python - Efficiently check if an element occurs at least n times in a list

Question

How to best write a Python function (check_list) to efficiently test if an element (x) occurs at least n times in a list (l)?

My first thought was:

def check_list(l, x, n):
    return l.count(x) >= n

But this doesn't short-circuit once x has been found n times and is always O(n).

A simple approach that does short-circuit would be:

def check_list(l, x, n):
    count = 0
    for item in l:
        if item == x:
            count += 1
            if count == n:
                return True
    return False

I also have a more compact short-circuiting solution with a generator:

def check_list(l, x, n):
    gen = (1 for item in l if item == x)
    return all(next(gen,0) for i in range(n))

Are there other good solutions? What is the best efficient approach?

Thank you

Answer 1

Instead of incurring extra overhead with the setup of a range object and using all which has to test the truthiness of each item, you could use itertools.islice to advance the generator n steps ahead, and then return the next item in the slice if the slice exists or a default False if not:

from itertools import islice

def check_list(lst, x, n):
    gen = (True for i in lst if i==x)
    return next(islice(gen, n-1, None), False)

Note that like list.count, itertools.islice also runs at C speed. And this has the extra advantage of handling iterables that are not lists.

---

Some timing:

In [1]: from itertools import islice

In [2]: from random import randrange

In [3]: lst = [randrange(1,10) for i in range(100000)]

In [5]: %%timeit # using list.index
   ....: check_list(lst, 5, 1000)
   ....:
1000 loops, best of 3: 736 μs per loop

In [7]: %%timeit # islice
   ....: check_list(lst, 5, 1000)
   ....:
1000 loops, best of 3: 662 μs per loop

In [9]: %%timeit # using list.index
   ....: check_list(lst, 5, 10000)
   ....:
100 loops, best of 3: 7.6 ms per loop

In [11]: %%timeit # islice
   ....: check_list(lst, 5, 10000)
   ....:
100 loops, best of 3: 6.7 ms per loop