[Tutor] longest common substring

[Joel Goldstick]

On Sat, Nov 12, 2011 at 11:40 AM, Andreas Perstinger <
andreas.perstinger at gmx.net> wrote:

> On 2011-11-12 16:24, lina wrote:
>
>> Thanks, ^_^, now better.
>>
>
> No, I'm afraid you are still not understanding.
>
>
>  I checked, the sublist (list) here can't be as a key of the results
>> (dict).
>>
>
> "result" isn't a dictionary. It started as an empty list and later becomes
> a null object ("NoneType").
>
> You must not forget that you are inside a for-loop. Simplified your
> situation is like this:
>
> >>> result = []
> >>> for i in range(1,10):
> ...     print("Iteration {0}, result = {1}".format(i, result))
> ...     result = result.append(i)
> ...
> Iteration 1, result = []
> Iteration 2, result = None
>
> Traceback (most recent call last):
>  File "<stdin>", line 3, in <module>
>
> AttributeError: 'NoneType' object has no attribute 'append'
>
> As you see the error happens in the *second* iteration, because result is
> no list any more.
> Dave gave you already the explanation: functions and method always return
> a value in Python. If the don't have a return statement they return "None".
>
> Another simple example:
>
> >>> a = print("Test")
> Test
>
> "print" is a function which prints out the text you passed to it and you
> usually aren't interested in its return value. But every function/method in
> Python returns something. You save this value in "a"
>
> >>> print(a)
> None
>
> As you see the return value of "print" is "None".
>
> >>> a.append(x)
>
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
>
> AttributeError: 'NoneType' object has no attribute 'append'
>
> Same error as above, because "NoneType" objects (null objects) don't have
> a method "append".
>
> I also think you mix two different ways to add an element to a list:
>
> result.append(x)
>
> is equivalent to
>
> result = result + [x] (that's what you will use in other languages)
>
> HTH, Andreas
>
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This is a fascinating thread in the same way that people can't help slowing
down and looking at a car crash on the side of the road is fascinating.

The original poster it seems is new to programming and has offered that he
likes to run before he learns to walk.  That doesn't seem like a good
proclamation to make when you are asking people to help you.  Anyway, this
particular piece of code is pretty tricky stuff.  It involves understanding
list comprehensions, multidimensional lists, and slices.  All things that
take more than a passing interest in to grasp.

Furthermore, the algorithm itself is pretty tricky.  If you follow the link
to the wikipedia article:
http://en.wikipedia.org/wiki/Longest_common_substring you learn that
understanding the algorithm requires understanding of trees (Generalized
suffix trees at that!).

I copied the code from the original article and played around with it for
an hour or so to understand it.  I didn't get the answers that I expected
either.

If you are learning coding in general and python in particular this
exercise seems unproductive.  Work through basic concepts.  If you don't
like one set of tutorials, find a different one.  if you don't like to
read, check out google videos for learning python, or youtube for that
matter.  But taking on a concise algorithm that solves a problem that would
challenge a 3rd year CS student doesn't seem like a good idea



-- 
Joel Goldstick
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