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Wrapping around a list in Python.

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  Wrapping around a list in Python. shengjie.shengjie@live.com - 2013-12-15 20:38 -0800
    Re: Wrapping around a list in Python. Ben Finney <ben+python@benfinney.id.au> - 2013-12-16 15:59 +1100
      Re: Wrapping around a list in Python. shengjie.shengjie@live.com - 2013-12-15 21:07 -0800
        Re: Wrapping around a list in Python. shengjie.shengjie@live.com - 2013-12-15 21:10 -0800
          Re: Wrapping around a list in Python. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-12-16 09:33 +0000
      Re: Wrapping around a list in Python. Denis McMahon <denismfmcmahon@gmail.com> - 2013-12-16 10:12 +0000
    Re: Wrapping around a list in Python. Gary Herron <gary.herron@islandtraining.com> - 2013-12-15 21:10 -0800
      Re: Wrapping around a list in Python. shengjie.shengjie@live.com - 2013-12-15 21:26 -0800
        Re: Wrapping around a list in Python. Peter Otten <__peter__@web.de> - 2013-12-16 10:15 +0100
        Re: Wrapping around a list in Python. David Robinow <drobinow@gmail.com> - 2013-12-16 07:41 -0500
        Re: Wrapping around a list in Python. Dave Angel <davea@davea.name> - 2013-12-16 07:51 -0500
    Re: Wrapping around a list in Python. shengjie.shengjie@live.com - 2013-12-15 21:26 -0800

#61995 — Wrapping around a list in Python.

Hi guys, I am trying to create a fixed list which would allow my values to be wrapped around it.
For example i have 10 values : 0,1,2,3,4,5,6,7,8,9
I need to create a list which contains 4 numbers and when the number exceeds the list, it would overwrite the first value.
[0,1,2,3]
[4,1,2,3]
[5,4,1,2]

Thanks in advance and much help appreciated.

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#62001

shengjie.shengjie@live.com writes:

> Hi guys, I am trying to create a fixed list which would allow my
> values to be wrapped around it.

This doesn't make a lot of sense to me, but I assume you have a purpose
in mind for this. What is the purpose? Perhaps it will help the
explanation if we know what it's for.

> For example i have 10 values : 0,1,2,3,4,5,6,7,8,9

Does this mean the input is a list, ‘[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]’? Or
do you mean something else? What is the input?

> I need to create a list which contains 4 numbers and when the number
> exceeds the list, it would overwrite the first value.

> [0,1,2,3]
> [4,1,2,3]
> [5,4,1,2]

That's three different lists. What is the input in each case? Under what
circumstances would you expect each one to be produced?

-- 
 \     “As scarce as truth is, the supply has always been in excess of |
  `\                                       the demand.” —Josh Billings |
_o__)                                                                  |
Ben Finney

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#62003

On Monday, 16 December 2013 12:59:32 UTC+8, Ben Finney  wrote:
> shengjie.shengjie@live.com writes:
> 
> 
> 
> > Hi guys, I am trying to create a fixed list which would allow my
> 
> > values to be wrapped around it.
> 
> 
> 
> This doesn't make a lot of sense to me, but I assume you have a purpose
> 
> in mind for this. What is the purpose? Perhaps it will help the
> 
> explanation if we know what it's for.
> 
> 
> 
> > For example i have 10 values : 0,1,2,3,4,5,6,7,8,9
> 
> 
> 
> Does this mean the input is a list, ‘[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]’? Or
> 
> do you mean something else? What is the input?
> 
> 
> 
> > I need to create a list which contains 4 numbers and when the number
> 
> > exceeds the list, it would overwrite the first value.
> 
> 
> 
> > [0,1,2,3]
> 
> > [4,1,2,3]
> 
> > [5,4,1,2]
> 
> 
> 
> That's three different lists. What is the input in each case? Under what
> 
> circumstances would you expect each one to be produced?
> 
> 
> 
> -- 
> 
>  \     “As scarce as truth is, the supply has always been in excess of |
> 
>   `\                                       the demand.” —Josh Billings |
> 
> _o__)                                                                  |
> 
> Ben Finney

Im currently creating a logging function for my GUI in pyqt.
I saved its feedback data values in a .txt file and I access these values by placing them in a list. However, this list gets really big and I am trying to find a work around.

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#62005

On Monday, 16 December 2013 13:07:46 UTC+8, shengjie...@live.com  wrote:
> On Monday, 16 December 2013 12:59:32 UTC+8, Ben Finney  wrote:
> 
> > shengjie.shengjie@live.com writes:
> 
> > 
> 
> > 
> 
> > 
> 
> > > Hi guys, I am trying to create a fixed list which would allow my
> 
> > 
> 
> > > values to be wrapped around it.
> 
> > 
> 
> > 
> 
> > 
> 
> > This doesn't make a lot of sense to me, but I assume you have a purpose
> 
> > 
> 
> > in mind for this. What is the purpose? Perhaps it will help the
> 
> > 
> 
> > explanation if we know what it's for.
> 
> > 
> 
> > 
> 
> > 
> 
> > > For example i have 10 values : 0,1,2,3,4,5,6,7,8,9
> 
> > 
> 
> > 
> 
> > 
> 
> > Does this mean the input is a list, ‘[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]’? Or
> 
> > 
> 
> > do you mean something else? What is the input?
> 
> > > The input is from [0 to 9] however I want a display which displays [0-3] and when the value reaches 3, it replaces the 4th value with the first value and so on.
> 
> > 
> 
> > 
> 
> > > I need to create a list which contains 4 numbers and when the number
> 
> > 
> 
> > > exceeds the list, it would overwrite the first value.
> 
> > 
> 
> > 
> 
> > 
> 
> > > [0,1,2,3]
> 
> > 
> 
> > > [4,1,2,3]
> 
> > 
> 
> > > [5,4,1,2]
> 
> > 
> 
> > 
> 
> > 
> 
> > That's three different lists. What is the input in each case? Under what
> 
> > 
> 
> > circumstances would you expect each one to be produced?
> 
> > 
> 
> > 
> 
> > 
> 
> > -- 
> 
> > 
> 
> >  \     “As scarce as truth is, the supply has always been in excess of |
> 
> > 
> 
> >   `\                                       the demand.” —Josh Billings |
> 
> > 
> 
> > _o__)                                                                  |
> 
> > 
> 
> > Ben Finney
> 
> 
> 
> Im currently creating a logging function for my GUI in pyqt.
> 
> I saved its feedback data values in a .txt file and I access these values by placing them in a list. However, this list gets really big and I am trying to find a work around.

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#62031

On 16/12/2013 05:10, shengjie.shengjie@live.com wrote:
> On Monday, 16 December 2013 13:07:46 UTC+8, shengjie...@live.com  wrote:

Would you please read and action this 
https://wiki.python.org/moin/GoogleGroupsPython to prevent us seeing 
double line spacing, thanks.

-- 
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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#62037

On Mon, 16 Dec 2013 15:59:32 +1100, Ben Finney wrote:

> shengjie.shengjie@live.com writes:
> 
>> Hi guys, I am trying to create a fixed list which would allow my values
>> to be wrapped around it.
> 
> This doesn't make a lot of sense to me, but I assume you have a purpose
> in mind for this. What is the purpose? Perhaps it will help the
> explanation if we know what it's for.
> 
>> For example i have 10 values : 0,1,2,3,4,5,6,7,8,9
> 
> Does this mean the input is a list, ‘[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]’? Or
> do you mean something else? What is the input?
> 
>> I need to create a list which contains 4 numbers and when the number
>> exceeds the list, it would overwrite the first value.
> 
>> [0,1,2,3]
>> [4,1,2,3]
>> [5,4,1,2]
> 
> That's three different lists. What is the input in each case? Under what
> circumstances would you expect each one to be produced?

I suspect the OP means:

first input is 0, list = [ 0 ]
next input is 1, list = [ 0, 1 ]
next input is 2, list = [ 0, 1, 2 ]
next input is 3, list = [ 0, 1, 2, 3 ]
next input is 4, list = [ 4, 1, 2, 3 ]
next input is 5, list = [ 5, 4, 1, 2 ]

But this is a bit daft, because he starts by appending, and when he hits 
overflow he starts prepending.

What I think he should do is use collections.dequeue and a couple of 
helper functions to add and get data items.

For a queue moving from position 0 to position 3 (left to right):

from collections import deque

q = dequeue([])

def add_to_queue( item, q ):
	if len( q ) is 4:
		q.pop()
	q.appendleft( item )

def get_next( q ):
	if len( q ) > 0:
		return q.pop()
	return None

To move from position 3 to position 0 (right to left), swap pop and 
appendleft for popleft and append.

-- 
Denis McMahon, denismfmcmahon@gmail.com

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#62007

On 12/15/2013 08:38 PM, shengjie.shengjie@live.com wrote:
> Hi guys, I am trying to create a fixed list which would allow my values to be wrapped around it.
> For example i have 10 values : 0,1,2,3,4,5,6,7,8,9
> I need to create a list which contains 4 numbers and when the number exceeds the list, it would overwrite the first value.
> [0,1,2,3]
> [4,1,2,3]
> [5,4,1,2]
>
> Thanks in advance and much help appreciated.

Is the output really three lists as you show.  Or is that one list whose 
contents you have shown three snapshots of?  Then what was the point of 
putting 4 in the first spot when you are just going to move it to the 
second spot?  And why stop at 4 and 5?  What about 7, 8, and 9?

Are you really shifting elements onto the beginning of the list and off 
the end of the list?  (That's easy to do, but is that what you want?)

If I follow your example a few elements further I get [9,8,7,6], just 
the last four elements of the original list in reverse order -- so there 
is no need fill a list and "wrap-around"  -- just grab the last four 
elements and reverse them.

Or have I misunderstood the problem completely?  (I think that's 
likely.)  I'm sure Python is general enough to do what you want, but 
you'll have to do a much better job telling is what you want.  While you 
are at it, tell us what you've already done, and how it fails to do 
whatever it is you want.

Gary Herron

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#62008

On Monday, 16 December 2013 13:10:22 UTC+8, Gary Herron  wrote:
> On 12/15/2013 08:38 PM, shengjie.shengjie@live.com wrote:
> 
> > Hi guys, I am trying to create a fixed list which would allow my values to be wrapped around it.
> 
> > For example i have 10 values : 0,1,2,3,4,5,6,7,8,9
> 
> > I need to create a list which contains 4 numbers and when the number exceeds the list, it would overwrite the first value.
> 
> > [0,1,2,3]
> 
> > [4,1,2,3]
> 
> > [5,4,1,2]
> 
> >
> 
> > Thanks in advance and much help appreciated.
> 
> 
> 
> Is the output really three lists as you show.  Or is that one list whose 
> 
> contents you have shown three snapshots of?  Then what was the point of 
> 
> putting 4 in the first spot when you are just going to move it to the 
> 
> second spot?  And why stop at 4 and 5?  What about 7, 8, and 9?
> 
> 
> 
> Are you really shifting elements onto the beginning of the list and off 
> 
> the end of the list?  (That's easy to do, but is that what you want?)
> 
> 
> 
> If I follow your example a few elements further I get [9,8,7,6], just 
> 
> the last four elements of the original list in reverse order -- so there 
> 
> is no need fill a list and "wrap-around"  -- just grab the last four 
> 
> elements and reverse them.
> 
> 
> 
> Or have I misunderstood the problem completely?  (I think that's 
> 
> likely.)  I'm sure Python is general enough to do what you want, but 
> 
> you'll have to do a much better job telling is what you want.  While you 
> 
> are at it, tell us what you've already done, and how it fails to do 
> 
> whatever it is you want.
> 
> 
> 
> Gary Herron

The idea is to grab the last 4 elements of the array. However i have an array that contains a few hundred elements in it. And the values continues to .append over time. How would i be able to display the last 4 elements of the array under such a condition?

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#62025

shengjie.shengjie@live.com wrote:

> The idea is to grab the last 4 elements of the array. However i have an
> array that contains a few hundred elements in it. And the values continues
> to .append over time. How would i be able to display the last 4 elements
> of the array under such a condition?

Use a deque:

>>> from collections import deque
>>> last_four = deque(maxlen=4)
>>> for i in range(10):
...     last_four.append(i)
... 
>>> last_four
deque([6, 7, 8, 9], maxlen=4)
>>> last_four.extend(range(100, 200))
>>> last_four
deque([196, 197, 198, 199], maxlen=4)
>>> last_four.append(42)
>>> last_four
deque([197, 198, 199, 42], maxlen=4)

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#62055

On Mon, Dec 16, 2013 at 12:26 AM,  <shengjie.shengjie@live.com> wrote:
> The idea is to grab the last 4 elements of the array. However i have an array that contains a few hundred elements in it. And the values continues to .append over time. How would i be able to display the last 4 elements of the array under such a condition?

I assume you mean 'list' rather than 'array'.
If all you want to do is 'display' the last 4 elements:

big_list = [0,1,2,3,4,5,6,7,8,9]
last4 =  big_list[-4:]
print(last4)

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#62056

On Sun, 15 Dec 2013 21:26:49 -0800 (PST), shengjie.shengjie@live.com 
wrote:
> The idea is to grab the last 4 elements of the array. However i 
have an array that contains a few hundred elements in it. And the 
values continues to .append over time. How would i be able to display 
the last 4 elements of the array under such a condition?

Your earlier example showed [5, 4, 1, 2] as one of the results, which 
is not "the last four". But assuming your goal has now changed and 
you really have an array (or list) of a few hundred elements,  then 
you can just use data [-4:] to get them. 

But if you're being imprecise and these values are not really in an 
array, then follow Peter ' advice and use a deque. Or fake it with a 
list,  appending to the end and popping from the beginning.

-- 
DaveA

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#62009

On Monday, 16 December 2013 12:38:14 UTC+8, shengjie...@live.com  wrote:
> Hi guys, I am trying to create a fixed list which would allow my values to be wrapped around it.
> 
> For example i have 10 values : 0,1,2,3,4,5,6,7,8,9
> 
> I need to create a list which contains 4 numbers and when the number exceeds the list, it would overwrite the first value.
> 
> [0,1,2,3]
> 
> [4,1,2,3]
> 
> [5,4,1,2]
> 
> 
> 
> Thanks in advance and much help appreciated.

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