如何在循环时访问上一个/下一个元素？

有没有办法在循环使用for循环时访问列表(或元组,或其他可迭代)的下一个或前一个元素？

l=[1,2,3]
for item in l:
    if item==2:
        get_previous(l,item)

Markus Jarde.. 68

表示为生成器函数:

def neighborhood(iterable):
    iterator = iter(iterable)
    prev_item = None
    current_item = next(iterator)  # throws StopIteration if empty.
    for next_item in iterator:
        yield (prev_item, current_item, next_item)
        prev_item = current_item
        current_item = next_item
    yield (prev_item, current_item, None)

用法:

for prev,item,next in neighborhood(l):
    print prev, item, next

---

Vicky Liau.. 29

一个简单的方法.

l=[1,2,3]
for i,j in zip(l, l[1:]):
    print i, j

我使用了这个,但扩展了以避免删除开始/结束项:`for prev,cur,next in zip([None] + l [: - 1],l,l [1:] + [None]):` (10认同)

---

Rudiger Wolf.. 11

l=[1,2,3]
for i,item in enumerate(l):
    if item==2:
        get_previous=l[i-1]
        print get_previous

>>>1

---

Brian.. 8

在处理需要某些上下文的生成器时,我经常使用下面的实用程序函数在迭代器上给出一个滑动窗口视图:

import collections, itertools

def window(it, winsize, step=1):
    """Sliding window iterator."""
    it=iter(it)  # Ensure we have an iterator
    l=collections.deque(itertools.islice(it, winsize))
    while 1:  # Continue till StopIteration gets raised.
        yield tuple(l)
        for i in range(step):
            l.append(it.next())
            l.popleft()

它将一次生成序列N个项目的视图,将步骤移位.例如.

>>> list(window([1,2,3,4,5],3))
[(1, 2, 3), (2, 3, 4), (3, 4, 5)]

在前瞻/后方情况下使用时,如果您还需要处理数字而没有下一个或上一个值,则可能需要使用适当的值(例如"无")填充序列.

l= range(10)
# Print adjacent numbers
for cur, next in window(l + [None] ,2):
    if next is None: print "%d is the last number." % cur
    else: print "%d is followed by %d" % (cur,next)

---

DuckPuncher.. 7

我知道这是旧的,但为什么不用enumerate呢？

l = ['adam', 'rick', 'morty', 'adam', 'billy', 'bob', 'wally', 'bob', 'jerry']

for i, item in enumerate(l):
    if i == 0:
        previous_item = None
    else:
        previous_item = l[i - 1]

    if i == len(l) - 1:
        next_item = None
    else:
        next_item = l[i + 1]

    print('Previous Item:', previous_item)
    print('Item:', item)
    print('Next Item:', next_item)
    print('')

    pass

如果你运行它,你会看到它抓住上一个和下一个项目,而不关心重复列表中的项目.

1> Markus Jarde..：

---

表示为生成器函数:

def neighborhood(iterable):
    iterator = iter(iterable)
    prev_item = None
    current_item = next(iterator)  # throws StopIteration if empty.
    for next_item in iterator:
        yield (prev_item, current_item, next_item)
        prev_item = current_item
        current_item = next_item
    yield (prev_item, current_item, None)

用法:

for prev,item,next in neighborhood(l):
    print prev, item, next

2> Vicky Liau..：

---

一个简单的方法.

l=[1,2,3]
for i,j in zip(l, l[1:]):
    print i, j

---

我使用了这个,但扩展了以避免删除开始/结束项:`for prev,cur,next in zip([None] + l [: - 1],l,l [1:] + [None]):`

3> Rudiger Wolf..：

---

l=[1,2,3]
for i,item in enumerate(l):
    if item==2:
        get_previous=l[i-1]
        print get_previous

>>>1

4> Brian..：

---

在处理需要某些上下文的生成器时,我经常使用下面的实用程序函数在迭代器上给出一个滑动窗口视图:

import collections, itertools

def window(it, winsize, step=1):
    """Sliding window iterator."""
    it=iter(it)  # Ensure we have an iterator
    l=collections.deque(itertools.islice(it, winsize))
    while 1:  # Continue till StopIteration gets raised.
        yield tuple(l)
        for i in range(step):
            l.append(it.next())
            l.popleft()

它将一次生成序列N个项目的视图,将步骤移位.例如.

>>> list(window([1,2,3,4,5],3))
[(1, 2, 3), (2, 3, 4), (3, 4, 5)]

在前瞻/后方情况下使用时,如果您还需要处理数字而没有下一个或上一个值,则可能需要使用适当的值(例如"无")填充序列.

l= range(10)
# Print adjacent numbers
for cur, next in window(l + [None] ,2):
    if next is None: print "%d is the last number." % cur
    else: print "%d is followed by %d" % (cur,next)

5> DuckPuncher..：

---

我知道这是旧的,但为什么不用enumerate呢？

l = ['adam', 'rick', 'morty', 'adam', 'billy', 'bob', 'wally', 'bob', 'jerry']

for i, item in enumerate(l):
    if i == 0:
        previous_item = None
    else:
        previous_item = l[i - 1]

    if i == len(l) - 1:
        next_item = None
    else:
        next_item = l[i + 1]

    print('Previous Item:', previous_item)
    print('Item:', item)
    print('Next Item:', next_item)
    print('')

    pass

如果你运行它,你会看到它抓住上一个和下一个项目,而不关心重复列表中的项目.

6> codeape..：

---

查看Tempita项目中的looper实用程序.它为循环项提供了一个包装器对象,它提供了上一个,下一个,第一个,最后一个等属性.

看一下looper类的源代码,很简单.还有其他这样的循环助手,但我现在不记得其他任何人.

例:

> easy_install Tempita
> python
>>> from tempita import looper
>>> for loop, i in looper([1, 2, 3]):
...     print loop.previous, loop.item, loop.index, loop.next, loop.first, loop.last, loop.length, loop.odd, loop.even
... 
None 1 0 2 True False 3 True 0
1 2 1 3 False False 3 False 1
2 3 2 None False True 3 True 0