python range xrange

引用

Help on built-in function range in module __builtin__:

range(...)
    range([start,] stop[, step]) -> list of integers
  
    Return a list containing an arithmetic progression of integers.
    range(i, j) returns [i, i+1, i+2, ..., j-1]; start (!) defaults to 0.
    When step is given, it specifies the increment (or decrement).
    For example, range(4) returns [0, 1, 2, 3].  The end point is omitted!
    These are exactly the valid indices for a list of 4 elements.

根据start与stop指定的范围以及step设定的步长，生成一个list。

>>> range(0,6)
[0, 1, 2, 3, 4, 5]

step为2的时候

>>> range(0,6,2)
[0, 2, 4]

>>> l=range(0,6)
>>> l
[0, 1, 2, 3, 4, 5]

引用

Help on class xrange in module __builtin__:

class xrange(object)
|  xrange([start,] stop[, step]) -> xrange object
|
|  Like range(), but instead of returning a list, returns an object that
|  generates the numbers in the range on demand.  For looping, this is
|  slightly faster than range() and more memory efficient.
|
|  Methods defined here:
|
|  __getattribute__(...)
|      x.__getattribute__('name') <==> x.name
|
|  __getitem__(...)
|      x.__getitem__(y) <==> x[y]
|
|  __iter__(...)
|      x.__iter__() <==> iter(x)
|
|  __len__(...)
|      x.__len__() <==> len(x)
|
|  __reduce__(...)
|
|  __repr__(...)
|      x.__repr__() <==> repr(x)
|
|  __reversed__(...)
|      Returns a reverse iterator.
|
|  ----------------------------------------------------------------------
|  Data and other attributes defined here:
|
|  __new__ = <built-in method __new__ of type object>
|      T.__new__(S, ...) -> a new object with type S, a subtype of T

xrange用法与range完全相同，所不同的是xrange生成的不是一个数组，而是一个generators

>>> list(xrange(0,6))
[0, 1, 2, 3, 4, 5]
>>> list(xrange(0,6,2))
[0, 2, 4]

因为xrange生成的是一个generators而不是数组，所以当数据很大的时候，使用xrange是有很大优势的。

>>> lst = range(0,7)
>>> lst
[0, 1, 2, 3, 4, 5, 6]
>>> for i in range((len(lst)-2)/2,-1,-1):
...     print i
...
2
1
0

>>> for i in range(len(lst)-1,-1,-1):
...     print i
...
6
5
4
3
2
1
0

参考资料：
http://docs.python.org/2/library/functions.html
http://stackoverflow.com/questions/94935/what-is-the-difference-between-range-and-xrange