Traverse a list in reverse order in Python

Question

So I can start from len collection  and end in collection 0    I also want to be able to access the loop index

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An expressive way to achieve reverse enumerate collection   in python 3   zip reversed range len collection     reversed collection     in python 2   izip reversed xrange len collection     reversed collection     I m not sure why we don t have a shorthand for this  eg    def reversed enumerate collection       return zip reversed range len collection     reversed collection     or why we don t have reversed range

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Use list reverse   and then iterate as you normally would   http   docs python org tutorial datastructures html

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for what ever it s worth you can do it like this too  very simple   a    1  2  3  4  5  6  7  for x in xrange len a        x    1     print a -x

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gt  gt  gt  l     a   b   c   d    gt  gt  gt  l reverse    gt  gt  gt  l   d    c    b    a     OR   gt  gt  gt  print l   -1    d    c    b    a

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def reverse spam       k          for i in spam          k insert 0 i      return    join k

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the reverse function comes in handy here   myArray    1 2 3 4  myArray reverse   for x in myArray      print x

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I think the most elegant way is to transform enumerate and reversed using the following generator   - ri 1   val  for ri  val in enumerate reversed foo     which generates a the reverse of the enumerate iterator  Example   foo    1 2 3  bar    3 6 9        bar i  - val     for i  val in   - ri 1   val  for ri  val in enumerate reversed foo        Result    6  4  2

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Assuming task is to find last element that satisfies some condition in a list  i e  first when looking backwards   I m getting following numbers    gt  gt  gt  min timeit repeat  for i in xrange len xs -1 -1 -1   n    if 128    xs i   break   setup  xs  n   range 256   0   repeat 8   4 6937971115112305  gt  gt  gt  min timeit repeat  for i in reversed xrange 0  len xs     n    if 128    xs i   break   setup  xs  n   range 256   0   repeat 8   4 809093952178955  gt  gt  gt  min timeit repeat  for i  x in enumerate reversed xs   1   n    if 128    x  break   setup  xs  n   range 256   0   repeat 8   4 931743860244751  gt  gt  gt  min timeit repeat  for i  x in enumerate xs   -1    n    if 128    x  break   setup  xs  n   range 256   0   repeat 8   5 548468112945557  gt  gt  gt  min timeit repeat  for i in xrange len xs   0  -1   n    if 128    xs i - 1   break   setup  xs  n   range 256   0   repeat 8   6 286104917526245  gt  gt  gt  min timeit repeat  i   len xs  nwhile 0  lt  i  n    i -  1 n    if 128    xs i   break   setup  xs  n   range 256   0   repeat 8   8 384078979492188   So  the ugliest option xrange len xs -1 -1 -1  is the fastest

User · Answer

It can be done like this  for i in range len collection -1  -1  -1       print collection i         print collection i   for python 3     So your guess was pretty close    A little awkward but it s basically saying  start with 1 less than len collection   keep going until you get to just before -1  by steps of -1  Fyi  the help function is very useful as it lets you view the docs for something from the Python console  eg  help range

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How about without recreating a new list  you can do by indexing    gt  gt  gt  foo     1a   2b   3c   4d    gt  gt  gt  for i in range len foo            print foo - i 1       4d 3c 2b 1a  gt  gt  gt    OR   gt  gt  gt  length   len foo   gt  gt  gt  for i in range length           print foo length-i-1      4d 3c 2b 1a  gt  gt  gt

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You can use a negative index in an ordinary for loop    gt  gt  gt  collection     ham    spam    eggs    baked beans    gt  gt  gt  for i in range 1  len collection    1           print collection -i        baked beans eggs spam ham   To access the index as though you were iterating forward over a reversed copy of the collection  use i - 1    gt  gt  gt  for i in range 1  len collection    1           print i-1  collection -i        0 baked beans 1 eggs 2 spam 3 ham   To access the original  un-reversed index  use len collection  - i    gt  gt  gt  for i in range 1  len collection    1           print len collection -i  collection -i        3 baked beans 2 eggs 1 spam 0 ham

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input list     foo   bar   baz   for i in range -1 -len input list -1 -1      print input list i     i think this one is also simple way to do  it    read from end and keep decrementing till the length of list  since we never execute the  end  index hence added -1 also

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You can also use a while loop   i   len collection -1 while i gt  0      value   collection i      index   i     i- 1

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If you need the index and your list is small  the most readable way is to do reversed list enumerate your list    like the accepted answer says  But this creates a copy of your list  so if your list is taking up a large portion of your memory you ll have to subtract the index returned by enumerate reversed    from len  -1   If you just need to do it once   a     b    d    c    a    for index  value in enumerate reversed a        index   len a -1 - index      do something index  value    or if you need to do this multiple times you should use a generator   def enumerate reversed lyst       for index  value in enumerate reversed lyst            index   len lyst -1 - index         yield index  value  for index  value in enumerate reversed a       do something index  value

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To use negative indices  start at -1 and step back by -1 at each iteration    gt  gt  gt  a     foo    bar    baz    gt  gt  gt  for i in range -1  -1  len a  1   -1           print i  a i       -1 baz -2 bar -3 foo

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I like the one-liner generator approach     i  sequence i   for i in reversed xrange len sequence

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An approach with no imports   for i in range 1 len arr  1       print arr -i     or  for i in arr   -1       print i

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The reversed builtin function is handy   for item in reversed sequence     The documentation for reversed explains its limitations   For the cases where I have to walk a sequence in reverse along with the index  e g  for in-place modifications changing the sequence length   I have this function defined an my codeutil module   import itertools def reversed enumerate sequence       return itertools izip          reversed xrange len sequence             reversed sequence           This one avoids creating a copy of the sequence  Obviously  the reversed limitations still apply

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If you don t mind the index being negative  you can do    gt  gt  gt  a     foo    bar    baz    gt  gt  gt  for i in range len a            print  i  a  i    -1 baz -2 bar -3 foo

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A simple way     n   int input    arr   list map int  input   split      for i in reversed range 0  n        print   d  d    i  arr i

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If you need the loop index  and don t want to traverse the entire list twice  or use extra memory  I d write a generator     def reverse enum L      for index in reversed xrange len L           yield index  L index   L     foo    bar    bas   for index  item in reverse enum L      print index  item

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Also  you could use either  range  or  count  functions   As follows   a     foo    bar    baz   for i in range len a -1  -1  -1       print i  a i    3 baz 2 bar 1 foo   You could also use  count  from itertools as following   a     foo    bar    baz   from itertools import count  takewhile  def larger than 0 x       return x  gt  0  for x in takewhile larger than 0  count 3  -1        print x  a x-1    3 baz 2 bar 1 foo

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you can use a generator   li    1 2 3 4 5 6  len li   len li  gen    len li-1-i for i in range len li     finally   for i in gen      print li i     hope this help you

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Use the built-in reversed   function    gt  gt  gt  a     foo    bar    baz    gt  gt  gt  for i in reversed a           print i       baz bar foo   To also access the original index  use enumerate   on your list before passing it to reversed      gt  gt  gt  for i  e in reversed list enumerate a             print i  e       2 baz 1 bar 0 foo   Since enumerate   returns a generator and generators can t be reversed  you need to convert it to a list first

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You can do   for item in my list   -1       print item    Or whatever you want to do in the for loop    The    -1  slice reverses the list in the for loop  but won t actually modify your list  permanently

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The other answers are good  but if you want to do as  List comprehension style  collection     a   b   c    item for item in reversed  collection

[python] Traverse a list in reverse order in Python

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