Split list into smaller lists split in half

Question

I am looking for a way to easily split a python list in half   So that if I have an array   A    0 1 2 3 4 5    I would be able to get   B    0 1 2   C    3 4 5

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A little more generic solution  you can specify the number of parts you want  not just split  in half     EDIT  updated post to handle odd list lengths  EDIT2  update post again based on Brians informative comments   def split list alist  wanted parts 1       length   len alist      return   alist i length    wanted parts   i 1  length    wanted parts                for i in range wanted parts     A    0 1 2 3 4 5 6 7 8 9   print split list A  wanted parts 1  print split list A  wanted parts 2  print split list A  wanted parts 8

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If you have a big list  It s better to use itertools and write a function to yield each part as needed   from itertools import islice  def make chunks data  SIZE       it   iter data        use  xragne  if you are in python 2 7      for i in range 0  len data   SIZE           yield  k for k in islice it  SIZE     You can use this like   A    0  1  2  3  4  5  6   size   len A     2  for sample in make chunks A  size       print sample    The output is    0  1  2   3  4  5   6    Thanks to  thefourtheye and  Bede Constantinides

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B C A  len A  2  A len A  2

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While the answers above are more or less correct  you may run into trouble if the size of your array isn t divisible by 2  as the result of a   2  a being odd  is a float in python 3 0  and in earlier version if you specify from   future   import division at the beginning of your script  You are in any case better off going for integer division  i e  a    2  in order to get  forward  compatibility of your code

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With hints from  ChristopheD  def line split N  K 1       length   len N      return  N i length K  i 1  length K  for i in range K    A    0 1 2 3 4 5 6 7 8 9  print line split A 1  print line split A 2

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If you don t care about the order       def split list         return list   2   list 1  2    list   2  gets every second element in the list starting from the 0th element  list 1  2  gets every second element in the list starting from the 1st element

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def splitter A       B   A 0 len A   2      C   A len A   2     return  B C    I tested  and the double slash is required to force int division in python 3  My original post was correct  although wysiwyg broke in Opera  for some reason

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for python 3     A    0 1 2 3 4 5      l   len A  2     B   A  int l       C   A int l

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10 years later   I thought - why not add another   arr    Some random string    10  n   4 print  arr e e n  for e in range 0 len arr  n

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f   lambda A  n 3   A i i n  for i in range 0  len A   n   f A    n - the predefined length of result arrays

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def split arr  size        arrs           while len arr   gt  size           pice   arr  size           arrs append pice           arr     arr size        arrs append arr       return arrs   Test   x  1  2  3  4  5  6  7  8  9  10  11  12  13  print split x  5     result     1  2  3  4  5    6  7  8  9  10    11  12  13

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A    1 2 3 4 5 6  B   A  len A   2  C   A len A   2     If you want a function   def split list a list       half   len a list   2     return a list  half   a list half    A    1 2 3 4 5 6  B  C   split list A

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Here is a common solution  split arr into count part  def split arr  count        return  arr i  count  for i in range count

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This is similar to other solutions  but a little faster     Usage  split half  1 2 3 4 5   Result    1  2    3  4  5    def split half a       half   len a   gt  gt  1     return a  half   a half

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Using list slicing  The syntax is basically my list start index end index    gt  gt  gt  i    0 1 2 3 4 5   gt  gt  gt  i  3    same as i 0 3  - grabs from first to third index  0- gt 2   0  1  2   gt  gt  gt  i 3     same as i 3 len i   - grabs from fourth index to end  3  4  5    To get the first half of the list  you slice from the first index to len i   2  where    is the integer division - so 3  2 will give the floored result of1  instead of the invalid list index of1 5      gt  gt  gt  i  len i   2   0  1  2      and the swap the values around to get the second half    gt  gt  gt  i len i   2    3  4  5

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Another take on this problem in 2020     Here s a generalization of the problem  I interpret the  divide a list in half  to be     i e  two lists only and there shall be no spillover to a third array in case of an odd one out etc   For instance  if the array length is 19 and a division by two using    operator gives 9  and we will end up having two arrays of length 9 and one array  third  of length 1  so in total three arrays   If we d want a general solution to give two arrays all the time  I will assume that we are happy with resulting duo arrays that are not equal in length  one will be longer than the other   And that its assumed to be ok to have the order mixed  alternating in this case         arrayinput -- gt  is an array of length N that you wish to split 2 times     ctr   1   lets initialize a counter  holder 1      holder 2       for i in range len arrayinput          if ctr    1           holder 1 append arrayinput i       elif ctr    2           holder 2 append arrayinput i        ctr    1       if ctr  gt  2     if it exceeds 2 then we reset          ctr   1    This concept works for any amount of list partition as you d like  you d have to tweak the code depending on how many list parts you want   And is rather straightforward to interpret  To speed things up   you can even write this loop in cython   C   C   to speed things up  Then again  I ve tried this code on relatively small lists   10 000 rows and it finishes in a fraction of second    Just my two cents    Thanks

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There is an official Python receipe for the more generalized case of splitting an array into smaller arrays of size n   from itertools import izip longest def grouper n  iterable  fillvalue None        Collect data into fixed-length chunks or blocks        grouper 3   ABCDEFG    x   -- gt  ABC DEF Gxx     args    iter iterable     n     return izip longest fillvalue fillvalue   args    This code snippet is from the python itertools doc page

[python] Split list into smaller lists (split in half)

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