Useful Functions

In [1]:

    

def primes_upto(n):
    """
    Generator for primes up to n.
    For primes under 'n', just use n-1.
    """
    if n < 2:
        return
    marked = [0] * (n+1);
    value = 3
    yield 2
    while value <= n:
        if marked[value] == 0:
            yield value
            i = value
            while i <= n:
                marked[i] = 1
                i += value
        value += 2

list(primes_upto(20))


    

    
Out[1]:





[2, 3, 5, 7, 11, 13, 17, 19]

In [2]:

    

def num_digits(n, base=10):
    """
    Number of digits for the number 'n' given some 'base'.
    """
    if n < 0:
        return num_digits(-n, base)
    if n < base:
        return 1
    return 1 + num_digits(n//base, base)

for b in [2, 8, 10, 16]:
    print("255 in base %d has %d digits."%(b, num_digits(255, b)))


    

    




255 in base 2 has 8 digits.
255 in base 8 has 3 digits.
255 in base 10 has 3 digits.
255 in base 16 has 2 digits.

In [3]:

    

import itertools

print('Permutations:', list(itertools.permutations([1,2,3])))


    

    




Permutations: [(1, 2, 3), (1, 3, 2), (2, 1, 3), (2, 3, 1), (3, 1, 2), (3, 2, 1)]

In [2]:

    

import math
def is_pentagonal(x):
    # See test for pentagonal numbers
    # https://en.wikipedia.org/wiki/Pentagonal_number
    n = math.sqrt(24 * x + 1) + 1
    return n/6 == n//6


    

In [3]:

    

import math
def is_hexagonal(x):
    # See test for pentagonal numbers
    # https://en.wikipedia.org/wiki/Hexagonal_number
    n = math.sqrt(8 * x + 1) + 1
    return n/4 == n//4


    

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