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How to access the low digits of a list

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  How to access the low digits of a list fl <rxjwg98@gmail.com> - 2015-06-02 05:23 -0700
    Re: How to access the low digits of a list Joel Goldstick <joel.goldstick@gmail.com> - 2015-06-02 08:32 -0400
    Re: How to access the low digits of a list Frank Stutzman <stutzman@cat2.kjsl.com> - 2015-06-02 12:34 +0000
    Re: How to access the low digits of a list Rustom Mody <rustompmody@gmail.com> - 2015-06-02 05:35 -0700
      Re: How to access the low digits of a list Ian Kelly <ian.g.kelly@gmail.com> - 2015-06-02 08:19 -0600
        Re: How to access the low digits of a list Rustom Mody <rustompmody@gmail.com> - 2015-06-03 14:08 -0700
          Re: How to access the low digits of a list Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-06-03 22:41 +0100
          Re: How to access the low digits of a list Ian Kelly <ian.g.kelly@gmail.com> - 2015-06-03 16:36 -0600
          Re: How to access the low digits of a list Steven D'Aprano <steve@pearwood.info> - 2015-06-04 23:27 +1000
    Re: How to access the low digits of a list Steven D'Aprano <steve@pearwood.info> - 2015-06-03 01:11 +1000

#91826 — How to access the low digits of a list

Hi,

I have a list:






>>> lines
['12', '42', '49', '156', '225', '36', '49', '164', '11181', '3100']



I want to access the last two digits. That is:

['12', '42', '49', '56', '25', '36', '49', '64', '81', '00']


When I try to use lines[3][0] is '1'
lines[3][1] is '5'
lines[3][2] is '6'


I don't know whether there is a way to know the length of lines[3]. Then, I
can use a -1 step to get the last two digits. Or, you may have much better 
ways to do that. Python is really too versatile I feel.

Thanks,

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#91828

On Tue, Jun 2, 2015 at 8:23 AM, fl <rxjwg98@gmail.com> wrote:
> Hi,
>
> I have a list:
>
>
>
>
>
>
>>>> lines
> ['12', '42', '49', '156', '225', '36', '49', '164', '11181', '3100']
>
>
>
> I want to access the last two digits. That is:
>
> ['12', '42', '49', '56', '25', '36', '49', '64', '81', '00']
>
>
> When I try to use lines[3][0] is '1'
> lines[3][1] is '5'
> lines[3][2] is '6'
>
>
> I don't know whether there is a way to know the length of lines[3]. Then, I
> can use a -1 step to get the last two digits. Or, you may have much better
> ways to do that. Python is really too versatile I feel.
>
> Thanks,
> --
> https://mail.python.org/mailman/listinfo/python-list

You should read about slices

lines[3][-2:]  will give the last two characters in the 3rd group
(starting from 0)

The notation means to start with 2 from the end, and proceed to the end


-- 
Joel Goldstick
http://joelgoldstick.com

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#91829

fl <rxjwg98@gmail.com> wrote:

---snip----
>>>> lines
> ['12', '42', '49', '156', '225', '36', '49', '164', '11181', '3100']
> 
> 
> 
> I want to access the last two digits. That is:
> 
> ['12', '42', '49', '56', '25', '36', '49', '64', '81', '00']
> 
> 
> When I try to use lines[3][0] is '1'
> lines[3][1] is '5'
> lines[3][2] is '6'

Is there something wrong with using:
   lines[3][-2:]
   

-- 
Frank Stutzman

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#91830

On Tuesday, June 2, 2015 at 5:53:34 PM UTC+5:30, fl wrote:
> Hi,
> 
> I have a list:
> 
> 
> 
> 
> 
> 
> >>> lines
> ['12', '42', '49', '156', '225', '36', '49', '164', '11181', '3100']
> 
> 
> 
> I want to access the last two digits. That is:
> 
> ['12', '42', '49', '56', '25', '36', '49', '64', '81', '00']
> 
> 
> When I try to use lines[3][0] is '1'
> lines[3][1] is '5'
> lines[3][2] is '6'
> 
> 
> I don't know whether there is a way to know the length of lines[3]. Then, I
> can use a -1 step to get the last two digits. Or, you may have much better 
> ways to do that. Python is really too versatile I feel.

len(lines[3])  ??

You can do this: [Hope I am not doing your homework!]

>>> [(x[-2:] if len(x) > 2 else x)  for x in lines]
['12', '42', '49', '56', '25', '36', '49', '64', '81', '00']
>>> 

For that matter even this works
But I am not sure whats happening or that I like it

>>> [x[-2:]  for x in lines]
['12', '42', '49', '56', '25', '36', '49', '64', '81', '00']
>>> 

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#91843

On Tue, Jun 2, 2015 at 6:35 AM, Rustom Mody <rustompmody@gmail.com> wrote:
> For that matter even this works
> But I am not sure whats happening or that I like it
>
>>>> [x[-2:]  for x in lines]
> ['12', '42', '49', '56', '25', '36', '49', '64', '81', '00']

x[-2:] selects all items in the sequence with index i such that len(x)
- 2 <= i < len(x). For a sequence of length 2 or less, that's the
entire sequence.

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#91996

On Tuesday, June 2, 2015 at 7:50:58 PM UTC+5:30, Ian wrote:
> On Tue, Jun 2, 2015 at 6:35 AM, Rustom Mody  wrote:
> > For that matter even this works
> > But I am not sure whats happening or that I like it
> >
> >>>> [x[-2:]  for x in lines]
> > ['12', '42', '49', '56', '25', '36', '49', '64', '81', '00']
> 
> x[-2:] selects all items in the sequence with index i such that len(x)
> - 2 <= i < len(x). For a sequence of length 2 or less, that's the
> entire sequence.

Thanks -- learn something
So it means that indices can give indexerror; slices cannot?
Seems fair enough put that way, but is visually counterintuitive

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#92001

On 03/06/2015 22:08, Rustom Mody wrote:
> On Tuesday, June 2, 2015 at 7:50:58 PM UTC+5:30, Ian wrote:
>> On Tue, Jun 2, 2015 at 6:35 AM, Rustom Mody  wrote:
>>> For that matter even this works
>>> But I am not sure whats happening or that I like it
>>>
>>>>>> [x[-2:]  for x in lines]
>>> ['12', '42', '49', '56', '25', '36', '49', '64', '81', '00']
>>
>> x[-2:] selects all items in the sequence with index i such that len(x)
>> - 2 <= i < len(x). For a sequence of length 2 or less, that's the
>> entire sequence.
>
> Thanks -- learn something
> So it means that indices can give indexerror; slices cannot?
> Seems fair enough put that way, but is visually counterintuitive
>

Are you seriously trying to say that you teach Python but don't 
understand a basic that is here 
https://docs.python.org/3/tutorial/introduction.html, "Slice indices 
have useful defaults; an omitted first index defaults to zero, an 
omitted second index defaults to the size of the string being sliced."?

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence

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#92010

On Wed, Jun 3, 2015 at 3:08 PM, Rustom Mody <rustompmody@gmail.com> wrote:
> On Tuesday, June 2, 2015 at 7:50:58 PM UTC+5:30, Ian wrote:
>> On Tue, Jun 2, 2015 at 6:35 AM, Rustom Mody  wrote:
>> > For that matter even this works
>> > But I am not sure whats happening or that I like it
>> >
>> >>>> [x[-2:]  for x in lines]
>> > ['12', '42', '49', '56', '25', '36', '49', '64', '81', '00']
>>
>> x[-2:] selects all items in the sequence with index i such that len(x)
>> - 2 <= i < len(x). For a sequence of length 2 or less, that's the
>> entire sequence.
>
> Thanks -- learn something
> So it means that indices can give indexerror; slices cannot?
> Seems fair enough put that way, but is visually counterintuitive

Yes. The rule I paraphrased above is stated at
https://docs.python.org/3/library/stdtypes.html#common-sequence-operations
-- scroll down to note 4. I don't know if there's anything that
clearly states that sequence slicing can't raise IndexError, but it is
at least implied by the above, and it is certainly true of all builtin
sequence types.

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#92047

On Thu, 4 Jun 2015 07:08 am, Rustom Mody wrote:

> So it means that indices can give indexerror; slices cannot?

If you write your own class with a __getitem__ method, you can have it do
anything you like, including raise an exception.

Built-in sequence types like list, str and tuple, however, allow slicing to
extend beyond the first and last indexes without error.



-- 
Steven

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#91848

On Tue, 2 Jun 2015 10:23 pm, fl wrote:

> I don't know whether there is a way to know the length of lines[3].

The same way as to know the length of anything else:

len(x)  # the length of x
len("hello")  # the length of "hello"
len(lines[3])  # the length of lines[3]

> Then, 
> I can use a -1 step to get the last two digits. Or, you may have much
> better ways to do that. Python is really too versatile I feel.

Last two digits of a string:

mystring = "123456"
mystring[-2:]


-- 
Steven

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