python中使用itertools的组合

Question

Hello there is there a way to do 5C2 in python? I am a noob in python and i can't find the answer anywhere. The answer has to be 10. Ive been trying to play around with but i can only get errors, Here's my code I need to get 10

from itertools import *
    print(combinations(5,2))

It gives me:

TypeError: 'int' object is not iterable

Any help is appreciated

Answer 1

In [35]: def choose(n,k):
   ....:     return math.factorial(n)/(math.factorial(k)*math.factorial(n-k))
   ....: 

In [36]: choose(5,3)
Out[36]: 10.0

Don't forget to import math

Answer 2

Since you are asking explicitely about finding the number of combinations using itertools , the solution would be

import itertools
print len(list(itertools.combinations(range(5),2)))

Let me explain: itertools.combinations(iterator, k) takes as arguments an iterator (which can be a list) and the number of elements to chose from that list. Instead of answering the question of how many combinations of k element tuples you can draw from that list, it returns that list directly. So given an n element list from which all k -tuples should be drawn and looking at the length of that list, gives you the binomial coefficient.

Of course there are easier ways to calculate the binomial coefficient, using eg scipy.special.binom

import scipy
print scipy.special.binom(5,2) 

Answer 3

If the goal is to find number of combinations (n choose k) as one number, in my opinion, the following is the simplest way, if you can use Python 3.8.
From Python 3.8 there is the math.comb(n, k) function in math module.

import math

print(math.comb(5,2))

Gives

10

Some references:
https://www.w3schools.com/python/ref_math_comb.asp
https://www.geeksforgeeks.org/python-math-comb-method/