Statistics: combinations in Python

Question

I need to compute combinatorials (nCr) in Python but cannot find the function to do that in math, numpy or stat libraries. Something like a function of the type:

comb = calculate_combinations(n, r)

I need the number of possible combinations, not the actual combinations, so itertools.combinations does not interest me.

Finally, I want to avoid using factorials, as the numbers I'll be calculating the combinations for can get too big and the factorials are going to be monstrous.

This seems like a REALLY easy to answer question, however I am being drowned in questions about generating all the actual combinations, which is not what I want.

Answer 1

Updated answer in 2023: Use the math.comb function, which exists since Python 3.8 and has gotten much faster in 3.11.

---

Old answer: See scipy.special.comb (scipy.misc.comb in older versions of scipy). When exact is False, it uses the gammaln function to obtain good precision without taking much time. In the exact case it returns an arbitrary-precision integer, which might take a long time to compute.

Answer 2

Why not write it yourself? It's a one-liner or such:

from operator import mul    # or mul=lambda x,y:x*y
from fractions import Fraction

def nCk(n,k): 
  return int( reduce(mul, (Fraction(n-i, i+1) for i in range(k)), 1) )

Test - printing Pascal's triangle:

>>> for n in range(17):
...     print ' '.join('%5d'%nCk(n,k) for k in range(n+1)).center(100)
...     
                                                   1                                                
                                                1     1                                             
                                             1     2     1                                          
                                          1     3     3     1                                       
                                       1     4     6     4     1                                    
                                    1     5    10    10     5     1                                 
                                 1     6    15    20    15     6     1                              
                              1     7    21    35    35    21     7     1                           
                           1     8    28    56    70    56    28     8     1                        
                        1     9    36    84   126   126    84    36     9     1                     
                     1    10    45   120   210   252   210   120    45    10     1                  
                  1    11    55   165   330   462   462   330   165    55    11     1               
               1    12    66   220   495   792   924   792   495   220    66    12     1            
            1    13    78   286   715  1287  1716  1716  1287   715   286    78    13     1         
         1    14    91   364  1001  2002  3003  3432  3003  2002  1001   364    91    14     1      
      1    15   105   455  1365  3003  5005  6435  6435  5005  3003  1365   455   105    15     1   
    1    16   120   560  1820  4368  8008 11440 12870 11440  8008  4368  1820   560   120    16     1
>>> 

PS. edited to replace int(round(reduce(mul, (float(n-i)/(i+1) for i in range(k)), 1))) with int(reduce(mul, (Fraction(n-i, i+1) for i in range(k)), 1)) so it won't err for big N/K

Answer 3

A quick search on google code gives (it uses formula from @Mark Byers's answer):

def choose(n, k):
    """
    A fast way to calculate binomial coefficients by Andrew Dalke (contrib).
    """
    if 0 <= k <= n:
        ntok = 1
        ktok = 1
        for t in xrange(1, min(k, n - k) + 1):
            ntok *= n
            ktok *= t
            n -= 1
        return ntok // ktok
    else:
        return 0

choose() is 10 times faster (tested on all 0 <= (n,k) < 1e3 pairs) than scipy.misc.comb() if you need an exact answer.

def comb(N,k): # from scipy.comb(), but MODIFIED!
    if (k > N) or (N < 0) or (k < 0):
        return 0L
    N,k = map(long,(N,k))
    top = N
    val = 1L
    while (top > (N-k)):
        val *= top
        top -= 1
    n = 1L
    while (n < k+1L):
        val /= n
        n += 1
    return val

Answer 4

If you want exact results and speed, try gmpy -- gmpy.comb should do exactly what you ask for, and it's pretty fast (of course, as gmpy's original author, I am biased;-).

Answer 5

If you want an exact result, use sympy.binomial. It seems to be the fastest method, hands down.

x = 1000000
y = 234050

%timeit scipy.misc.comb(x, y, exact=True)
1 loops, best of 3: 1min 27s per loop

%timeit gmpy.comb(x, y)
1 loops, best of 3: 1.97 s per loop

%timeit int(sympy.binomial(x, y))
100000 loops, best of 3: 5.06 µs per loop

Answer 6

A literal translation of the mathematical definition is quite adequate in a lot of cases (remembering that Python will automatically use big number arithmetic):

from math import factorial

def calculate_combinations(n, r):
    return factorial(n) // factorial(r) // factorial(n-r)

For some inputs I tested (e.g. n=1000 r=500) this was more than 10 times faster than the one liner reduce suggested in another (currently highest voted) answer. On the other hand, it is out-performed by the snippit provided by @J.F. Sebastian.

Answer 7

Starting Python 3.8, the standard library now includes the math.comb function to compute the binomial coefficient:

math.comb(n, k)

which is the number of ways to choose k items from n items without repetition
n! / (k! (n - k)!):

import math
math.comb(10, 5) # 252

Answer 8

Here's another alternative. This one was originally written in C++, so it can be backported to C++ for a finite-precision integer (e.g. __int64). The advantage is (1) it involves only integer operations, and (2) it avoids bloating the integer value by doing successive pairs of multiplication and division. I've tested the result with Nas Banov's Pascal triangle, it gets the correct answer:

def choose(n,r):
  """Computes n! / (r! (n-r)!) exactly. Returns a python long int."""
  assert n >= 0
  assert 0 <= r <= n

  c = 1L
  denom = 1
  for (num,denom) in zip(xrange(n,n-r,-1), xrange(1,r+1,1)):
    c = (c * num) // denom
  return c

Rationale: To minimize the # of multiplications and divisions, we rewrite the expression as

    n!      n(n-1)...(n-r+1)
--------- = ----------------
 r!(n-r)!          r!

To avoid multiplication overflow as much as possible, we will evaluate in the following STRICT order, from left to right:

n / 1 * (n-1) / 2 * (n-2) / 3 * ... * (n-r+1) / r

We can show that integer arithmatic operated in this order is exact (i.e. no roundoff error).

Answer 9

You can write 2 simple functions that actually turns out to be about 5-8 times faster than using scipy.special.comb. In fact, you don't need to import any extra packages, and the function is quite easily readable. The trick is to use memoization to store previously computed values, and using the definition of nCr

# create a memoization dictionary
memo = {}
def factorial(n):
    """
    Calculate the factorial of an input using memoization
    :param n: int
    :rtype value: int
    """
    if n in [1,0]:
        return 1
    if n in memo:
        return memo[n]
    value = n*factorial(n-1)
    memo[n] = value
    return value

def ncr(n, k):
    """
    Choose k elements from a set of n elements - n must be larger than or equal to k
    :param n: int
    :param k: int
    :rtype: int
    """
    return factorial(n)/(factorial(k)*factorial(n-k))

If we compare times

from scipy.special import comb
%timeit comb(100,48)
>>> 100000 loops, best of 3: 6.78 µs per loop

%timeit ncr(100,48)
>>> 1000000 loops, best of 3: 1.39 µs per loop

Answer 10

Using dynamic programming, the time complexity is Θ(n*m) and space complexity Θ(m):

def binomial(n, k):
""" (int, int) -> int

         | c(n-1, k-1) + c(n-1, k), if 0 < k < n
c(n,k) = | 1                      , if n = k
         | 1                      , if k = 0

Precondition: n > k

>>> binomial(9, 2)
36
"""

c = [0] * (n + 1)
c[0] = 1
for i in range(1, n + 1):
    c[i] = 1
    j = i - 1
    while j > 0:
        c[j] += c[j - 1]
        j -= 1

return c[k]

Answer 11

If your program has an upper bound to n (say n <= N) and needs to repeatedly compute nCr (preferably for >>N times), using lru_cache can give you a huge performance boost:

from functools import lru_cache

@lru_cache(maxsize=None)
def nCr(n, r):
    return 1 if r == 0 or r == n else nCr(n - 1, r - 1) + nCr(n - 1, r)

Constructing the cache (which is done implicitly) takes up to O(N^2) time. Any subsequent calls to nCr will return in O(1).

Answer 12

It's pretty easy with sympy.

import sympy

comb = sympy.binomial(n, r)

Answer 13

The direct formula produces big integers when n is bigger than 20.

So, yet another response:

from math import factorial

reduce(long.__mul__, range(n-r+1, n+1), 1L) // factorial(r)

short, accurate and efficient because this avoids python big integers by sticking with longs.

It is more accurate and faster when comparing to scipy.special.comb:

 >>> from scipy.special import comb
 >>> nCr = lambda n,r: reduce(long.__mul__, range(n-r+1, n+1), 1L) // factorial(r)
 >>> comb(128,20)
 1.1965669823265365e+23
 >>> nCr(128,20)
 119656698232656998274400L  # accurate, no loss
 >>> from timeit import timeit
 >>> timeit(lambda: comb(n,r))
 8.231969118118286
 >>> timeit(lambda: nCr(128, 20))
 3.885951042175293

Answer 14

Using only standard library distributed with Python:

import itertools

def nCk(n, k):
    return len(list(itertools.combinations(range(n), k)))

Answer 15

This function is very optimized.

def nCk(n,k):
    m=0
    if k==0:
        m=1
    if k==1:
        m=n
    if k>=2:
        num,dem,op1,op2=1,1,k,n
        while(op1>=1):
            num*=op2
            dem*=op1
            op1-=1
            op2-=1
        m=num//dem
    return m

Answer 16

That's probably as fast as you can do it in pure python for reasonably large inputs:

def choose(n, k):
    if k == n: return 1
    if k > n: return 0
    d, q = max(k, n-k), min(k, n-k)
    num =  1
    for n in xrange(d+1, n+1): num *= n
    denom = 1
    for d in xrange(1, q+1): denom *= d
    return num / denom

Answer 17

Here is an efficient algorithm for you

for i = 1.....r

   p = p * ( n - i ) / i

print(p)

For example nCr(30,7) = fact(30) / ( fact(7) * fact(23)) = ( 30 * 29 * 28 * 27 * 26 * 25 * 24 ) / (1 * 2 * 3 * 4 * 5 * 6 * 7)

So just run the loop from 1 to r can get the result.

---

In python:

n,r=5,2
p=n
for i in range(1,r):
   p = p*(n - i)/i
else:
   p = p/(i+1)
print(p)

Answer 18

I timed 17 different functions from this thread and libraries linked here.

Since I feel it's a bit much to dump here, I put the code for the functions in a pastebin here.

The first test I did was to build pascal's triangle to the 100th row. I used timeit to do this 100 times. The numbers below are the average time it took in seconds to build the triangle once.

gmpy2.gmpy2.comb 0.0012259269999998423
math.comb 0.007063110999999935
__main__.stdfactorial2 0.011469491
__main__.scipybinom 0.0120114319999999
__main__.stdfactorial 0.012105122
__main__.scipycombexact 0.012569045999999844
__main__.andrewdalke 0.01825201100000015
__main__.rabih 0.018472497000000202
__main__.kta 0.019374668000000383
__main__.wirawan 0.029312811000000067
scipy.special._basic.comb 0.03221609299999954
__main__.jfsmodifiedscipy 0.04332894699999997
__main__.rojas 0.04395155400000021
sympy.functions.combinatorial.factorials.binomial 0.3233529779999998
__main__.nasbanov 0.593365528
__main__.pantelis300 1.7780402499999999

You may notice that there are only 16 functions here. That's because the recursive() function couldn't complete this even once in a reasonable amount of time, so I had to exclude it from the timeit tests. seriously, it's been going for hours.

I also timed various other types of inputs that not all of the above functions supported. Keep in mind that I only ran the test for each 10 times because nCr is computationally expensive and I'm impatient

Fractional values for n

__main__.scipybinom 0.011481370000000001
__main__.kta 0.01869513999999999
sympy.functions.combinatorial.factorials.binomial 6.33897291

Fractional values for r

__main__.scipybinom 0.010960040000000504
scipy.special._basic.comb 0.03681254999999908
sympy.functions.combinatorial.factorials.binomial 3.2962564499999987

Fractional values for n and r

__main__.scipybinom 0.008623409999998444
sympy.functions.combinatorial.factorials.binomial 3.690936439999999

Negative values for n

gmpy2.gmpy2.comb 0.010770989999997482
__main__.kta 0.02187850000000253
__main__.rojas 0.05104292999999984
__main__.nasbanov 0.6153183200000001
sympy.functions.combinatorial.factorials.binomial 3.0460310799999943

Negative fractional values for n, fractional values for r

sympy.functions.combinatorial.factorials.binomial 3.7689941699999965

the best solution currently for maximum speed and versatility would be a hybrid function to choose between different algorithms depending on the inputs

def hybrid(n: typing.Union[int, float], k: typing.Union[int, float]) -> typing.Union[int, float]:
    # my own custom hybrid solution
    def is_integer(n):
        return isinstance(n, int) or n.is_integer()
    if k < 0:
        raise ValueError("k cannot be negative.")
    elif n == 0:
        return 0
    elif k == 0 or k == n:
        return 1
    elif is_integer(n) and is_integer(k):
        return int(gmpy2.comb(int(n), int(k)))
    elif n > 0:
        return scipy.special.binom(n, k)
    else:
        return float(sympy.binomial(n, k))

Since sympy.binomial() is so slow, the true ideal solution would be to combine the code of scipy.special.binom() which performs well for fractions and gmpy2.comb() which performs well for ints. scipy's func and gympy2's func are both written in C which I am not very familiar with.

Answer 19

This is @killerT2333 code using the builtin memoization decorator.

from functools import lru_cache

@lru_cache()
def factorial(n):
    """
    Calculate the factorial of an input using memoization
    :param n: int
    :rtype value: int
    """
    return 1 if n in (1, 0) else n * factorial(n-1)

@lru_cache()
def ncr(n, k):
    """
    Choose k elements from a set of n elements,
    n must be greater than or equal to k.
    :param n: int
    :param k: int
    :rtype: int
    """
    return factorial(n) // (factorial(k) * factorial(n - k))

print(ncr(6, 3))

Answer 20

Very simple. Just import comb function from math module and get the result!!

Complete code is below:

from math import comb
n, r = 7, 3
print(comb(n,r))