Finding first and last index of some value in a list in Python

Question

Is there any built-in methods that are part of lists that would give me the first and last index of some value  like   verts IndexOf 12 345  verts LastIndexOf 12 345

User · Accepted Answer

Sequences have a method index value  which returns index of first occurrence - in your case this would be verts index value     You can run it on verts   -1  to find out the last index  Here  this would be len verts  - 1 - verts   -1  index value

User · Answer

s index x   i   j      index of the first occurrence of x in s  at or after index i and before index j

User · Answer

Python lists have the index   method  which you can use to find the position of the first occurrence of an item in a list  Note that list index   raises ValueError when the item is not present in the list  so you may need to wrap it in try except  try      idx   lst index value  except ValueError      idx   None  To find the position of the last occurrence of an item in a list efficiently  i e  without creating a reversed intermediate list  you can use this function  def rindex lst  value       for i  v in enumerate reversed lst            if v    value              return len lst  - i - 1    return the index in the original list     return None      print rindex  1  2  3   3         2 print rindex  3  2  1  3   3      3 print rindex  3  2  1  3   4      None

User · Answer

Perhaps the two most efficient ways to find the last index  def rindex lst  value       lst reverse       i   lst index value      lst reverse       return len lst  - i - 1  def rindex lst  value       return len lst  - operator indexOf reversed lst   value  - 1  Both take only O 1  extra space and the two in-place reversals of the first solution are much faster than creating a reverse copy  Let s compare it with the other solutions posted previously  def rindex lst  value       return len lst  - lst   -1  index value  - 1  def rindex lst  value       return len lst  - next i for i  val in enumerate reversed lst   if val    value  - 1  Benchmark results  my solutions are the red and green ones   This is for searching a number in a list of a million numbers  The x-axis is for the location of the searched element  0  means it s at the start of the list  100  means it s at the end of the list  All solutions are fastest at location 100   with the two reversed solutions taking pretty much no time for that  the double-reverse solution taking a little time  and the reverse-copy taking a lot of time  A closer look at the right end   At location 100   the reverse-copy solution and the double-reverse solution spend all their time on the reversals  index   is instant   so we see that the two in-place reversals are about seven times as fast as creating the reverse copy  The above was with lst   list range 1 000 000  2 000 001    which pretty much creates the int objects sequentially in memory  which is extremely cache-friendly  Let s do it again after shuffling the list with random shuffle lst   probably less realistic  but interesting     All got a lot slower  as expected  The reverse-copy solution suffers the most  at 100  it now takes about 32 times     as long as the double-reverse solution  And the enumerate-solution is now second-fastest only after location 98   Overall I like the operator indexOf solution best  as it s the fastest one for the last half or quarter of all locations  which are perhaps the more interesting locations if you re actually doing rindex for something  And it s only a bit slower than the double-reverse solution in earlier locations  All benchmarks done with CPython 3 9 0 64-bit on Windows 10 Pro 1903 64-bit

User · Answer

If you are searching for the index of the last occurrence of myvalue in mylist   len mylist  - mylist   -1  index myvalue  - 1

User · Answer

As a small helper function   def rindex mylist  myvalue       return len mylist  - mylist   -1  index myvalue  - 1

User · Answer

This method can be more optimized than above  def rindex iterable  value       try          return len iterable  - next i for i  val in enumerate reversed iterable   if val    value  - 1     except StopIteration          raise ValueError

[python] Finding first and last index of some value in a list in Python

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