Computer Scientists Invent an Efficient New Way to Count
Imagine that you’re sent to a pristine rainforest to carry out a wildlife census. Every time you see an animal, you snap a photo. Your digital camera will track the total number of shots, but you’re only interested in the number of unique animals — all the ones that you haven’t counted already. What’s the best way to get that number? “The obvious solution requires remembering every animal you’ve seen so far and comparing each new animal to the list,” said Lance Fortnow, a computer scientist at the Illinois Institute of Technology. But there are cleverer ways to proceed, he added, because if you have thousands of entries, the obvious approach is far from easy.
It gets worse. What if you’re Facebook, and you want to count the number of distinct users who log in each day, even if some of them log in from multiple devices and at multiple times? Now we’re comparing each new login to a list that could run to the billions.
In a recent paper, computer scientists have described a new way to approximate the number of distinct entries in a long list, a method that requires remembering only a small number of entries. The algorithm will work for any list where the items come in one at a time — think words in a speech, goods on a conveyor belt or cars on the interstate.
