How to find all positions of the maximum value in a list

Question

I have a list   a    32  37  28  30  37  25  27  24  35  55  23  31  55  21  40  18  50               35  41  49  37  19  40  41  31    max element is 55  two elements on position 9 and 12   I need to find on which position s  the maximum value is situated  Please  help

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I can t reproduce the  SilentGhost-beating performance quoted by  martineau  Here s my effort with comparisons       maxelements py      a    32  37  28  30  37  25  27  24  35  55  23  31  55  21  40  18  50               35  41  49  37  19  40  41  31  b   range 10000  c   range 10000 - 1  -1  -1  d   b   c  def maxelements s seq      SilentGhost         Return list of position s  of largest element         m   max seq      return  i for i  j in enumerate seq  if j    m   def maxelements m seq      martineau         Return list of position s  of largest element         max indices          if len seq           max val   seq 0          for i  val in   i  val  for i  val in enumerate seq  if val  gt   max val               if val    max val                  max indices append i              else                  max val   val                 max indices    i      return max indices  def maxelements j seq      John Machin         Return list of position s  of largest element         if not seq  return        max val   seq 0  if seq 0   gt   seq -1  else seq -1      max indices          for i  val in enumerate seq           if val  lt  max val  continue         if val    max val              max indices append i          else              max val   val             max indices    i      return max indices   Results from a beat-up old laptop running Python 2 7 on Windows XP SP3    gt  python27 python -mtimeit -s import maxelements as me   me maxelements s me a   100000 loops  best of 3  6 88 usec per loop   gt  python27 python -mtimeit -s import maxelements as me   me maxelements m me a   100000 loops  best of 3  11 1 usec per loop   gt  python27 python -mtimeit -s import maxelements as me   me maxelements j me a   100000 loops  best of 3  8 51 usec per loop   gt  python27 python -mtimeit -s import maxelements as me a100 me a 100   me maxelements s a100   1000 loops  best of 3  535 usec per loop   gt  python27 python -mtimeit -s import maxelements as me a100 me a 100   me maxelements m a100   1000 loops  best of 3  558 usec per loop   gt  python27 python -mtimeit -s import maxelements as me a100 me a 100   me maxelements j a100   1000 loops  best of 3  489 usec per loop

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This code is not as sophisticated as the answers posted earlier but it will work   m   max a  n   0      frequency of max  a  for number in a       if number    m           n   n   1 ilist    None    n    a list containing index values of maximum number in list a  ilistindex   0 aindex   0    required index value      for number in a       if number    m           ilist ilistindex    aindex         ilistindex   ilistindex   1     aindex   aindex   1  print ilist   ilist in the above code would contain all the positions of the maximum number in the list

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You can do it in various ways   The old conventional way is   maxIndexList   list    this list will store indices of maximum values maximumValue   max a   get maximum value of the list length   len a         calculate length of the array  for i in range length    loop through 0 to length-1  because  0 based indexing      if a i   maximumValue   if any value of list a is equal to maximum value then store its index to maxIndexList         maxIndexList append i   print maxIndexList   finally print the list   Another way without calculating the length of the list and storing maximum value to any variable   maxIndexList   list   index   0  variable to store index for i in a   iterate through the list  actually iterating through the value of list  not index       if i  max a    max a  returns a maximum value of list          maxIndexList append index   store the index of maximum value index   index 1  increment the index  print maxIndexList    We can do it in Pythonic and smart way  Using list comprehension just in one line   maxIndexList    i for i j in enumerate a  if j  max a    here i index and j   value of that index   All my codes are in Python 3

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Here is the max value and the indexes it appears at    gt  gt  gt  from collections import defaultdict  gt  gt  gt  d   defaultdict list   gt  gt  gt  a    32  37  28  30  37  25  27  24  35  55  23  31  55  21  40  18  50  35  41  49  37  19  40  41  31   gt  gt  gt  for i  x in enumerate a           d x  append i        gt  gt  gt  k   max d keys     gt  gt  gt  print k  d k  55  9  12      Later  for the satisfaction of  SilentGhost   gt  gt  gt  from itertools import takewhile  gt  gt  gt  import heapq  gt  gt  gt    gt  gt  gt  def popper heap           while heap              yield heapq heappop heap        gt  gt  gt  a    32  37  28  30  37  25  27  24  35  55  23  31  55  21  40  18  50  35  41  49  37  19  40  41  31   gt  gt  gt  h     -x  i  for i  x in enumerate a    gt  gt  gt  heapq heapify h   gt  gt  gt    gt  gt  gt  largest   heapq heappop h   gt  gt  gt  indexes    largest 1      x 1  for x in takewhile lambda large  large 0     largest 0   popper h     gt  gt  gt  print -largest 0   indexes 55  9  12

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shash answered this elsewhere     A Pythonic way to find the index of the maximum list element would be   position   max enumerate a   key lambda x  x 1   0     Which does one pass  Yet  it is slower than the solution by  Silent Ghost and  even more so   nmichaels   for i in s m j n  do echo  i   python -mtimeit -s import maxelements as me   me maxelements   i  me a    done s 100000 loops  best of 3  3 13 usec per loop m 100000 loops  best of 3  4 99 usec per loop j 100000 loops  best of 3  3 71 usec per loop n 1000000 loops  best of 3  1 31 usec per loop

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import operator  def max positions iterable  key None  reverse False     if key is None      def key x         return x   if reverse      better   operator lt   else      better   operator gt    it   enumerate iterable    for pos  item in it      break   else      raise ValueError  max positions  empty iterable         note this is the same exception type raised by max       cur max   key item    cur pos    pos     for pos  item in it      k   key item      if better k  cur max         cur max   k       cur pos    pos      elif k    cur max        cur pos append pos     return cur max  cur pos  def min positions iterable  key None  reverse False     return max positions iterable  key  not reverse       gt  gt  gt  L   range 10    2  gt  gt  gt  L  0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5  6  7  8  9   gt  gt  gt  max positions L   9   9  19    gt  gt  gt  min positions L   0   0  10    gt  gt  gt  max positions L  key lambda x  x    2  reverse True   0   0  1  10  11

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gt  gt  gt  m   max a   gt  gt  gt   i for i  j in enumerate a  if j    m   9  12

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Just one line   idx   max range len a    key   lambda i  a i

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The chosen answer  and most others  require at least two passes through the list  Here s a one pass solution which might be a better choice for longer lists    Edited  To address the two deficiencies pointed out by  John Machin  For  2  I attempted to optimize the tests based on guesstimated probability of occurrence of each condition and inferences allowed from predecessors  It was a little tricky figuring out the proper initialization values for max val and max indices which worked for all possible cases  especially if the max happened to be the first value in the list     but I believe it now does   def maxelements seq           Return list of position s  of largest element         max indices          if seq          max val   seq 0          for i val in   i val  for i val in enumerate seq  if val  gt   max val               if val    max val                  max indices append i              else                  max val   val                 max indices    i       return max indices

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Similar idea with a list comprehension but without enumerate  m   max a   i for i in range len a   if a i     m

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I came up with the following and it works as you can see with max  min and others functions over lists like these   So  please consider the next example list find out the position of the maximum in the list a    gt  gt  gt  a    3 2 1  4 5    Using the generator enumerate and making a casting   gt  gt  gt  list enumerate a     0  3    1  2    2  1    3  4    4  5     At this point  we can extract the position of max with    gt  gt  gt  max enumerate a   key  lambda x  x 1     4  5    The above tells us  the maximum is in the position 4 and his value is 5     As you see  in the key argument  you can find the maximum over any iterable object by defining a lambda appropriate    I hope that it contributes    PD  As  PaulOyster noted in a comment  With Python 3 x the min and max allow a new keyword default that avoid the raise exception ValueError when argument is empty list  max enumerate list   key  lambda x x 1    default   -1

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You can also use the numpy package   import numpy as np A   np array a  maximum indices   np where A  max a     This will return an numpy array of all the indices that contain the max value  if you want to turn this to a list   maximum indices list   maximum indices tolist

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Also a solution  which gives only the first appearance  can be achieved by using numpy    gt  gt  gt  import numpy as np  gt  gt  gt  a np   np array a   gt  gt  gt  np argmax a np  9

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a index max a     will tell you the index of the first instance of the largest valued element of list a

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If you want to get the indices of the largest n numbers in a list called data  you can use Pandas sort values   pd Series data  sort values ascending False  index 0 n

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gt  gt  gt  max enumerate  1 2 3 32 1 5 7 9   key lambda x  x 1    gt  gt  gt   3  32

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a    32  37  28  30  37  25  27  24  35            55  23  31  55  21  40  18  50           35  41  49  37  19  40  41  31   import pandas as pd  pd Series a  idxmax    9   That is how I usually do it

[python] How to find all positions of the maximum value in a list?

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