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Creating a list with holes

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  Creating a list with holes Larry Martell <larry.martell@gmail.com> - 2014-01-03 10:19 -0500
    Re: Creating a list with holes eneskristo@gmail.com - 2014-01-03 07:30 -0800
      Re: Creating a list with holes Larry Martell <larry.martell@gmail.com> - 2014-01-03 10:41 -0500
        Re: Creating a list with holes Denis McMahon <denismfmcmahon@gmail.com> - 2014-01-03 18:07 +0000
          Re: Creating a list with holes Larry Martell <larry.martell@gmail.com> - 2014-01-03 19:15 -0500
            Re: Creating a list with holes Roy Smith <roy@panix.com> - 2014-01-03 20:18 -0500
              Re: Creating a list with holes Denis McMahon <denismfmcmahon@gmail.com> - 2014-01-04 02:03 +0000
    Re: Creating a list with holes Roy Smith <roy@panix.com> - 2014-01-03 10:38 -0500
      Re: Creating a list with holes Chris Angelico <rosuav@gmail.com> - 2014-01-04 02:46 +1100

#63058 — Creating a list with holes

I think I know the answer is no, but is there any package that allows
creating a list with holes in it? E.g. I'd want to do something like:

x[10] = 12
x[20] = 30

I'm thinking of something like defaultdict but for lists (I know
that's very different, but ... )

Thanks!
-larry

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#63059

On Friday, January 3, 2014 4:19:09 PM UTC+1, Larry....@gmail.com wrote:
> I think I know the answer is no, but is there any package that allows
> 
> creating a list with holes in it? E.g. I'd want to do something like:
> 
> 
> 
> x[10] = 12
> 
> x[20] = 30
> 
> 
> 
> I'm thinking of something like defaultdict but for lists (I know
> 
> that's very different, but ... )
> 
> 
> 
> Thanks!
> 
> -larry

Hello Larry!

The thing is, where to put the holes? A costum function can be made if you want the hole to be placed for example:
1. In random
2. Every nth hole(Or with another sequence)
3. In the beginning or end.

Please tell me how do you want them, and I will try my best to help!
- Enes

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#63064

On Fri, Jan 3, 2014 at 10:30 AM,  <eneskristo@gmail.com> wrote:
> On Friday, January 3, 2014 4:19:09 PM UTC+1, Larry....@gmail.com wrote:
>> I think I know the answer is no, but is there any package that allows
>>
>> creating a list with holes in it? E.g. I'd want to do something like:
>>
>> x[10] = 12
>> x[20] = 30
>>
>> I'm thinking of something like defaultdict but for lists (I know
>>
>> that's very different, but ... )
>
> Hello Larry!
>
> The thing is, where to put the holes? A costum function can be made if you want the hole to be placed for example:
> 1. In random
> 2. Every nth hole(Or with another sequence)
> 3. In the beginning or end.
>
> Please tell me how do you want them, and I will try my best to help!

The holes would be between the items I put in. In my example above, if
I assigned to [10] and [20], then the other items ([0..9] and
[11..19]) would have None.

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#63079

On Fri, 03 Jan 2014 10:41:21 -0500, Larry Martell wrote:

> The holes would be between the items I put in. In my example above, if I
> assigned to [10] and [20], then the other items ([0..9] and [11..19])
> would have None.

>>> dic = { 10:6, 20:11}
>>> dic.get(10)
6
>>> dic.get(14)
>>> dic.get(27,"oh god there's nothing here")
"oh god there's nothing here"
>>> dic.get(99,None)
>>> dic.get(168,False)
False
>>> dic.get(20,"Boo Yah")
11
>>>

So a standard dictionary does this returning None (or any other default 
value you care to pass it) as long as you use the dict.get(key[,default]) 
method rather than dict[key] to return the value.

See also: 

http://stackoverflow.com/questions/6130768/return-none-if-dictionary-key-
is-not-available

-- 
Denis McMahon, denismfmcmahon@gmail.com

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#63090

On Fri, Jan 3, 2014 at 1:07 PM, Denis McMahon <denismfmcmahon@gmail.com> wrote:
> On Fri, 03 Jan 2014 10:41:21 -0500, Larry Martell wrote:
>
>> The holes would be between the items I put in. In my example above, if I
>> assigned to [10] and [20], then the other items ([0..9] and [11..19])
>> would have None.
>
>>>> dic = { 10:6, 20:11}
>>>> dic.get(10)
> 6
>>>> dic.get(14)
>>>> dic.get(27,"oh god there's nothing here")
> "oh god there's nothing here"
>>>> dic.get(99,None)
>>>> dic.get(168,False)
> False
>>>> dic.get(20,"Boo Yah")
> 11
>>>>
>
> So a standard dictionary does this returning None (or any other default
> value you care to pass it) as long as you use the dict.get(key[,default])
> method rather than dict[key] to return the value.
>
> See also:
>
> http://stackoverflow.com/questions/6130768/return-none-if-dictionary-key-
> is-not-available

Thanks, but I know all that about dicts. I need to use a list for
compatibility with existing code.

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#63093

In article <mailman.4871.1388794533.18130.python-list@python.org>,
 Larry Martell <larry.martell@gmail.com> wrote:

> 
> Thanks, but I know all that about dicts. I need to use a list for
> compatibility with existing code.

Generalizing what I think the situation is, "A dict is the best data 
structure for the parsing phase, but I need a list later to hand off to 
legacy interfaces".

No problem.  Parse the data using a dict, then convert the dict to a 
list later.  I haven't been following all the details here, but just 
wanted to point out that using different data structures to hold the 
same data at different phases of a program is a perfectly reasonable 
approach.

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#63097

On Fri, 03 Jan 2014 20:18:06 -0500, Roy Smith wrote:

> In article <mailman.4871.1388794533.18130.python-list@python.org>,
>  Larry Martell <larry.martell@gmail.com> wrote:
> 
> 
>> Thanks, but I know all that about dicts. I need to use a list for
>> compatibility with existing code.
> 
> Generalizing what I think the situation is, "A dict is the best data
> structure for the parsing phase, but I need a list later to hand off to
> legacy interfaces".
> 
> No problem.  Parse the data using a dict, then convert the dict to a
> list later.  I haven't been following all the details here, but just
> wanted to point out that using different data structures to hold the
> same data at different phases of a program is a perfectly reasonable
> approach.

Indeed, assuming the requirement is to have a list of some length n units 
representing integer keys into a range of values from start to end, and 
he creates a dict initially:

list = [ dict.get(x,None) for x in range(start,end + 1) ]

Examples:

>>> dic = {1:"fred", 9:"jim", 15:"susan", 25:"albert" }
>>> l = [ dic.get(x,None) for x in range(1,20) ]
>>> l
['fred', None, None, None, None, None, None, None, 'jim', None, None, 
None, None, None, 'susan', None, None, None, None]
>>> l = [ dic.get(x,None) for x in range(-10,50) ]
>>> l
[None, None, None, None, None, None, None, None, None, None, None, 'fred', 
None, None, None, None, None, None, None, 'jim', None, None, None, None, 
None, 'susan', None, None, None, None, None, None, None, None, None, 
'albert', None, None, None, None, None, None, None, None, None, None, 
None, None, None, None, None, None, None, None, None, None, None, None, 
None, None]
>>> len(l)
60

then the value of l[offset] will either be None or some string depending 
on the offset into the list

-- 
Denis McMahon, denismfmcmahon@gmail.com

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#63063

In article <mailman.4852.1388762356.18130.python-list@python.org>,
 Larry Martell <larry.martell@gmail.com> wrote:

> I think I know the answer is no, but is there any package that allows
> creating a list with holes in it? E.g. I'd want to do something like:
> 
> x[10] = 12
> x[20] = 30

Whenever you ask, "What data structure do I want", you need to be able 
to answer, "What operations do I want to perform?, and, "What 
constraints do I have on memory use?"

Why do you want holes?  Is the issue that you're storing sparse data and 
don't want to waste memory on unused keys?  If so, a dictionary should 
do you fine.

Do you need to be able to read the values back out in a specific order?  
You can still do that with a dictionary if you're willing to re-sort the 
keys; that's O(n log n) on the number of keys, but if n is small, it 
doesn't matter.

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#63066

On Sat, Jan 4, 2014 at 2:38 AM, Roy Smith <roy@panix.com> wrote:
> Why do you want holes?  Is the issue that you're storing sparse data and
> don't want to waste memory on unused keys?  If so, a dictionary should
> do you fine.
>
> Do you need to be able to read the values back out in a specific order?
> You can still do that with a dictionary if you're willing to re-sort the
> keys; that's O(n log n) on the number of keys, but if n is small, it
> doesn't matter.

There's another factor, which is iteration time. Maybe you don't care
about the memory usage, but compare these:

foo = [None]*1000
foo[123] = 234
foo[543] = 432

for key,value in enumerate(foo):
    if value: print("Slot %d is %d"%(key,value))

# versus

foo = {}
foo[123] = 234
foo[543] = 432

for key in sorted(foo.keys()):
    value = foo[key]
    if value: print("Slot %d is %d"%(key,value))

Which one's going to be faster? The dictionary, by far, in this
example. I'm not sure how populated the list would have to be to beat
it (as the dict will get slower on O(n log n) on keys, as you mention,
while the list will run at O(n) on the highest element, which may well
be a constant), but it's something to consider.

ChrisA

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