当使用for循环迭代时，我如何处理输入的最后一个元素?特别是，如果有代码应该只出现在元素之间(而不是在最后一个元素之后)，我该如何构造代码?
目前，我写的代码是这样的:
for i, data in enumerate(data_list):
code_that_is_done_for_every_element
if i != len(data_list) - 1:
code_that_is_done_between_elements
我如何简化或改进它?
当前回答
假设input是一个迭代器，下面是使用itertools中的tee和izip的方法:
from itertools import tee, izip
items, between = tee(input_iterator, 2) # Input must be an iterator.
first = items.next()
do_to_every_item(first) # All "do to every" operations done to first item go here.
for i, b in izip(items, between):
do_between_items(b) # All "between" operations go here.
do_to_every_item(i) # All "do to every" operations go here.
演示:
>>> def do_every(x): print "E", x
...
>>> def do_between(x): print "B", x
...
>>> test_input = iter(range(5))
>>>
>>> from itertools import tee, izip
>>>
>>> items, between = tee(test_input, 2)
>>> first = items.next()
>>> do_every(first)
E 0
>>> for i,b in izip(items, between):
... do_between(b)
... do_every(i)
...
B 0
E 1
B 1
E 2
B 2
E 3
B 3
E 4
>>>
其他回答
我发现在循环表达式之前定义循环值很方便。对于这个方框示例，匹配循环中的值，或者其他可能需要它的地方。
numberofboxes = 1411
for j in range(1,numberofboxes):
if j != numberofboxes - 1:
print ("},")
else:
print("}")
虽然这个问题很老了，但我通过谷歌来到这里，我发现了一个非常简单的方法:列表切片。假设您想在所有列表条目之间放一个“&”。
s = ""
l = [1, 2, 3]
for i in l[:-1]:
s = s + str(i) + ' & '
s = s + str(l[-1])
返回'1 & 2 & 3'。
假设input是一个迭代器，下面是使用itertools中的tee和izip的方法:
from itertools import tee, izip
items, between = tee(input_iterator, 2) # Input must be an iterator.
first = items.next()
do_to_every_item(first) # All "do to every" operations done to first item go here.
for i, b in izip(items, between):
do_between_items(b) # All "between" operations go here.
do_to_every_item(i) # All "do to every" operations go here.
演示:
>>> def do_every(x): print "E", x
...
>>> def do_between(x): print "B", x
...
>>> test_input = iter(range(5))
>>>
>>> from itertools import tee, izip
>>>
>>> items, between = tee(test_input, 2)
>>> first = items.next()
>>> do_every(first)
E 0
>>> for i,b in izip(items, between):
... do_between(b)
... do_every(i)
...
B 0
E 1
B 1
E 2
B 2
E 3
B 3
E 4
>>>
“code between”是头尾模式的一个例子。
你有一个项目，后面是一系列(项目之间)对。您还可以将其视为(item, between)对的序列，后跟一个item。通常更简单的做法是将第一个元素作为特殊元素，而将所有其他元素作为“标准”情况。
此外，为了避免重复代码，必须提供一个函数或其他对象来包含不想重复的代码。在循环中嵌入if语句，除非有一次总是假的，这有点傻。
def item_processing( item ):
# *the common processing*
head_tail_iter = iter( someSequence )
head = next(head_tail_iter)
item_processing( head )
for item in head_tail_iter:
# *the between processing*
item_processing( item )
这更可靠，因为它更容易证明，它不会创建额外的数据结构(即列表的副本)，也不需要大量浪费if条件的执行，if条件总是假的，只有一次例外。
除了向上数，你也可以向下数:
nrToProcess = len(list)
for s in list:
s.doStuff()
nrToProcess -= 1
if nrToProcess==0: # this is the last one
s.doSpecialStuff()
