python for循环，如何找到下一个值（对象）？ [英] python for loop, how to find next value(object)?

问题描述

我试图使用for循环找到每两个对象之间的差异减去彼此。
所以，如何找到for循环中的下一个值？

 用于输入条目：
 first = entry＃现值
 last = ?????? ＃最后一个值怎么说？ 
 diff = last = first 
  

解决方案

请注意，这些解决方案的 none 都适用于发电机。为此，请参阅Glenn Maynards高级解决方案。

使用小型列表中的zip：

 当前，最后一个zip（条目[1：]，条目）：
 diff = current  -  last 
  

这个列表的副本（以及列表的两个副本中的一个元组列表）的副本，所以最好使用itertools来处理更大的列表。

  import itertools as 
 
 items = it.izip（it.islice（entries，1，None），entries）
 for current，last在项目中：
 diff = current  -  last 
  

这样可以避免复制list 和创建一个元组列表。

另一种不复制的方法是

  entry_iter = iter（entries）
 entry_iter.next（）＃丢弃第一个版本
 for i，enumerate（entry_iter）：
 diff = entry  -  entries [i] 
  

另一种方法是：

  for xrange（len（entries） -  1）：
 diff = entries [i + 1]  -  entries [i] 
  

这将创建一个迭代器，索引条目，并将其前进1。然后使用 enumerate 来获得该项目的指示。而且，正如Tyler在评论中指出的那样，一个循环可能会过度杀伤对于这样一个简单的问题，如果你只是想迭代差异。

  diffs =（current  -  last for current，last in 
 it.izip（it.islice（entries，1，None），entries））
  

HI, I'm trying to use for loop to find the difference between every two object by minus each other. So, how can I find the next value in a for loop?

for entry in entries:
    first = entry      # Present value
    last = ??????      # The last value how to say?
    diff = last = first

解决方案

It should be noted that none of these solutions work for generators. For that see Glenn Maynards superior solution.

use zip for small lists:

 for current, last in zip(entries[1:], entries):
     diff = current - last

This makes a copy of the list (and a list of tuples from both copies of the list) so it's good to use itertools for handling larger lists

import itertools as it

items = it.izip(it.islice(entries, 1, None), entries)
for current, last in items:
    diff = current - last

This will avoid both making a copy of the list and making a list of tuples.

Another way to do it without making a copy is

entry_iter = iter(entries)
entry_iter.next() # Throw away the first version
for i, entry in enumerate(entry_iter):
    diff = entry - entries[i]

And yet another way is:

for i in xrange(len(entries) - 1):
    diff = entries[i+1] - entries[i]

This creates an iterator that indexes entries and advances it by one. It then uses enumerate to get an indice with the item. The indice starts at 0 and so points to the previous element because we the loop one item in.

Also, as Tyler pointed out in the comment, a loop might be overkill for such a simple problem if you just want to iterate over the differences.

diffs = (current - last for current, last in 
         it.izip(it.islice(entries, 1, None), entries))

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