Many newbie questions regarding python

[gregfull at gmail.com]

On Oct 7, 6:10 pm, Rogério Brito <rbr... at ime.usp.br> wrote:
> Hi there.
>
> I am used to some languages like C, but I am just a complete newbie with Python
> and, while writing some small snippets, I had encountered some problems, with
> which I would sincerely appreciate any help, since I appreciate this language to
> write my "running pseudocode in" and I am seriously thinking of using it to
> teach some algorithms classes.
>
> 1 - The first issue that I am having is that I don't seem to be able to, say,
> use something that would be common for people writing programs in C: defining a
> one-dimensional vector and only initializing it when needed.
>
> For instance, in C, I would write something like:
>
> int v[20];
> for (i = 0; i < 20; i++)
>     v[i] = 0;
>
> Note that I only define the vector v (and its size) at the beginning but
> initialize it latter during the code per-se.
>
> My first try to write it in Python was something like this:
>
> v = []
> for i in range(20):
>     v[i] = 0
>
> Unfortunately, this doesn't work, as I get an index out of bounds when trying to
> index the v list. Of course, the main difference between the two snippets is
> that, in C, I declared v to have 20 positions, while in python I initialized it
> to be the empty list and, indeed, it has an empty set of indexes.
>
> What is the Pythonic way of writing code like this? So far, I have found many
> alternatives and I would like to write code that others in the Python community
> would find natural to read. Some of the things that crossed my mind:
>
>     v = [0 for i in range(20)]
>
>     v = [0] * 20
>
>     v = []
>     for i in range(20): v.append(0)
>
> What should I prefer? Any other alternative?
>
> If possible, I would like to simply declare the list and fill it latter in my
> program, as lazily as possible (this happens notoriously when one is using a
> technique of programming called dynamic programming where initializing all
> positions of a table may take too much time in comparison to the filling of the
> array).
>
> 2 - If I declare a class with some member variables, is is strictly necessary
> for me to qualify those members in a method in that class? For instance, if I
> define:
>
> class C:
>     f = 1
>     def g(self):
>         return f
>
> I get an annoying message when I try to call the g method in an object of type
> C, telling me that there's no global symbol called f. If I make g return self.f
> instead, things work as expected, but the code loses some readability.
>
> Is there any way around this or is that simply "a matter of life"?
>
> I have some other questions, but I will save them for latter.
>
> Please, keep in mind that I am a newbie in Python. Despite that, I am enjoying
> the little that I know.
>
> Thank you very much in advance,
>
> --
> Rogério Brito : rbrito@{ime.usp.br,gmail.com} : GPG key 4096R/BCFCAAAAhttp://rb.doesntexist.org: Packages for LaTeX : algorithms.berlios.de
> DebianQA:http://qa.debian.org/developer.php?login=rbrito%40ime.usp.br

How about:

 v = [None] * 20

That way, you're not initializing with an artifical value like 0.