dracula571 wrote: > s[i:j] slice of s from i to j (3), (4) > > (3) > If i or j is negative, the index is relative to the end of the string: > len(s) + i or len(s) + j is substituted. But note that -0 is still 0. > > > (4) > The slice of s from i to j is defined as the sequence of items with > index k such that i <= k < j. If i or j is greater than len(s), use > len(s). If i is omitted or None, use 0. If j is omitted or None, use > len(s). If i is greater than or equal to j, the slice is empty. > > i can't follow (3) very well. > for example:k = [1,2,3,4,5] > k[-1:2]=[] > k[-5:2]=[1,2] > but k[-6:2] = [1,2] > why k[-6:2] is [1,2]not [].
Because 6 is greater then len(k), so when you ask for i=-6, you get i=-5 instead. And relative -5 is actually absolute 0 which is less than 2, so you do not get an empty slice. > i do follow (3),to make i positive by > plusing len(k) twice.But the result is not what i expect.why,or did i > misunderstand the rule of (3). See (4). -- http://mail.python.org/mailman/listinfo/python-list