(2:1001:end))
ans = -6345.5