learning big o notation

def count_ones(a_list)
  total = 0  # O(1) + O(1)  -- constant store + constant fetch for object '0'
  a_list.each do |e| # O(n) -- because it increases linearly with input
    if e == 1        # O(1) -- because binary operations are constants
      total += 1     # O(1) + O(1) + O(1) + O(1) -- constant fetch, constant fetch, binary operation (+), constant store
    end
  end
  return total
end

# O(1) + O(1) + O(n) * ( O(1) + O(1) + O(1) + O(1) + O(1) )
# O(1) + O(1) + O(5n)
# we only care about the largest term : 0(5n)