Loop backwards using indices in Python

Question

I am trying to loop from 100 to 0  How do I do this in Python   for i in range  100 0  doesn t work

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a   10 for i in sorted range a   reverse True       print i

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I tried this in one of the codeacademy exercises  reversing chars in a string without using reversed nor    -1   def reverse text       chars         l   len text      last   l-1     for i in range  l           chars append text last           last- 1      result            for c in chars          result    c     return result print reverse  hola

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You can also create a custom reverse mechanism in python  Which can be use anywhere for looping an iterable backwards  class Reverse         Iterator for looping over a sequence backwards        def   init   self  seq           self seq   seq         self index   len seq       def   iter   self           return self      def   next   self           if self index    0              raise StopIteration         self index -  1         return self seq self index     gt  gt  gt  d    1 2 3 4 5   gt  gt  gt  for i in Reverse d         print i       5 4 3 2 1

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for var in range 10 -1 -1  works

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You can always do increasing range and subtract from a variable in your case 100 - i where i in range  0  101         for i in range  0  101        print 100 - i

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Short and sweet  This was my solution when doing codeAcademy course  Prints a string in rev order    def reverse text       string          for i in range len text -1 -1 -1           string    text i      return string

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for i in range 100  -1  -1    and some slightly longer  and slower  solution   for i in reversed range 101    for i in range 101    -1

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Another solution   z   10 for x in range  z      y   z-x    print y   Result   10 9 8 7 6 5 4 3 2 1   Tip  If you are using this method to count back indices in a list  you will want to -1 from the  y  value  as your list indices will begin at 0

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I wanted to loop through a two lists backwards at the same time so I needed the negative index  This is my solution   a   1 3 4 5 2  for i in range -1  -len a   -1       print i  a i     Result   -1 2 -2 5 -3 4 -4 3 -5 1

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The simple answer to solve your problem could be like this   for i in range 100       k   100 - i     print k

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Oh okay read the question wrong  I guess it s about going backward in an array  if so  I have this   array     ty    rogers    smith    davis    tony    jack    john    jill    harry    tom    jane    hilary    jackson    andrew    george    rachel     counter   0     for loop in range len array        if loop  lt   len array           counter   -1         reverseEngineering   loop   counter         print array reverseEngineering

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You might want to use the reversed function in python  Before we jump in to the code we must remember that the range function always returns a list  or a tuple I don t know  so range 5  will return  0  1  2  3  4   The reversed function reverses a list or a tuple so reversed range 5   will be  4  3  2  1  0  so your solution might be  for i in reversed range 100        print i

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Generally in Python  you can use negative indices to start from the back   numbers    10  20  30  40  50  for i in xrange len numbers        print numbers -i - 1    Result   50 40 30 20 10

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In my opinion  this is the most readable   for i in reversed xrange 101        print i

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Why your code didn t work  You code for i in range  100  0  is fine  except  the third parameter  step  is by default  1  So you have to specify 3rd parameter to range   as -1 to step backwards   for i in range 100  -1  -1       print i    NOTE  This includes 100  amp  0 in the output   There are multiple ways   Better Way  For pythonic way  check PEP 0322   This is Python3 pythonic example to print from 100 to 0  including 100  amp  0    for i in reversed range 101        print i

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Try range 100 -1 -1   the 3rd argument being the increment to use  documented here        range  options  start  stop  step are documented here

[python] Loop backwards using indices in Python?

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