Gah, hit the reply button instead of reply-all. :-(
--
I enjoy haiku
but sometimes they don't make sense;
refrigerator?
---------- Forwarded message ----------
From: Matthew Wood <woodm1...@gmail.com>
Date: Wed, Jun 9, 2010 at 2:30 PM
Subject: Re: [Tutor] treeTraversal, nested return?
To: jjcr...@uw.edu
Well, I've taken a look at your code. In your last example, the appending
last line here:
> else:
> for x in getWorks(id):
> result.extend(makeNestedLists(x[1]))
uses, a list.extend call. If you switch that to append, you SHOULD see the
result you're looking for. ...at least as best I can guess without
replicating the code and having some good test cases.
Here's some of your other questions:
>To my surprise, I say, because it looks like it shouldn't work at all
>with the result being set to the empty list with each recursion;
Each time you call the function 'makeNestedLists' a new copy of all the
local variables is created. Thus, you aren't blanking the 'result' list
each time; instead you're creating a new list each time. Then, when you
return the list, you then shove it into the parent-function-instance list.
Just take care to understand the difference between:
[1, 2, 3].append([4, 5, 6]) -> [1, 2, 3, [4, 5, 6]]
[1, 2, 3].extend([4, 5, 6]) -> [1, 2, 3, 4, 5, 6]
Your list comprehension example above adds an extra list wrapper around
things, which is why the extend worked.
--
I enjoy haiku
but sometimes they don't make sense;
refrigerator?
On Wed, Jun 9, 2010 at 12:40 PM, <jjcr...@uw.edu> wrote:
> Thanks to Matthew and Denis for trying to enlighten me. Thanks to your
> clues I've made some progress; however, I can't seem to get the hang of the
> usual recursion trick of keeping track of the level one is on. It doesn't
> work, I suppose, because the data structure I'm traversing isn't actually
> nested, its a flat series of lists of tuples.
>
> I did finally get a function to traverse the "tree-like" data coming from
> the db
>
> getRecord() returns a tuple of record-type, record-id, parent-id, and
> title, thus:
> ('work', 47302, 47301, 'record title')
>
> getImages(id) returns a list of 'image' tuples whose parent-id is id
> getWorks(id) returns a list of 'work' tuples whose parent-id is id
>
> works can be nodes, images can't
>
> so:
>
> def makeDecendentList(id, result):
> result.append(getRecord(id))
> if getImages(id):
> for x in getImages(id):
> result.append(x)
> if not getWorks(id):
> return
> else:
> for y in getWorks(id):
> makeDecendentList(y[1],result)
> return result
>
> called with the empty list as the second argument.
>
> Well and good. This walks the 'tree' (following the parent-ids) and returns
> me a list of all the decendents of id. But what I wanted was a nested data
> structure.
>
> In the fashion of an ape given a typewriter and sufficient time, I trial
> and errored my way to this:
>
> def makeNestedLists(id):
> result = []
> result.append(getRecord(id))
> result.extend(getImages(id))
> if not getWorks(id):
> return result
> else:
> result.extend([makeNestedLists(x[1]) for x in getWorks(id)])
> return result
>
> To my surprise, this gets me exactly what I wanted, eg:
>
> [ ('work', 47301, 47254, 'a title'),
> ('image', 36871, 47301, 'a title'),
> ('image', 36872, 47301, 'a title'),
> [ ('work', 47302, 47301, 'a title'),
> ('image', 10706, 47302, 'a title'),
> ('image', 10725, 47302, 'a title')
> ]
> ]
>
> To my surprise, I say, because it looks like it shouldn't work at all with
> the result being set to the empty list with each recursion; moreover, if I
> spell out the loop in the list comp like this:
>
> def makeNestedLists(id):
> result = []
> result.append(getRecord(id))
> result.extend(getImages(id))
> if not getWorks(id):
> return result
> else:
> for x in getWorks(id):
> result.extend(makeNestedLists(x[1]))
> return result
>
> I once again get a flat list of all the decendents of id.
>
> I know it's sad, but can any wise tutor explain to me what's going on in
> the code that I myself wrote?
>
> Thanks,
> Jon
>
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