New submission from airwin: Note 5 (at <https://docs.python.org/2/library/stdtypes.html#sequence-types-str-unicode-list-tuple-bytearray-buffer-xrange> for Python 2 and at <https://docs.python.org/3/library/stdtypes.html#common-sequence-operations> for Python 3) concerning s[i:j:k] (the slice of s from i to j with step k) contains the following two sentences:
"The slice of s from i to j with step k is defined as the sequence of items with index x = i + n*k such that 0 <= n < (j-i)/k. In other words, the indices are i, i+k, i+2*k, i+3*k and so on, stopping when j is reached (but never including j)." That limit, "(j-i)/k" is demonstrably wrong when the integer division has a non-zero remainder. For example, for i=0, j=3, and k=2, n must be less than int(3/2) = 1 (i.e., the slice only has one element) according to that limit, but in fact the slice has two elements in agreement with the second sentence and which can be seen from the following python (2 or 3) code example: >>> x = [0,1,2,3,4,5] >>> x[0:3:2] [0, 2] The following Python result is also instructive for negative steps. >>> y=[5,4,3,2,1,0] >>> y[-1:-4:-2] [0, 2] For this case as well the result is consistent with the second sentence of the documentation but inconsistent with the first sentence since according to that limit = int((len-4 - (len-1))/-2) = int(3/2) = 1 implying only one element in the slice in contradiction to the above result. Therefore, I suggest removing the incorrect mathematical limit "(j-i)/k" from note 5 by replacing the above two sentences as follows: "The slice of s from i to j with positive or negative step k is defined as the sequence of items with index x = i + n*k such that the indices are i, i+k, i+2*k, i+3*k and so on, stopping when j is reached (but never including j)." Alternatively, one could replace the current incorrect limit by the correct mathmatical expression. My guess is the limit should be "(j-i)/k" when there is a zero remainder for that division, and "(j-i)/k + 1" when there is a non-zero remainder. That limit works for the above two examples, but I don't know how to prove that in general for this integer limit case. Furthermore, even if somebody else can provide that proof, I still think the above single sentence documents the slice limits exactly, is simpler, and therefore is preferred. N.B. I am formally reporting this issue as a Python 3 bug but as shown above the same two sentences occur in the Python 2 documentation so when this bug is fixed for the Python 3 documentation it should also be consistently fixed for Python 2. ---------- assignee: docs@python components: Documentation messages: 280068 nosy: airwin, docs@python priority: normal severity: normal status: open title: Slice limit documentation is incorrect versions: Python 3.7 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue28614> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
