Python list sublist selection -1 weirdness

Question

So I ve been playing around with python and noticed something that seems a bit odd  The semantics of -1 in selecting from a list don t seem to be consistent   So I have a list of numbers  ls   range 1000    The last element of the list if of course ls -1  but if I take a sublist of that so that I get everything from say the midpoint to the end I would do  ls 500 -1    but this does not give me a list containing the last element in the list  but instead a list containing everything UP TO the last element  However if I do  ls 0 10    I get a list containing also the tenth element  so the selector ought to be inclusive   why then does it not work for -1   I can of course do ls 500   or ls 500 len ls    which would be silly   I was just wondering what the deal with -1 was  I realise that I don t need it there

User · Accepted Answer

In list first last   last is not included     The 10th element is ls 9   in ls 0 10  there isn t ls 10

User · Answer

-1 isn t special in the sense that the sequence is read backwards  it rather wraps around the ends  Such that minus one means zero minus one  exclusive  and  for a positive step value  the sequence is read  from left to right    so for i    1  2  3  4   i 2 -1  means from item two to the beginning minus one  or   around to the end    which results in  3   The -1th element  or element 0 backwards 1 is the last 4  but since it s exclusive  we get 3   I hope this is somewhat understandable

User · Answer

I get consistent behaviour for both instances    gt  gt  gt  ls 0 10   0  1  2  3  4  5  6  7  8  9   gt  gt  gt  ls 10 -1   10  11  12  13  14  15  16  17  18    Note  though  that tenth element of the list is at index 9  since the list is 0-indexed  That might be where your hang-up is   In other words   0 10  doesn t go from index 0-10  it effectively goes from 0 to the tenth element  which gets you indexes 0-9  since the 10 is not inclusive at the end of the slice

User · Answer

It seems pretty consistent to me  positive indices are also non-inclusive  I think you re doing it wrong  Remembering that range   is also non-inclusive  and that Python arrays are 0-indexed  here s a sample python session to illustrate    gt  gt  gt  d   range 10   gt  gt  gt  d  0  1  2  3  4  5  6  7  8  9   gt  gt  gt  d 9  9  gt  gt  gt  d -1  9  gt  gt  gt  d 0 9   0  1  2  3  4  5  6  7  8   gt  gt  gt  d 0 -1   0  1  2  3  4  5  6  7  8   gt  gt  gt  len d  10

User · Answer

If you want to get a sub list including the last element  you leave blank after colon    gt  gt  gt  ll range 10   gt  gt  gt  ll  0  1  2  3  4  5  6  7  8  9   gt  gt  gt  ll 5    5  6  7  8  9   gt  gt  gt  ll     0  1  2  3  4  5  6  7  8  9

User · Answer

when slicing an array    ls y x      takes the slice from element y upto and but not including x  when you use the negative indexing it is equivalent to using   ls y -1     ls y len ls -1    so it so the slice would be upto the last element  but it wouldn t include it  as per the slice

[python] Python list / sublist selection -1 weirdness

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