Getting next element while cycling through a list

Question

li    0  1  2  3   running   True while running      for elem in li          thiselem   elem         nextelem   li li index elem  1    When this reaches the last element  an IndexError is raised  as is the case for any list  tuple  dictionary  or string that is iterated   I actually want at that point for nextelem to equal li 0   My rather cumbersome solution to this was  while running      for elem in li          thiselem   elem         nextelem   li li index elem -len li  1      negative index   Is there a better way of doing this

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li    0  1  2  3          for elem in li              if  li index elem   1    len li                   thiselem   elem                 nextelem   li li index elem  1                  print  thiselem  thiselem                 print  nextel  nextelem             else                  print  thiselem  li li index elem                   print  nextel  li li index elem

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I ve used enumeration to handle this problem  storage      for num  value in enumerate result  start 0       content   value     if  A     content          storage   result num   1   I ve used num as Index here  when it finds the correct value it adds up one to the current index of actual list  Which allows me to maneuver to the next index  I hope this helps your purpose

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while running      lenli   len li      for i  elem in enumerate li           thiselem   elem         nextelem   li  i 1  lenli

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while running      for elem next elem in zip li  li 1    li 0

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while running          lenli   len li          for i  elem in enumerate li               thiselem   elem             nextelem   li  i 1  lenli    This line is vital

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The simple solution is to remove IndexError by incorporating the condition  if index lt  len li -1    The error  index out of range  will not occur now as the last index will not be reached  The idea is to access the next element while iterating  On reaching the penultimate element  you can access the last element  Use enumerate method to add index or counter to an iterable list  tuple  etc    Now using the index 1  we can access the next element while iterating through the list  li    0  1  2  3   running   True while running      for index  elem in enumerate li           if index lt  len li -1                thiselem   elem             nextelem   li index 1

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After thinking this through carefully  I think this is the best way   It lets you step off in the middle easily without using break  which I think is important  and it requires minimal computation  so I think it s the fastest   It also doesn t require that li be a list or tuple   It could be any iterator   from itertools import cycle  li    0  1  2  3   running   True licycle   cycle li    Prime the pump nextelem   next licycle  while running      thiselem  nextelem   nextelem  next licycle      I m leaving the other solutions here for posterity   All of that fancy iterator stuff has its place  but not here   Use the   operator   li    0  1  2  3   running   True while running      for idx  elem in enumerate li           thiselem   elem         nextelem   li  idx   1    len li     Now  if you intend to infinitely cycle through a list  then just do this   li    0  1  2  3   running   True idx   0 while running      thiselem   li idx      idx    idx   1    len li      nextelem   li idx    I think that s easier to understand than the other solution involving tee  and probably faster too   If you re sure the list won t change size  you can squirrel away a copy of len li  and use that   This also lets you easily step off the ferris wheel in the middle instead of having to wait for the bucket to come down to the bottom again   The other solutions  including yours  require you check running in the middle of the for loop and then break

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For strings list from 1 or whatever  gt  0  until end  itens     car    house    moon    sun    v   0 for item in itens      b   itens 1   v      print b      print  any other command       if b    itens -1           print  End           break     v    1

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As simple as this  li    0  1  2  3  while 1     for i  item in enumerate x         k   i   1 if i    len x  - 1 else 0       print  Current index          format i li k

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You can use a pairwise cyclic iterator   from itertools import izip  cycle  tee  def pairwise seq       a  b   tee seq      next b      return izip a  b   for elem  next elem in pairwise cycle li

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A rather different way to solve this            li    0 1 2 3      for i in range len li            if i  lt  len li -1                until end is reached            print  this   li i             print  next   li i 1          else                end            print  this   li i

User · Answer

Use the zip method in Python  This function returns a list of tuples  where the i-th tuple contains the i-th element from each of the argument sequences or iterables      while running          for thiselem nextelem in zip li  li 1       li    1                 Do whatever you want with thiselem and nextelem

[python] Getting next element while cycling through a list

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