# Re: get the sum of differences between integers in a list

```Hi, first of all, let me rephase the problem.

For an arbitrary list of integers (the integers in the list are not
necessary to be sequential), e.g. [1,2,3,6,8,9,10,11,13],```
```
if a set of consecutive integers having a difference of 1 between them, put
them in a list, i.e. there are two such lists in this example,

[1,2,3],

[8,9,10,11],

and then put such lists in another list (i.e. [[1,2,3], [8,9,10,11]]). Put
the rest integers (non-sequential) in a separated list, i.e.

`[6, 13]`.

Note that, the problem only considers sequential integers with step_size =
1.

I tried to use itertools.groupby and itertools.count,

from itertools import groupby, count
lst = [1,2,3,6,8,9,10,11,13]
c = count()
result = [list(g) for i, g in groupby(lst, key=lambda x: x-next(c))]

but the result is close to the requirement shown as a list of lists,

[[1, 2, 3], [6], [8, 9, 10, 11], [13]]

but it didn't separate sequential lists from non-sequential ones.
Also, I couldn't find a way to put [6] and [13] in a single list.

I have also tried to separate sequential lists from non-sequential ones,

result = [list(g) for i, g in groupby(lst, key=lambda x: x-next(c) == 1)] #
tried to extract [1,2,3] and [8,9,10,11] from the list

or

result = [list(g) for i, g in groupby(lst, key=lambda x: x-next(c) > 1)] #
tried to extract [6] and [13] from the list

but they all ended up with

[[1, 2, 3], [6, 8, 9, 10, 11, 13]]

So two questions here,

1. How does itertools.groupby key function work in my case?

2. How to improve the program to achieve my goals?

Many thanks

On 21 September 2016 at 00:19, John Pote <johnp...@jptechnical.co.uk> wrote:

> On 20/09/2016 12:52, Daiyue Weng wrote:
>
> Hi, I have a list numbers say,
>>
>> [1,2,3,4,6,8,9,10,11]
>>
>> First, I want to calculate the sum of the differences between the numbers
>> in the list.
>>
> At least for this first part a little pencil, paper and algebra yields a
> simple formula of constant and minimal calculation time. I had an intuitive
> guess and wrote down the sum of differences for a couple of examples,
> [1, 2, 5]       => 4
> [9, 11, 17, 19] => 10
> It works for negative differences as well,
> [1, 2, 5, 1]    => 0
> The trick is to spot the relation between the sum of differences and the
> numbers in the list. A few lines of algebra then gave a very simple formula.
>
> As for the rest it's down to code as others have hinted at.
>
> Second, if a sequence of numbers having a difference of 1, put them in a
>> list, i.e. there are two such lists,
>>
>> [1,2,3]
>>
>> [8,9,10,11]
>>
>> and also put the rest numbers in another list, i.e. there is only one such
>> list in the example,
>>
>> [6].
>>
>> Third, get the lists with the max/min sizes from above, i.e. in this
>> example, the max list is,
>>
>> [8,9,10,11]
>>
>> min list is
>>
>> [1,2,3].
>>
>> What's the best way to implement this?
>>
>> cheers
>>
>
> --
> https://mail.python.org/mailman/listinfo/python-list
>
--
https://mail.python.org/mailman/listinfo/python-list
```