Re: Testing if an index is in a slice
Steven D'Aprano wrote: Here's a less verbose version which passes your test cases: def inslice(index, slc, len): """Return True if index would be part of slice slc of a sequence of length len, otherwise return False. """ start, stop, stride = slc.indices(len) if stride < 0: return (start >= index > stop) and ((start-index) % -stride == 0) else: return (start <= index < stop) and ((index-start) % stride == 0) (Hint: help(slice) is your friend.) I should really think about abandoning my strategy of doing everything the hard way. -- --Bryan -- http://mail.python.org/mailman/listinfo/python-list
Re: Testing if an index is in a slice
On Sat, 03 Jan 2009 14:53:12 -0800, Bryan Olson wrote about testing whether or not an index is in a slice: > I'm leaning towards mmanns' idea that this should be built in. What's the use-case for it? > Handling > all the cases is remarkably tricky. Here's a verbose version, with a > little test: [snip code] Here's a less verbose version which passes your test cases: def inslice(index, slc, len): """Return True if index would be part of slice slc of a sequence of length len, otherwise return False. """ start, stop, stride = slc.indices(len) if stride < 0: return (start >= index > stop) and ((start-index) % -stride == 0) else: return (start <= index < stop) and ((index-start) % stride == 0) (Hint: help(slice) is your friend.) -- Steven -- http://mail.python.org/mailman/listinfo/python-list
Re: Testing if an index is in a slice
On Jan 3, 8:53 pm, Bryan Olson wrote: > If we add a parameter for the length of the list to which the slice is > applied, then inslice() is well-defined. Cool! > I thought it would easy to write, Heh, I gave up on the example I mentioned above :) > but that was hours ago when I knew > less about Python slice indexing than I do now. And I thought I new a > bunch from writing a virtual slice class. Maybe you could write about what you learned in the docs? > I'm leaning towards mmanns' idea that this should be built in. Handling > all the cases is remarkably tricky. Even without testing your code, I agree. I suggest that you add this as a feature request to the tracker at http://bugs.python.org/. I hope Raymond Hettinger likes it and adds an islice version :) -- http://mail.python.org/mailman/listinfo/python-list
Re: Testing if an index is in a slice
ajaksu wrote: On Jan 1, 4:12 pm, mma...@gmx.net wrote: I would like to check if an index is in a slice or not without iterating over the slice. Something like: isinslice(36, slice(None, 34, -1)) True I think it'd be feasible for slices that can be mapped to ranges[1], but slices are more flexible than that. If we add a parameter for the length of the list to which the slice is applied, then inslice() is well-defined. I thought it would easy to write, but that was hours ago when I knew less about Python slice indexing than I do now. And I thought I new a bunch from writing a virtual slice class. I'm leaning towards mmanns' idea that this should be built in. Handling all the cases is remarkably tricky. Here's a verbose version, with a little test: def inslice(i, slc, length): """ Would index i be part of slice slc of a list of len length? """ step = slc.step or 1 if step > 0: if slc.start is None: start = 0 elif slc.start >= 0: start = slc.start else: start = max(0, length + slc.start) if slc.stop is None: stop = length elif slc.stop >= 0: stop = min(length, slc.stop) else: stop = length + slc.stop return start <= i < stop and (i - start) % step == 0 else: if slc.start is None: start = length - 1 elif slc.start >= 0: start = min(length - 1, slc.start) else: start = length + slc.start if slc.stop is None: stop = -1 elif slc.stop >= 0: stop = slc.stop else: stop = max(-1, length + slc.stop) return start >= i > stop and (start - i) % (-step) == 0 # test for start in [None, 0, 1, -1, -3, 4]: for stop in [None, 0, 1, -1, 3, -5]: for step in [None, 1, -1, 2, -3]: for n in [0, 1, 2, 3, 5, 11, 16]: slc = slice(start, stop, step) s1 = [i for i in range(-3, n + 5) if inslice(i, slc, n)] s2 = sorted(range(n)[slc]) assert s1 == s2 -- --Bryan -- http://mail.python.org/mailman/listinfo/python-list
Re: Testing if an index is in a slice
On Thu, 1 Jan 2009 11:34:53 -0800 (PST) ajaksu wrote: > http://article.gmane.org/gmane.comp.python.python-3000.devel/8732 I will build upon this code. Thanks for your help Martin -- http://mail.python.org/mailman/listinfo/python-list
Re: Testing if an index is in a slice
On Jan 1, 12:12 pm, mma...@gmx.net wrote: > Hi > > I would like to check if an index is in a slice or not without > iterating over the slice. > > Something like: > > >>> isinslice(36, slice(None, 34, -1)) > > True > > I would like to use the batteries if possible. > However, I looked in the docs, pypi and in Usenet without luck. > > Does anyone have a solution? > > Martin Untested: if slice.end< 0: slice.end= len( obj )- slice.end+ 1 if slice.step< 0 and slice.start<= target< slice.end: return True if slice.step> 0 and slice.start>= target> slice.end: return True return False -- http://mail.python.org/mailman/listinfo/python-list
Re: Testing if an index is in a slice
On Jan 1, 4:12 pm, mma...@gmx.net wrote: > I would like to check if an index is in a slice or not without > iterating over the slice. > > Something like: > > >>> isinslice(36, slice(None, 34, -1)) > > True I think it'd be feasible for slices that can be mapped to ranges[1], but slices are more flexible than that. What would the answer be for: >>> isinslice(36, slice(0, -1, 1)) ? This shows the issue: >>> sli = slice(0, -1, 1) >>> range(4)[sli] [0, 1, 2] >>> range(10)[sli] [0, 1, 2, 3, 4, 5, 6, 7, 8] HTH, Daniel [1] http://article.gmane.org/gmane.comp.python.python-3000.devel/8732 -- http://mail.python.org/mailman/listinfo/python-list
Testing if an index is in a slice
Hi I would like to check if an index is in a slice or not without iterating over the slice. Something like: >>> isinslice(36, slice(None, 34, -1)) True I would like to use the batteries if possible. However, I looked in the docs, pypi and in Usenet without luck. Does anyone have a solution? Martin -- http://mail.python.org/mailman/listinfo/python-list
Testing if an index is in a slice
Hi I would like to check if an index is in a slice or not without iterating over the slice. Something like: >>> isinslice(36, slice(None, 34, -1)) True I would like to use the batteries if possible. However, I looked in the docs, pypi and in Usenet without luck. Does anyone have a solution? Martin -- http://mail.python.org/mailman/listinfo/python-list
