def nCr(n, r):
    """
    Calculate the number of combination (nCr = nPr/r!).
    The parameters need to meet the condition of n >= r >= 0.
    It returns 1 if r == 0, which means there is one pattern
    to choice 0 items out of the number of n.
    """

    # 10C7 = 10C3
    r = min(r, n-r)

    # Calculate the numerator.
    numerator = 1
    for i in range(n, n-r, -1):
        numerator *= i

    # Calculate the denominator. Should use math.factorial?
    denominator = 1
    for i in range(r, 1, -1):
        denominator *= i

    return numerator // denominator

for n,r in [(0,0), (0,1), (1,0), (1,1), (2,2), (3,1), (3,2), (5,2), (5,3), (10, 3)]:
    print('nCr({},{})={}'.format(n, r, nCr(n, r)))