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Newbie questions on Python

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  Newbie questions on Python idkfaidkfaidkfa@gmail.com - 2013-04-16 08:20 -0700
    Re: Newbie questions on Python Chris Angelico <rosuav@gmail.com> - 2013-04-17 01:30 +1000
      Re: Newbie questions on Python Walter Hurry <walterhurry@lavabit.com> - 2013-04-16 21:40 +0000
        Re: Newbie questions on Python Chris Angelico <rosuav@gmail.com> - 2013-04-17 07:53 +1000
    Re: Newbie questions on Python Matt Jones <matt.walker.jones@gmail.com> - 2013-04-16 10:33 -0500
    Re: Newbie questions on Python Neil Cerutti <neilc@norwich.edu> - 2013-04-16 15:41 +0000
      Re: Newbie questions on Python Lele Gaifax <lele@metapensiero.it> - 2013-04-16 18:06 +0200
        Re: Newbie questions on Python Neil Cerutti <neilc@norwich.edu> - 2013-04-16 17:16 +0000

#43672 — Newbie questions on Python

Hi all,
i'm programming in python for the first time (usually i use C as programming language). I don't understand these results:

>>> a=[1,2,3,4,5]
>>> a[:-1]
[1, 2, 3, 4]
>>> a[::-1]
[5, 4, 3, 2, 1]
>>> a[2::-1]
[3, 2, 1]

what does a[2::-1] means?
Thanks

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#43673

On Wed, Apr 17, 2013 at 1:20 AM,  <idkfaidkfaidkfa@gmail.com> wrote:
> Hi all,
> i'm programming in python for the first time (usually i use C as programming language). I don't understand these results:
>
>>>> a=[1,2,3,4,5]
>>>> a[:-1]
> [1, 2, 3, 4]
>>>> a[::-1]
> [5, 4, 3, 2, 1]
>>>> a[2::-1]
> [3, 2, 1]
>
> what does a[2::-1] means?

That's taking a slice. This page has something to say on the subject:

http://docs.python.org/3.3/tutorial/introduction.html

By the way, regarding your email address: there are no cheat codes in
Python... either that, or Python *is* a cheat code. :)

ChrisA
[[ VERY HAPPY CODING ADDED ]]

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#43705

On Wed, 17 Apr 2013 01:30:03 +1000, Chris Angelico wrote:

> By the way, regarding your email address: there are no cheat codes in
> Python

ROFLMAO. Incidentally, my son used to use IDDQD rather than IDKFA.

I of course spurned all such, since I preferred to do it the hard way. 
Thus I was Doomed.

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#43706

On Wed, Apr 17, 2013 at 7:40 AM, Walter Hurry <walterhurry@lavabit.com> wrote:
> On Wed, 17 Apr 2013 01:30:03 +1000, Chris Angelico wrote:
>
>> By the way, regarding your email address: there are no cheat codes in
>> Python
>
> ROFLMAO. Incidentally, my son used to use IDDQD rather than IDKFA.
>
> I of course spurned all such, since I preferred to do it the hard way.
> Thus I was Doomed.

I'd sometimes use IDDQD/IDFA (no K) and then see how quickly I could
blitz the levels, with proper navigation. There's something inherently
fun about blasting through everything with the rocket launcher.

Mind you, I was really cheap, so I never bought Doom... and was just
playing through the demo over and over.

ChrisA

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#43674

[Multipart message — attachments visible in raw view] — view raw

When slicing: l[start:end:step]

In your example of "a[2::-1]" you are reversing the list by using a step of
-1, then you are slicing at index 2 (third element).

*Matt Jones*


On Tue, Apr 16, 2013 at 10:30 AM, Chris Angelico <rosuav@gmail.com> wrote:

> On Wed, Apr 17, 2013 at 1:20 AM,  <idkfaidkfaidkfa@gmail.com> wrote:
> > Hi all,
> > i'm programming in python for the first time (usually i use C as
> programming language). I don't understand these results:
> >
> >>>> a=[1,2,3,4,5]
> >>>> a[:-1]
> > [1, 2, 3, 4]
> >>>> a[::-1]
> > [5, 4, 3, 2, 1]
> >>>> a[2::-1]
> > [3, 2, 1]
> >
> > what does a[2::-1] means?
>
> That's taking a slice. This page has something to say on the subject:
>
> http://docs.python.org/3.3/tutorial/introduction.html
>
> By the way, regarding your email address: there are no cheat codes in
> Python... either that, or Python *is* a cheat code. :)
>
> ChrisA
> [[ VERY HAPPY CODING ADDED ]]
> --
> http://mail.python.org/mailman/listinfo/python-list
>

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#43676

On 2013-04-16, idkfaidkfaidkfa@gmail.com <idkfaidkfaidkfa@gmail.com> wrote:
> Hi all,
> i'm programming in python for the first time (usually i use C as programming language). I don't understand these results:
>
>>>> a=[1,2,3,4,5]
>>>> a[:-1]
> [1, 2, 3, 4]
>>>> a[::-1]
> [5, 4, 3, 2, 1]
>>>> a[2::-1]
> [3, 2, 1]

The third item is the "step". The default value is 1. If you
provide a negative step, your slice will be in reverse. So you
are getting item 2 through 0 in reverse order in your result
slice.

Imagine something like the following for loop taking place
somewhere:

for (int i = 2; i <= 0; --i) {
    fprintf(a[i]);
}
   
-- 
Neil Cerutti

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#43679

Neil Cerutti <neilc@norwich.edu> writes:

> Imagine something like the following for loop taking place
> somewhere:
>
> for (int i = 2; i <= 0; --i) {
>     fprintf(a[i]);
> }

Neil most probably meant

for (int i = 2; i >= 0; --i) {
    fprintf(a[i]);
}

where "fprintf" is actually a fictitious "do_something" function.

ciao, lele.
-- 
nickname: Lele Gaifax | Quando vivrò di quello che ho pensato ieri
real: Emanuele Gaifas | comincerò ad aver paura di chi mi copia.
lele@metapensiero.it  |                 -- Fortunato Depero, 1929.

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#43688

On 2013-04-16, Lele Gaifax <lele@metapensiero.it> wrote:
> Neil Cerutti <neilc@norwich.edu> writes:
>
>> Imagine something like the following for loop taking place
>> somewhere:
>>
>> for (int i = 2; i <= 0; --i) {
>>     fprintf(a[i]);
>> }
>
> Neil most probably meant
>
> for (int i = 2; i >= 0; --i) {
>     fprintf(a[i]);
> }
>
> where "fprintf" is actually a fictitious "do_something" function.
>
> ciao, lele.

Thanks for the correction.

-- 
Neil Cerutti

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