Question

Given a vector of ones and zeros, I would like to count the number of entries with a value one. However, the vector may be very long and I only care to know if the vector has zero, one, or more entries with a value o f one.

Using the approach given here, I can count the number of ones in the vector.

(count (filter #{1} [1 0 1 0 0 1 1]))

Can I limit filter (or use some other approach) to avoid visiting any more than three elements of the vector, in this case?

Answer 1

Filter is lazy, so will only do as much work as required. Since you only care about having no 1’s, one 1’s or two or more ones, you only need to examine up to two elements of the filtered sequence of 1’s, so just take 2 before you count:

user=> (count (take 2 (filter #{1} [1 0 1 0 0 1 1])))
2
user=> (count (take 2 (filter #{1} [0 0 0 0 0 0 0])))
0
user=> (count (take 2 (filter #{1} [0 0 0 0 0 0 1])))
1
user=> (def rare (repeatedly #(if (< (rand) 0.0001) 1 0)))
#'user/rare
user=> (take 10 rare)
(0 0 0 0 0 0 0 0 0 0)
user=> (count (take 2 (filter #{1} rare)))
2