Problem 1: TakeWhile (100pts)

Problem

Implement takeWhile, which takes in an iterator t and a predicate p. takeWhile yields the values from t up to the first element that does not satisfy p.

A predicate p is just a pure function with one argument and returns either True or False.

实现 takeWhile，它接受一个迭代器 t 和一个谓词 p。 takeWhile 产出来自 t 的值，直到第一个不满足 p 的元素。

谓词 p 只是一个接受一个参数并返回 True 或 False 的纯函数。

def takeWhile(t, p):
    """Take elements from t until p is not satisfied.

    >>> s = iter([10, 9, 10, 9, 9, 10, 8, 8, 8, 7])
    >>> list(takeWhile(s, lambda x: x == 10))
    [10]
    >>> s2 = iter([1, 1, 2, 3, 5, 8, 13])
    >>> list(takeWhile(s2, lambda x: x % 2 == 1))
    [1, 1]
    >>> s = iter(['a', '', 'b', '', 'c'])
    >>> list(takeWhile(s, lambda x: x != ''))
    ['a']
    >>> list(takeWhile(s, lambda x: x != ''))
    ['b']
    >>> next(s)
    'c'
    """
    "*** YOUR CODE HERE ***"

Hints

• 如果你的程序看起来没有问题，但是 OJ 只有 80/100，可以考虑使用以下结构。原因见 PEP-0479。

try:
    "*** YOUR CODE HERE ***"
except StopIteration:
    return

Solutions

使用 try：先存储 next(t) 的值，判断是否符合条件再输出

def takeWhile(t, p):
    while True:
        try:
            i = next(t)
            if p(i): yield i
            else: return
        except StopIteration: return

---

或者使用 for

def takeWhile(t, p):
    for i in t:
        if p(i): yield i
        else: return

---

one-liner:

使用 next 的 default 参数，递归

def takeWhile(t, p):
    if (i := next(t, o := object())) is not o and p(i): yield from [] if (yield i) and False else takeWhile(t, p)