NumPy first and last element from array

Question

I am trying to dynamically get the first and last element from an array    So  let us suppose the array has 6 elements    test    1 23 4 6 7 8    If I am trying to get the first and last   1 8  23 7 and 4 6  Is there a way to get elements in this order   I looked at a couple of questions Link Link2  I took help of these links and I came up with this prototype       usr bin env python  import numpy  test    1 23 4 6 7 8  test1   numpy array  1 23 4 6 7 8   len test   len test  first list    0 1 2  len first   len first list  second list    -1 -2 -3  len second   len second list   for a in range len first           print numpy array test   first list a    second list a            print test1  first list a   second list a      But this prototype won t scale for if you have more than 6 elements  So  I was wondering if there is way to dynamically get the pair of elements   Thanks

User · Answer

You can simply use take method and index of element  Last index can be -1    arr   np array  1 2 3    last   arr take -1    3

User · Answer

This does it  Note that with an odd number of elements the one in the middle won t be included   test    1  23  4  6  7  8  5      for i in range len test  2       print  test i   test -1-i     Output    1  5   23  8   4  7

User · Answer

How about   In  10   arr   numpy array  1 23 4 6 7 8    In  11     arr i   arr -i-1   for i in range len arr     2   Out 11     1  8    23  7    4  6     Depending on the size of arr  writing the entire thing in NumPy may be more performant   In  41   arr   numpy array  1 23 4 6 7 8  100   In  42    timeit   arr i   arr -i-1   for i in range len arr     2   10000 loops  best of 3  167 us per loop  In  43    timeit numpy vstack  arr  arr   -1       len arr   2  100000 loops  best of 3  16 4 us per loop

User · Answer

I ended here  because I googled for  python first and last element of array   and found everything else but this  So here s the answer to the title question   a    1 2 3  a 0    first element  returns 1  a -1    last element  returns 3

User · Answer

How about this    gt  gt  gt  import numpy  gt  gt  gt  test1   numpy array  1 23 4 6 7 8    gt  gt  gt  forward   iter test1   gt  gt  gt  backward   reversed test1   gt  gt  gt  for a in range  len test1  1   2           print forward next    backward next        1 8 23 7 4 6   The  len test1  1   2 ensures that the middle element of odd length arrays is also returned    gt  gt  gt  test1   numpy array  1 23 4 9 6 7 8     additional element  9  in the middle  gt  gt  gt  forward   iter test1                                                         gt  gt  gt  backward   reversed test1   gt  gt  gt  for a in range  len test1  1   2           print forward next    backward next   1 8 23 7 4 6 9 9   Using just len test1   2 will drop the middle elemen of odd length arrays

User · Answer

Assuming the list has a even number of elements  you could do   test    1 23 4 6 7 8  test rest   reversed test  len test  2    for n in len test rest       print  test n   test test n

User · Answer

arr   np array  1 2 3 4   arr -1    last element

User · Answer

gt  gt  gt  test    1 23 4 6 7 8   gt  gt  gt  from itertools import izip longest  gt  gt  gt  for e in izip longest test  reversed test        print e    1  8   23  7   4  6   6  4   7  23   8  1    Another option   gt  gt  gt  test    1 23 4 6 7 8   gt  gt  gt  start  end   iter test   reversed test   gt  gt  gt  try      while True          print map next   start  end   except StopIteration      pass   1  8   23  7   4  6   6  4   7  23   8  1

User · Answer

Using Numpy s fancy indexing    gt  gt  gt  test array   1  23   4   6   7   8     gt  gt  gt  test   -1     test  reversed array   8   7   6   4  23   1     gt  gt  gt  numpy vstack  test  test   -1       stack test and its reverse array    1  23   4   6   7   8            8   7   6   4  23   1      gt  gt  gt    transpose  then take the first half   gt  gt  gt     1 to cater to odd-length arrays  gt  gt  gt  numpy vstack  test  test   -1    T   len test    1     2  array    1   8           23   7            4   6      vstack copies the array  but all the other operations are constant-time pointer tricks  including reversal  and hence are very fast

[python] NumPy first and last element from array

Examples related to python

Examples related to arrays

Examples related to numpy