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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

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

User · Answer

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

[python] Traverse a list in reverse order in Python

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