Python reverse order

发布时间： 2013 年 08 月 14 日 /作者： admin /

Python可以很简单实现C语言中for(i = len-1; i>=0; –i)的逆序循环，而且有不止一种写法：

第一种，for i in range(len-1, -1, -1) ，简单易懂

第二种，for i in range(len)[::-1]，利用list的[start:end:step]排序功能，当然在大数组下性能堪忧

What is Python's range() Function?

As an experienced Python developer, or even a beginner, you've likely heard of the Python range() function. But what does it do? In a nutshell, it generates a list of numbers, which is generally used to iterate over with for loops. There's many use cases. Often you will want to use this when you want to perform an action X number of times, where you may or may not care about the index. Other times you may want to iterate over a list (or another iterable object), while being able to have the index available.

The range() function works a little bit differently between Python 2.x and 3.x under the hood, however the concept is the same. We'll get to that a bit later, however.

Python's range() Parameters

The range() function has two sets of parameters, as follows:

range(stop)

• stop: Number of integers (whole numbers) to generate, starting from zero. eg. range(3) == [0, 1, 2].

range([start], stop[, step])

• start: Starting number of the sequence.
• stop: Generate numbers up to, but not including this number.
• step: Difference between each number in the sequence.

Note that:

• All parameters must be integers.
• All parameters can be positive or negative.
• range() (and Python in general) is 0-index based, meaning list indexes start at 0, not 1. eg. The syntax to access the first element of a list is mylist[0]. Therefore the last integer generated by range() is up to, but not including, stop. For example range(0, 5) generates integers from 0 up to, but not including, 5.

Python's range() Function Examples

Simple Usage

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

>>>   # One parameter >>>   for   i   in   range ( 5 ) : . . .       print ( i ) . . . 0 1 2 3 4 >>>   # Two parameters >>>   for   i   in   range ( 3 ,   6 ) : . . .       print ( i ) . . . 3 4 5 >>>   # Three parameters >>>   for   i   in   range ( 4 ,   10 ,   2 ) : . . .       print ( i ) . . . 4 6 8 >>>   # Going backwards >>>   for   i   in   range ( 0 ,   - 10 ,   - 2 ) : . . .       print ( i ) . . . 0 - 2 - 4 - 6 - 8

Iterating Lists

1 2 3 4 5 6 7 8 9 10

>>>   my_list   =   [ 'one' ,   'two' ,   'three' ,   'four' ,   'five' ] >>>   my_list_len   =   len ( my_list ) >>>   for   i   in   range ( 0 ,   my_list_len ) : . . .       print ( my_list [ i ] ) . . . one two three four five

99 Bottles of Beer on the Wall...

With the following code:

1 2 3 4 5 6 7 8 9 10

for   i   in   range ( 99 ,   0 ,   - 1 ) :          if   i   ==   1 :                  print ( '1 bottle of beer on the wall, 1 bottle of beer!' )                  print ( 'So take it down, pass it around, no more bottles of beer on the wall!' )          elif   i   ==   2 :                  print ( '2 more bottles of beer on the wall, 2 more bottles of beer!' )                  print ( 'So take one down, pass it around, 1 more bottle of beer on the wall!' )          else :                  print ( '{0} bottles of beer on the wall, {0} bottles of beer!' . format ( i ) )                  print ( 'So take it down, pass it around, {0} more bottles of beer on the wall!' . format ( i   -   1 ) )

We get the following output:

1 2 3 4 5 6 7 8 9 10 11 12 13

99   bottles  of  beer  on  the  wall ,   99   bottles  of  beer ! So  take  one  down ,   pass   it  around ,   98   more  bottles  of  beer  on  the  wall ! 98   bottles  of  beer  on  the  wall ,   98   bottles  of  beer ! So  take  one  down ,   pass   it  around ,   97   more  bottles  of  beer  on  the  wall ! 97   bottles  of  beer  on  the  wall ,   97   bottles  of  beer ! So  take  one  down ,   pass   it  around ,   96   more  bottles  of  beer  on  the  wall ! . . . 3   bottles  of  beer  on  the  wall ,   3   bottles  of  beer ! So  take  one  down ,   pass   it  around ,   2   more  bottles  of  beer  on  the  wall ! 2   more  bottles  of  beer  on  the  wall ,   2   more  bottles  of  beer ! So  take  one  down ,   pass   it  around ,   1   more  bottle  of  beer  on  the  wall ! 1   bottle  of  beer  on  the  wall ,   1   bottle  of  beer ! So  take  it  down ,   pass   it  around ,   no  more  bottles  of  beer  on  the  wall !

Brilliant! Finally you can see the true power of Python :). If you're a little confused, for reference see the Wikipedia article. 

Python's range() vs xrange() Functions

You may have heard of a function known as xrange(). This is a function that is present in Python 2.x, however it was renamed to range() in Python 3.x, and the original range() function was deprecated in Python 3.x. So what's the difference? Well, in Python 2.x range() produced a list, and xrange() returned an iterator - a sequence object. We can see this in the following example:

• Python 3.x
• Python 2.x

1 2 3 4

>>>   range ( 1 ) range ( 0 ,   1 ) >>>   type ( range ( 1 ) ) < class   'range' >

So in Python 3.x, the range() function got its own type. In basic terms, if you want to use range() in a for loop, then you're good to go. However you can't use it purely as a list object. For example you cannot slice a range type.

When you're using an iterator, every loop of the for statement produces the next number on the fly. Whereas the original range() function produced all numbers instantaneously, before the for loop started executing. The problem with the original range() function was that it used a very large amount of memory when producing a lot of numbers. However it tends to be quicker with a small amount of numbers. Note that in Python 3.x, you can still produce a list by passing the generator returned to the list() function. As follows:

• Python 3.x

1 2 3

>>>   list_of_ints   =   list ( range ( 3 ) ) >>>   list_of _ints [ 0 ,   1 ,   2 ]

To see the speed difference between the original range() and xrange() function, you may want to check out this article.

Using floats with Python's range() function

Unfortunately the range() function doesn't support the float type. However don't get upset too soon! We can easily implement it with a function. There's several ways it can be done, however here's one.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

>>>   # Note: All arguments are required. >>>   # We're not fancy enough to implement all. >>>   def   frange ( start ,   stop ,   step ) : . . .       i   =   start . . .       while   i   <   stop : . . .           yield   i . . .           i   +=   step . . . >>>   for   i   in   frange ( 0.5 ,   1.0 ,   0.1 ) : . . .           print ( i ) . . . 0.5 0.6 0.7 0.8 0.9 1.0