Re: [Pytables-users] Help on sorting tables

[Francesc A. <fa...@gm...>]

On 3/22/12 1:59 PM, Francesc Alted wrote:
> On 3/22/12 12:48 PM, sreeaurovindh viswanathan wrote:
>> But.. Can i get sort one column by descending and the other ascending.
>> say
>> if i have two columns and first i would like to sort the one in
>> ascending and then sort the second column based on the search from the
>> first.
>>
>>
>> I mean I i have
>>
>> 1 5
>> 2 6
>> 1 8
>> 2 9
>>
>> Could i get an output as
>>
>> 1 5
>> 1 8
>> 2 6
>> 2 9
> No, this is not supported by PyTables.
>
> But hey, you can always make use of the sorted iterator, and the 
> additonal sorting by yourselves.  In your example, let's suppose that 
> column 0 is named 'f0' and column 1 is named 'f1'.  Then, the next loop:
>
> prevval = None
> gf1 = []
> for r in t.itersorted('f0'):
>     if r['f0'] != prevval:
>         if gf1:
>             gf1.sort()
>             print prevval, gf1[::-1]  # reverse sorted
>         prevval = r['f0']
>         gf1 = []
>     gf1.append(r['f1'])
> if gf1:
>     gf1.sort()
>     print prevval, gf1[::-1]  # reverse sorted
Hmm, I just realized that there it is another, equivalent code that 
solves the same problem:

def field_selector(row):
     return row['f0']

for field, rows_grouped_by_field in 
itertools.groupby(t.itersorted('f0'), field_selector):
     group = [ r['f1'] for r in rows_grouped_by_field ]
     group.sort()
     print '%s -> %s' % (field, group[::-1])

The performance of both is similar, so use whatever you find more useful.

For the record, I'm attaching a couple of self-contained examples that 
exercises the different approaches.

  -- Francesc Alted