How can I reverse a list in Python

Question

How can I do the following in Python   array    0  10  20  40  for  i   array length   - 1  i  gt   0  i--    I need to have the elements of an array  but from the end to the beginning

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gt  gt  gt  L    0 10 20 40   gt  gt  gt  L   -1   40  20  10  0    Extended slice syntax is explained well in the Python What s new Entry for release 2 3 5  By special request in a comment this is the most current slice documentation

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Strictly speaking  the question is not how to return a list in reverse but rather how to reverse a list with an example list name array   To reverse a list named  array  use array reverse     The incredibly useful slice method as described can also be used to reverse a list in place by defining the list as a sliced modification of itself using array   array   -1

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Using some logic  Using some old school logic to practice for interviews       Swapping numbers front to back  Using two pointers index 0  and index last     def reverse array       n   array     first   0     last   len array  - 1     while first  lt  last        holder   n first        n first    n last        n last    holder       first    1       last -  1     return n  input - gt   -1  1  2  3  4  5  6  output - gt   6  1  2  3  4  5  -1

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You can also use the bitwise complement of the array index to step through the array in reverse   gt  gt  gt  array    0  10  20  40   gt  gt  gt   array  i  for i    in enumerate array    40  20  10  0   Whatever you do  don t do it this way

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array  0 10 20 40  for e in reversed array     print e

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Using slicing  e g  array   array   -1   is a neat trick and very Pythonic  but a little obscure for newbies maybe  Using the reverse   method is a good way to go in day to day coding because it is easily readable   However  if you need to reverse a list in place as in an interview question  you will likely not be able to use built in methods like these   The interviewer will be looking at how you approach the problem rather than the depth of Python knowledge  an algorithmic approach is required  The following example  using a classic swap  might be one way to do it -  def reverse in place lst          Declare a function     size   len lst                Get the length of the sequence     hiindex   size - 1     its   size 2                  Number of iterations required     for i in xrange 0  its        i is the low index pointer         temp   lst hiindex        Perform a classic swap         lst hiindex    lst i          lst i    temp         hiindex -  1              Decrement the high index pointer     print  Done      Now test it   array    2  5  8  9  12  19  25  27  32  60  65  1  7  24  124  654   print array                      Print the original sequence reverse in place array           Call the function passing the list print array                      Print reversed list     The result     2  5  8  9  12  19  25  27  32  60  65  1  7  24  124  654  Done   654  124  24  7  1  65  60  32  27  25  19  12  9  8  5  2    Note that this will not work on Tuples or string sequences  because strings and tuples are immutable  i e   you cannot write into them to change elements

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This class uses Python magic methods and iterators for reversing  and reverses a list   class Reverse object           Builds a reverse method using magic methods          def   init   self  data           self data   data         self index   len data        def   iter   self           return self      def   next   self           if self index    0              raise StopIteration          self index   self index - 1         return self data self index    REV INSTANCE   Reverse  0  10  20  40    iter REV INSTANCE   rev list      for i in REV INSTANCE      rev list append i   print rev list      Output   40  20  10  0

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gt  gt  gt  L    0 10 20 40   gt  gt  gt  L reverse    gt  gt  gt  L  40  20  10  0    Or    gt  gt  gt  L   -1   40  20  10  0

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gt  gt  gt  l    1  2  3  4  5   gt  gt  gt  print reduce lambda acc  x   x    acc  l        5  4  3  2  1

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list data    1 2 3 4 5  l   len list data  i l 1 rev data      while l gt 0    j i-l   l- 1   rev data append list data -j   print  After Rev -  s   rev data

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The most direct translation of your requirement into Python is this for statement   for i in xrange len array  - 1  -1  -1      print i  array i    This is rather cryptic but may be useful

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Can be done using   reverse     which returns a generator    gt  gt  gt  l    1 2 3 4 5   gt  gt  gt  for i in l   reversed            print i      5 4 3 2 1  gt  gt  gt

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With minimum amount of built-in functions  assuming it s interview settings  array    1  2  3  4  5  6 7  8  inverse       create container for inverse array length   len array    to iterate later  returns 8  counter   length - 1   because the 8th element is on position 7  as python starts from 0   for i in range length       inverse append array counter      counter -  1 print inverse

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Here s a way to lazily evaluate the reverse using a generator   def reverse seq       for x in range len seq   -1  -1    Iterate through a sequence starting from -1 and increasing by -1          yield seq x   Yield a value to the generator   Now iterate through like this   for x in reverse  1  2  3        print x    If you need a list   l   list reverse  1  2  3

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With reversed and list    gt  gt  gt  list1    1 2 3   gt  gt  gt  reversed list   list reversed list1    gt  gt  gt  reversed list  gt  gt  gt   3  2  1

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for x in array   -1       do stuff

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Reverse of a user input values in one line code   for i in input     -1   print i end

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You can make use of the reversed function for this as    gt  gt  gt  array  0 10 20 40   gt  gt  gt  for i in reversed array           print i    Note that reversed      does not return a list  You can get a reversed list using list reversed array

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Use list comprehension    array n  for n in range len array -1  -1  -1

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def reverse text       output          for i in range len text -1  -1  -1           output append text i       return output

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You could always treat the list like a stack just popping the elements off the top of the stack from the back end of the list  That way you take advantage of first in last out characteristics of a stack  Of course you are consuming the 1st array  I do like this method in that it s pretty intuitive in that you see one list being consumed from the back end while the other is being built from the front end    gt  gt  gt  l    1 2 3 4 5 6   nl     gt  gt  gt  while l          nl append l pop       gt  gt  gt  print nl  6  5  4  3  2  1

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For reversing the same list use    array reverse     To assign reversed list into some other list use   newArray   array   -1

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gt  gt  gt  L    1  2  3  4   gt  gt  gt  L    L -i  for i in range 1  len L    1    gt  gt  gt  L  4  3  2  1

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Another solution would be to use numpy flip for this  import numpy as np array    0  10  20  40  list np flip array    40  20  10  0

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use   print reversed list name

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def reverse my list     L   len my list    for i in range L 2       my list i   my list L-i - 1    my list L-i-1   my list i    return my list

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ORGANIZING VALUES  In Python  lists  order too can be manipulated with sort  organizing your variables in numerical alphabetical order  Temporarily  print sorted my list   Permanent  my list sort    print my list  You can sort with the flag  quot reverse True quot   print sorted my list  reverse True    or  my list sort reverse True   print my list   WITHOUT ORGANIZING Maybe you do not want to sort values  but only reverse the values  Then we can do it like this  print list reversed my list       Numbers have priority over alphabet in listing order  The Python values  organization is awesome  Edit 1  a mistaken moderator claimed that my answer was a copy and deleted my old post

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I find  contrary to some other suggestions  that l reverse   is by far the fastest way to reverse a long list in Python 3 and 2  I d be interested to know if others can replicate these timings   l   -1  is probably slower because it copies the list prior to reversing it  Adding the list   call around the iterator made by reversed l  must add some overhead  Of course if you want a copy of the list or an iterator then use those respective methods  but if you want to just reverse the list then l reverse   seems to be the fastest way   Functions  def rev list1 l       return l   -1   def rev list2 l       return list reversed l    def rev list3 l       l reverse       return l   List  l   list range 1000000     Python 3 5 timings  timeit lambda  rev list1 l   number 1000    6 48 timeit lambda  rev list2 l   number 1000    7 13 timeit lambda  rev list3 l   number 1000    0 44   Python 2 7 timings  timeit lambda  rev list1 l   number 1000    6 76 timeit lambda  rev list2 l   number 1000    9 18 timeit lambda  rev list3 l   number 1000    0 46

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Using reversed array  would be the likely best route     gt  gt  gt  array    1 2 3 4   gt  gt  gt  for item in reversed array    gt  gt  gt      print item   Should you need to understand how could implement this without using the built in reversed   def reverse a       midpoint   len a  2     for item in a  midpoint           otherside    len a  - a index item   - 1         temp   a otherside          a otherside    a a index item           a a index item     temp     return a   This should take O N  time

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Summary of Methods with Explanation and Timing Results There are three different built-in ways to reverse a list  Which method is best depends on whether you need to   Reverse an existing list in-place  altering the original list variable   Best solution is object reverse   method   Create an iterator of the reversed list  because you are going to feed it to a for-loop  a generator  etc    Best solution is reversed object  which creates the iterator   Create a copy of the list  just in the reverse order  to preserve the original list   Best solution is using slices with a -1 step size  object   -1     From a speed perspective  it is best to use the built-in functions to reverse a list  In this case  they are 2 to 8 times faster on short lists  10 items   and up to  300  times faster on long lists compared to a manually-created loop or generator  This makes sense as they are written in a native language  i e  C   have experts creating them  scrutiny  and optimization  They are also less prone to defects and more likely to handle edge and corner cases  Test Script Put all the code snippets in this answer together to make a script that will run the different ways of reversing a list that are described below  It will time each method while running it 100 000 times  The results are shown in the last section for lists of length 2  10  and 1000 items  from timeit import timeit from copy import copy  def time str ms t       return   0 8 2f  ms  format t   1000   Method 1  Reverse in place with obj reverse   If the goal is just to reverse the order of the items in an existing list  without looping over them or getting a copy to work with  use the  lt list gt  reverse   function  Run this directly on a list object  and the order of all items will be reversed  Note that the following will reverse the original variable that is given  even though it also returns the reversed list back  i e  you can create a copy by using this function output  Typically  you wouldn t make a function for this  but the timing script requires it  We test the performance of this two ways - first just reversing a list in-place  changes the original list   and then copying the list and reversing it afterward to see if that is the fastest way to create a reversed copy compared to the other methods  def rev in place mylist       mylist reverse       return mylist  def rev copy reverse mylist       a   copy mylist      a reverse       return a  Method 2  Reverse a list using slices obj   -1  The built-in index slicing method allows you to make a copy of part of any indexed object   It does not affect the original object It builds a full list  not an iterator  The generic syntax is   lt object gt  first index last index step   To exploit slicing to create a simple reversed list  use   lt list gt    -1   When leaving an option empty  it sets them to defaults of the first and last element of the object  reversed if the step size is negative   Indexing allows one to use negative numbers  which count from the end of the object s index backwards  i e  -2 is the second to last item   When the step size is negative  it will start with the last item and index backward by that amount  def rev slice mylist       a   mylist   -1      return a  Method 3  Reverse a list with the reversed obj  iterator function There is a reversed indexed object  function   This creates a reverse index iterator  not a list  Great if you are feeding it to a loop for better performance on large lists This creates a copy and does not affect the original object  Test with both a raw iterator  and creating a list from the iterator  def reversed iterator mylist       a   reversed mylist      return a  def reversed with list mylist       a   list reversed mylist       return a  Method 4  Reverse list with Custom Manual indexing As the timing shows  creating your own methods of indexing is a bad idea  Use the built-in methods unless you really do need to do something custom  This simply means learning the built-in methods  That said  there is not a huge penalty with smaller list sizes  but when you scale up the penalty becomes tremendous  The code below could be optimized  I m sure  but it can t ever match the built-in methods as they are directly implemented in a native language  def rev manual pos gen mylist       max index   len mylist  - 1     return   mylist max index - index  for index in range len mylist      def rev manual neg gen mylist          index is 0 to 9  but we need -1 to -10     return   mylist -index-1  for index in range len mylist      def rev manual index loop mylist       a          reverse index   len mylist  - 1     for index in range len mylist            a append mylist reverse index - index       return a      def rev manual loop mylist       a          reverse index   len mylist      for index    in enumerate mylist           reverse index -  1         a append mylist reverse index       return a  Timing each method Following is the rest of the script to time each method of reversing  It shows reversing in place with obj reverse   and creating the reversed obj  iterator are always the fastest  while using slices is the fastest way to create a copy  It also proves not to try to create a way of doing it on your own unless you have to  loops to test   100000 number of items   10 list to reverse   list range number of items   if number of items  lt  15      print  quot a     quot  format list to reverse   print  Loops        format loops to test     List of the functions we want to test with the timer  in print order fcns    rev in place  reversed iterator  rev slice  rev copy reverse          reversed with list  rev manual pos gen  rev manual neg gen          rev manual index loop  rev manual loop  max name string   max   len fcn   name    for fcn in fcns    for fcn in fcns      a   copy list to reverse    copy to start fresh each loop     out str       out       format fcn a   if number of items  lt  15 else          Time in ms for the given   of loops on this fcn     time str   time str ms timeit lambda  fcn a   number loops to test         Get the output string for this function     fcn str       a    format fcn   name          Add the correct string length to accommodate the maximum fcn name     format str      fx   s     time    rev    format max name string   4      print format str format fx fcn str  time time str  rev out str    Timing Results The results show that scaling works best with the built-in methods best suited for a given task  In other words  as the object element count increases  the built-in methods begin to have far superior performance results  You are also better off using the best built-in method that directly achieves what you need than to string things together  i e  slicing is best if you need a copy of the reversed list - it s faster than creating a list from the reversed   function  and faster than making a copy of the list and then doing an in-place obj reverse    But if either of those methods are really all you need  they are faster  but never by more than double the speed  Meanwhile - custom  manual methods can take orders of magnitude longer  especially with very large lists  For scaling  with a 1000 item list  the reversed  lt list gt   function call takes  30 ms to setup the iterator  reversing in-place takes just  55 ms  using the slice method takes  210 ms to create a copy of the full reversed list  but the quickest manual method I made took  8400 ms   With 2 items in the list  a   0  1  Loops  100 000 rev in place a               24 70 ms   out    1  0  reversed iterator a          30 48 ms   out    lt list reverseiterator object at 0x0000020242580408 gt  rev slice a                  31 65 ms   out    1  0  rev copy reverse a           63 42 ms   out    1  0  reversed with list a         48 65 ms   out    1  0  rev manual pos gen a         98 94 ms   out    1  0  rev manual neg gen a         88 11 ms   out    1  0  rev manual index loop a      87 23 ms   out    1  0  rev manual loop a            79 24 ms   out    1  0   With 10 items in the list  rev in place a               23 39 ms   out    9  8  7  6  5  4  3  2  1  0  reversed iterator a          30 23 ms   out    lt list reverseiterator object at 0x00000290A3CB0388 gt  rev slice a                  36 01 ms   out    9  8  7  6  5  4  3  2  1  0  rev copy reverse a           64 67 ms   out    9  8  7  6  5  4  3  2  1  0  reversed with list a         50 77 ms   out    9  8  7  6  5  4  3  2  1  0  rev manual pos gen a        162 83 ms   out    9  8  7  6  5  4  3  2  1  0  rev manual neg gen a        167 43 ms   out    9  8  7  6  5  4  3  2  1  0  rev manual index loop a     152 04 ms   out    9  8  7  6  5  4  3  2  1  0  rev manual loop a           183 01 ms   out    9  8  7  6  5  4  3  2  1  0   And with 1000 items in the list  rev in place a               56 37 ms reversed iterator a          30 47 ms rev slice a                 211 42 ms rev copy reverse a          295 74 ms reversed with list a        418 45 ms rev manual pos gen a       8410 01 ms rev manual neg gen a      11054 84 ms rev manual index loop a   10543 11 ms rev manual loop a         15472 66 ms

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If you want to store the elements of reversed list in some other variable  then you can use revArray   array   -1  or revArray   list reversed array     But the first variant is slightly faster   z   range 1000000  startTimeTic   time time   y   z   -1  print  Time   s s     time time   - startTimeTic    f   range 1000000  startTimeTic   time time   g   list reversed f   print  Time   s s     time time   - startTimeTic     Output   Time  0 00489711761475 s Time  0 00609302520752 s

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Reversing in-place by switching references of opposite indices    gt  gt  gt  l    1 2 3 4 5 6 7       gt  gt  gt  for i in range len l   2           l i   l -1-i    l -1-i   l i       gt  gt  gt  l  7  6  5  4  3  2  1

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There are 3 methods to get the reversed list    Slicing Method 1     reversed array   array -1  -1  Slicing Method 2  reversed array2   array   -1  Using the builtin function  reversed array   array reverse     The third function actually reversed the list object in place  That means no copy of pristine data is maintained  This is a good approach if you don t want to maintain the old version  But doesn t seem to be a solution if you do want the pristine and reversed version

[python] How can I reverse a list in Python?

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