### Question :

First of all, my purpose is to randomly get only one element in both known sets. So my original method is firstly intersect two sets. And then randomly pick up a element from the intersected set. But this is foolish, because that I only need a elements but a intersected set.

So I need to find the algorithm of set.intersection().

I compare the cost time between the methods of ‘set.intersection()’ and ‘for{for{}}’. Set.intersection() is more faster than other one(100 times). So using ‘for{for{}}’ to pick up a randomly elements is not a wise idea.

What’s the algorithm behind set.intersection() in python?

##
Answer #1:

The algorithm is as follows: the smaller set is looped over and every element is copied depending whether it’s found in the bigger set. So, it’s the C equivalent of

```
def intersect(a, b):
if len(a) > len(b):
a, b = b, a
c = set()
for x in a:
if x in b:
c.add(x)
return c
```

(Or: `return set(x for x in a if x in b)`

.)