Print a list in reverse order with range

Question

How can you produce the following list with range   in Python    9  8  7  6  5  4  3  2  1  0

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Use the  range  built-in function  The signature is range start  stop  step    This produces a sequence that yields numbers  starting with start  and ending if stop has been reached  excluding stop    gt  gt  gt  range 9 -1 -1          9  8  7  6  5  4  3  2  1  0   gt  gt  gt  range -2  6  2       -2  0  2  4    In Python 3  this produces a non-list range object  which functions effectively like a read-only list  but uses way less memory  particularly for large ranges

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For those who are interested in the  efficiency  of the options collected so far     Jaime RGP s answer led me to restart my computer after timing the somewhat  challenging  solution of Jason literally following my own suggestion  via comment   To spare the curious of you the downtime  I present here my results  worst-first    Jason s answer  maybe just an excursion into the power of list comprehension      python -m timeit   9-i for i in range 10    1000000 loops  best of 3  1 54 usec per loop   martineau s answer  readable if you are familiar with the extended slices syntax      python -m timeit  range 10    -1   1000000 loops  best of 3  0 743 usec per loop   Michal   rajer s answer  the accepted one  very readable      python -m timeit  reversed range 10    1000000 loops  best of 3  0 538 usec per loop   bene s answer  the very first  but very sketchy at that time      python -m timeit  range 9 -1 -1   1000000 loops  best of 3  0 401 usec per loop   The last option is easy to remember using the range n-1 -1 -1  notation by Val Neekman

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Readibility aside  reversed range n   seems to be faster than range n    -1      python -m timeit  reversed range 1000000000    1000000 loops  best of 3  0 598 usec per loop   python -m timeit  range 1000000000    -1   1000000 loops  best of 3  0 945 usec per loop   Just if anyone was wondering

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range 9 -1 -1       9  8  7  6  5  4  3  2  1  0    Is the correct form  If you use  reversed range 10     you wont get a 0 case  For instance  say your 10 isn t a magic number and a variable you re using to lookup start from reverse  If your n case is 0  reversed range 0   will not execute which is wrong if you by chance have a single object in the zero index

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No sense to use reverse because the range method can return reversed list   When you have iteration over n items and want to replace order of list returned by range start  stop  step  you have to use third parameter of range which identifies step and set it to -1  other parameters shall be adjusted accordingly    Provide stop parameter as -1 it s previous value of stop - 1  stop was equal to 0   As start parameter use n-1    So equivalent of range n  in reverse order would be   n   10 print range n-1 -1 -1     9  8  7  6  5  4  3  2  1  0

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range 9 -1 -1   9  8  7  6  5  4  3  2  1  0

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Very often asked question is whether range 9  -1  -1  better than reversed range 10   in Python 3  People who have worked in other languages with iterators immediately tend to think that reversed   must cache all values and then return in reverse order  Thing is that Python s reversed   operator doesn t work if the object is just an iterator  The object must have one of below two for reversed   to work    Either support len   and integer indexes via    Or have   reversed     method implemented    If you try to use reversed   on object that has none of above then you will get    gt  gt  gt   reversed  x for x in range 10     TypeError   generator  object is not reversible   So in short  Python s reversed   is only meant on array like objects and so it should have same performance as forward iteration   But what about range    Isn t that a generator  In Python 3 it is generator but wrapped in a class that implements both of above  So range 100000  doesn t take up lot of memory but it still supports efficient indexing and reversing   So in summary  you can use reversed range 10   without any hit on performance

User · Answer

for i in range 8  0  -1    will solve this problem  It will output 8 to 1  and -1 means a reversed list

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The requirement in this question calls for a list of integers of size 10 in descending order  So  let s produce a list in python     This meets the requirement    But it is a bit harder to wrap one s head around this  right   gt  gt  gt  range 10-1  -1  -1   9  8  7  6  5  4  3  2  1  0     let s find something that is a bit more self-explanatory  Sounds good    ----------------------------------------------------    This returns a list in ascending order    Opposite of what the requirement called for   gt  gt  gt  range 10   0  1  2  3  4  5  6  7  8  9     This returns an iterator in descending order    Doesn t meet the requirement as it is not a list   gt  gt  gt  reversed range 10    lt listreverseiterator object at 0x10e14e090 gt     This returns a list in descending order and meets the requirement  gt  gt  gt  list reversed range 10     9  8  7  6  5  4  3  2  1  0

User · Answer

Using without    -1  or reversed -  def reverse text       result          for index in range len text -1 -1 -1           c   text index          result append c      return    join result   print reverse  python

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You don t necessarily need to use the range function  you can simply do list   -1  which should return the list in reversed order swiftly  without using any additions

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I thought that many  as myself  could be more interested in a common case of traversing an existing list in reversed order instead  as it s stated in the title  rather than just generating indices for such traversal  Even though  all the right answers are still perfectly fine for this case  I want to point out that the performance comparison done in Wolf s answer is for generating indices only  So I ve made similar benchmark for traversing an existing list in reversed order  TL DR a   -1  is the fastest  NB  If you want more detailed analysis of different reversal alternatives and their performance  check out this great answer  Prerequisites  a   list range 10    Jason s answer   timeit  a 9-i  for i in range 10   1 27   s    61 5 ns per loop  mean    std  dev  of 7 runs  1000000 loops each   martineau s answer   timeit a   -1  135 ns    4 07 ns per loop  mean    std  dev  of 7 runs  10000000 loops each   Michal   rajer s answer   timeit list reversed a   374 ns    9 87 ns per loop  mean    std  dev  of 7 runs  1000000 loops each   bene s answer   timeit  a i  for i in range 9  -1  -1   1 09   s    11 1 ns per loop  mean    std  dev  of 7 runs  1000000 loops each   As you see  in this case there s no need to explicitly generate indices  so the fastest method is the one that makes less extra actions  NB  I tested in JupyterLab which has handy  quot magic command quot   timeit  It uses standard timeit timeit under the hood  Tested for Python 3 7 3

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Suppose you have a list call it  a  1 2 3 4 5  Now if you want to print the list in reverse then simply use the following code   a reverse for i in a     print i    I know you asked using range but its already answered

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9-i for i in range 10    9  8  7  6  5  4  3  2  1  0

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because range n  produces an iterable there are all sorts of nice things you can do which will produce the result you desire  such as  range n    -1   if loops are ok  we can make sort of a queue  a      for i in range n       a insert 0 a  return a  or maybe use the reverse   method on it  reverse range n

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You could use range 10    -1  which is the same thing as range 9  -1  -1  and arguably more readable  if you re familiar with the common sequence   -1  Python idiom

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You can do printing of reverse numbers with range   BIF Like     for number in range   10   0   -1         print   number      Output will be   10 9 8 7 6 5 4 3 2 1    range   - range   start   end   increment decrement    where start is inclusive   end is exclusive and increment can be any numbers and behaves like step

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i believe this can help   range 5    -1    below is Usage   for i in range 5    -1       print i

User · Answer

use reversed   function   reversed range 10     It s much more meaningful    Update   If you want it to be a list  as btk pointed out    list reversed range 10      Update   If you want to use only range to achieve the same result  you can use all its parameters  range start  stop  step   For example  to generate a list  5 4 3 2 1 0   you can use the following   range 5  -1  -1    It may be less intuitive but as the comments mention  this is more efficient and the right usage of range for reversed list

[python] Print a list in reverse order with range()?

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