John Salerno wrote: > Given: > > numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] > > can someone explain to me why > > numbers[10:0:-2] results in [10, 8, 6, 4, 2]?
I think the documentation is misleading/incomplete when it comes to negative strides for extended slices. The relevent sections are here: http://www.python.org/doc/2.3.5/ref/types.html [quote] Some sequences also support "extended slicing" with a third "step" parameter: a[i:j:k] selects all items of a with index x where x = i + n*k, n >= 0 and i <= x < j. [end quote] and from http://www.python.org/doc/2.3.5/ref/slicings.html [quote] It is not an error if i or j lie outside the range of valid indexes (such items don't exist so they aren't selected). [end quote] The documentation suggests that, given a slice [10:0:-2], Python looks up indices: 10 + 0*-2, 10 + 1*-2, 10 + 2*-2, ... or 10, 8, 6, 4, 2, 0, ... but since *none* of these indices is within the limits 10 <= x and x < 0, the documentation suggests that the result should be the empty list. The indices actually selected are: 9, 7, 5, 3, 1 at which point I give up and throw my hands in the air and promise never to use negative strides with extended slices. -- Steven. -- http://mail.python.org/mailman/listinfo/python-list
