Problem 2: Count van Count (100pts)

Problem

Consider the following implementations of count_factors and count_primes:

考虑以下 count_factors 和 count_primes 的实现：

def count_factors(n):
    """Return the number of positive factors that n has.
    >>> count_factors(6)
    4   # 1, 2, 3, 6
    >>> count_factors(4)
    3   # 1, 2, 4
    """
    i, count = 1, 0
    while i <= n:
        if n % i == 0:
            count += 1
        i += 1
    return count

def count_primes(n):
    """Return the number of prime numbers up to and including n.
    >>> count_primes(6)
    3   # 2, 3, 5
    >>> count_primes(13)
    6   # 2, 3, 5, 7, 11, 13
    """
    i, count = 1, 0
    while i <= n:
        if is_prime(i):
            count += 1
        i += 1
    return count

def is_prime(n):
    return count_factors(n) == 2 # only factors are 1 and n

The implementations look quite similar! Generalize this logic by writing a function count_cond, which takes in a two-argument predicate function condition(n, i). count_cond returns a one-argument function that takes in n, which counts all the numbers from 1 to n that satisfy condition when called.

这些实现看起来非常相似！通过编写一个函数 count_cond 来概括这个逻辑，该函数接受一个双参数谓词函数 condition(n, i)。count_cond 返回一个单参数函数，该单参数函数接受 n，并计算从 1 到 n 之间所有满足调用 condition 时的条件的数字个数。

def count_cond(condition):
    """Returns a function with one parameter N that counts all the numbers from
    1 to N that satisfy the two-argument predicate function Condition, where
    the first argument for Condition is N and the second argument is the
    number from 1 to N.

    >>> count_factors = count_cond(lambda n, i: n % i == 0)
    >>> count_factors(2)   # 1, 2
    2
    >>> count_factors(4)   # 1, 2, 4
    3
    >>> count_factors(12)  # 1, 2, 3, 4, 6, 12
    6

    >>> is_prime = lambda n, i: count_factors(i) == 2
    >>> count_primes = count_cond(is_prime)
    >>> count_primes(2)    # 2
    1
    >>> count_primes(3)    # 2, 3
    2
    >>> count_primes(4)    # 2, 3
    2
    >>> count_primes(5)    # 2, 3, 5
    3
    >>> count_primes(20)   # 2, 3, 5, 7, 11, 13, 17, 19
    8
    """
    "*** YOUR CODE HERE ***"

Hints

• 考虑编写一个从1到n的循环，对每一个数字判断是否满足条件

Solutions

使用 while 循环：

def count_cond(condition):
    def count(n):
        result, i = 0, 1
    while i <= n:
        if condition(n, i):
            result += 1
        i += 1
        return result
    return count

---

使用 for 循环：

def count_cond(condition):
    def count(n):
        result = 0
        for i in range(1, n + 1):
            if (condition(n, i)):
                result += 1
        return result
    return count