Sorting a multidimensional array by multiple keys

[Rehceb Rotkiv]

Hello everyone,

can I sort a multidimensiona l array in Python by multiple sort keys? A
litte code sample would be nice!

Thx,
Rehceb

[=?UTF-8?B?VGhvbWFzIEtyw7xnZXI=?=]

Re: Sorting a multidimensiona l array by multiple keys

Rehceb Rotkiv schrieb:

can I sort a multidimensiona l array in Python by multiple sort keys? A
litte code sample would be nice!

You can pass a function as argument to the sort method of a list.
The function should take two arguments and return -1, 0 or 1 as
comparison result. Just like the cmp function.

This will objects in list obj_lst by their id attributes:

def sorter(a, b):
return cmp(a.id, b.id)

obj_lst.sort(so rter)

Thomas

[bearophileHUGS@lycos.com]

Re: Sorting a multidimensiona l array by multiple keys

Rehceb Rotkiv:

can I sort a multidimensiona l array in Python by multiple sort keys? A
litte code sample would be nice!

If you want a good answer you have to give me/us more details, and an
example too.

Bye,
bearophile

[Rehceb Rotkiv]

Re: Sorting a multidimensiona l array by multiple keys

If you want a good answer you have to give me/us more details, and an

example too.

OK, here is some example data:

reaction is BUT by the
sodium , BUT it is
sea , BUT it is
this manner BUT the dissolved
pattern , BUT it is
rapid , BUT it is

As each line consists of 5 words, I would break up the data into an array
of five-field-arrays (Would you use lists or tuples or a combination in
Python?). The word "BUT" would be in the middle, with two fields/words
left and two fields/words right of it. I then want to sort this list by

- field 3
- field 4
- field 1
- field 0

in this hierarchy. This is the desired result:

pattern , BUT it is
rapid , BUT it is
sea , BUT it is
sodium , BUT it is
reaction is BUT by the
this manner BUT the dissolved

The first 4 lines all could not be sorted by fields 3 & 4, as they are
identical ("it", "is"), so they have been sorted first by field 1 (which
is also identical: ",") and then by field 0:

pattern
rapid
sea
sodium

I hope I have explained this in an understandable way. It would be cool
if you could show me how this can be done in Python!

Regards,
Rehceb

[Rehceb Rotkiv]

Re: Sorting a multidimensiona l array by multiple keys

Wait, I made a mistake. The correct result would be

reaction is BUT by the
pattern , BUT it is
rapid , BUT it is
sea , BUT it is
sodium , BUT it is
this manner BUT the dissolved

because "by the" comes before and "the dissolved" after "it is". Sorry
for the confusion.

[Steven Bethard]

Re: Sorting a multidimensiona l array by multiple keys

Rehceb Rotkiv wrote:

>If you want a good answer you have to give me/us more details, and an
>example too.

>
OK, here is some example data:
>
reaction is BUT by the
sodium , BUT it is
sea , BUT it is
this manner BUT the dissolved
pattern , BUT it is
rapid , BUT it is
>
As each line consists of 5 words, I would break up the data into an array
of five-field-arrays (Would you use lists or tuples or a combination in
Python?). The word "BUT" would be in the middle, with two fields/words
left and two fields/words right of it. I then want to sort this list by
>
- field 3
- field 4
- field 1
- field 0

You're probably looking for the key= argument to list.sort(). If your
function simply returns the fields in the order above, I believe you get
the right thing::

>>s = '''\

.... reaction is BUT by the
.... sodium , BUT it is
.... sea , BUT it is
.... this manner BUT the dissolved
.... pattern , BUT it is
.... rapid , BUT it is
.... '''

>>word_lists = [line.split() for line in s.splitlines()]
>>def key(word_list):

.... return word_list[3], word_list[4], word_list[1], word_list[0]
....

>>word_lists.so rt(key=key)
>>word_lists

[['reaction', 'is', 'BUT', 'by', 'the'],
['pattern', ',', 'BUT', 'it', 'is'],
['rapid', ',', 'BUT', 'it', 'is'],
['sea', ',', 'BUT', 'it', 'is'],
['sodium', ',', 'BUT', 'it', 'is'],
['this', 'manner', 'BUT', 'the', 'dissolved']]

STeVe

[Duncan Booth]

Re: Sorting a multidimensiona l array by multiple keys

Rehceb Rotkiv <rehceb@no.spam .plzwrote:

Wait, I made a mistake. The correct result would be
>
reaction is BUT by the
pattern , BUT it is
rapid , BUT it is
sea , BUT it is
sodium , BUT it is
this manner BUT the dissolved
>
because "by the" comes before and "the dissolved" after "it is". Sorry
for the confusion.

>>data = [

"reaction is BUT by the",
"sodium , BUT it is",
"sea , BUT it is",
"this manner BUT the dissolved",
"pattern , BUT it is",
"rapid , BUT it is",
]

>>data = [ s.split() for s in data]
>>from pprint import pprint
>>pprint(data )

[['reaction', 'is', 'BUT', 'by', 'the'],
['sodium', ',', 'BUT', 'it', 'is'],
['sea', ',', 'BUT', 'it', 'is'],
['this', 'manner', 'BUT', 'the', 'dissolved'],
['pattern', ',', 'BUT', 'it', 'is'],
['rapid', ',', 'BUT', 'it', 'is']]

>>from operator import itemgetter
>>data.sort(key =itemgetter(0))
>>data.sort(key =itemgetter(1))
>>data.sort(key =itemgetter(4))
>>data.sort(key =itemgetter(3))
>>pprint(data )

[['reaction', 'is', 'BUT', 'by', 'the'],
['pattern', ',', 'BUT', 'it', 'is'],
['rapid', ',', 'BUT', 'it', 'is'],
['sea', ',', 'BUT', 'it', 'is'],
['sodium', ',', 'BUT', 'it', 'is'],
['this', 'manner', 'BUT', 'the', 'dissolved']]

[attn.steven.kuo@gmail.com]

Re: Sorting a multidimensiona l array by multiple keys

On Mar 31, 6:42 am, Rehceb Rotkiv <reh...@no.spam .plzwrote:

(snipped)

As each line consists of 5 words, I would break up the data into an array
of five-field-arrays (Would you use lists or tuples or a combination in
Python?). The word "BUT" would be in the middle, with two fields/words
left and two fields/words right of it. I then want to sort this list by
>
- field 3
- field 4
- field 1
- field 0

import StringIO

buf = """
reaction is BUT by the
sodium , BUT it is
sea , BUT it is
this manner BUT the dissolved
pattern , BUT it is
rapid , BUT it is
""".lstrip( )

mockfile = StringIO.String IO(buf)

tokens = [ line.split() + [ line ] for line in mockfile ]
tokens.sort(key =lambda l: (l[3], l[4], l[1], l[0]))
for l in tokens:
print l[-1],


--
Hope this helps,
Steven

[Peter Otten]

Re: Sorting a multidimensiona l array by multiple keys

Duncan Booth wrote:

>>>from operator import itemgetter
>>>data.sort(ke y=itemgetter(0) )
>>>data.sort(ke y=itemgetter(1) )
>>>data.sort(ke y=itemgetter(4) )
>>>data.sort(ke y=itemgetter(3) )

Or, in Python 2.5:

>>data.sort(key =itemgetter(3, 4, 1, 0))

Peter

[Rehceb Rotkiv]

Re: Sorting a multidimensiona l array by multiple keys

Thank you all for your helpful solutions!

Regards,
Rehceb

[Duncan Booth]

Re: Sorting a multidimensiona l array by multiple keys

Peter Otten <__peter__@web. dewrote:

Duncan Booth wrote:
>

>>>>from operator import itemgetter
>>>>data.sort(k ey=itemgetter(0 ))
>>>>data.sort(k ey=itemgetter(1 ))
>>>>data.sort(k ey=itemgetter(4 ))
>>>>data.sort(k ey=itemgetter(3 ))

>
Or, in Python 2.5:
>

>>>data.sort(ke y=itemgetter(3, 4, 1, 0))

>

Thanks, I'd forgotten itemgetter had that strangley assymmetric behaviour
of returning either a single value or a tuple.

[Paulo da Silva]

Re: Sorting a multidimensiona l array by multiple keys

Rehceb Rotkiv escreveu:

Hello everyone,
>
can I sort a multidimensiona l array in Python by multiple sort keys? A
litte code sample would be nice!

class MyList(list):
# This is the index of the element to be compared
CmpIndex=0

# Comparision methods
@staticmethod
def __cmp_pars(x,y) :
if isinstance(x,My List):
x=x[MyList.CmpIndex]
if isinstance(y,My List):
y=y[MyList.CmpIndex]
return x,y
def __cmp__(self,ot her):
s,o=MyList.__cm p_pars(self,oth er)
return cmp(s,o)
def __lt__(self,oth er):
s,o=MyList.__cm p_pars(self,oth er)
return s<o
def __le__(self,oth er):
s,o=MyList.__cm p_pars(self,oth er)
return s<=o
def __gt__(self,oth er):
s,o=MyList.__cm p_pars(self,oth er)
return s>o
def __ge__(self,oth er):
s,o=MyList.__cm p_pars(self,oth er)
return s>=o
def __eq__(self,oth er):
s,o=MyList.__cm p_pars(self,oth er)
return s==o
def __ne__(self,oth er):
s,o=MyList.__cm p_pars(self,oth er)
return s!=o

Use:

x=MyList(<list of lists>)
MyList.CmpIndex =2 # Compare by index 2
x.sort()

May be there is a better solution ...
HTH
Paulo

[Alex Martelli]

Re: Sorting a multidimensiona l array by multiple keys

Thomas Krüger <newsgroups@nos pam.nowire.orgw rote:

Rehceb Rotkiv schrieb:

can I sort a multidimensiona l array in Python by multiple sort keys? A
litte code sample would be nice!

>
You can pass a function as argument to the sort method of a list.
The function should take two arguments and return -1, 0 or 1 as
comparison result. Just like the cmp function.
>
This will objects in list obj_lst by their id attributes:
>
def sorter(a, b):
return cmp(a.id, b.id)
>
obj_lst.sort(so rter)

A MUCH better way to obtain exactly the same semantics would be:

def getid(a):
return a.id

obj_list.sort(k ey=getid)


Alex

[=?ISO-8859-1?Q?Thomas_Kr=FCger?=]

Re: Sorting a multidimensiona l array by multiple keys

Alex Martelli schrieb:

Thomas Krüger <newsgroups@nos pam.nowire.orgw rote:

>def sorter(a, b):
> return cmp(a.id, b.id)
>>
>obj_lst.sort(s orter)

>
A MUCH better way to obtain exactly the same semantics would be:
>
def getid(a):
return a.id
>
obj_list.sort(k ey=getid)

Frankly speaking the purpose of the example was to show how to pass a
function as argument for the sort method.
Your code may be more efficient but it explains something different.

Thomas

[Steven Bethard]

Re: Sorting a multidimensiona l array by multiple keys

Thomas Krüger wrote:

Alex Martelli schrieb:

>Thomas Krüger <newsgroups@nos pam.nowire.orgw rote:

>>def sorter(a, b):
>> return cmp(a.id, b.id)
>>>
>>obj_lst.sort( sorter)

>A MUCH better way to obtain exactly the same semantics would be:
>>
>def getid(a):
> return a.id
>>
>obj_list.sort( key=getid)

>
Frankly speaking the purpose of the example was to show how to pass a
function as argument for the sort method.
Your code may be more efficient but it explains something different.

Yes, but there's almost never a reason to use the cmp= argument to
sort() anymore. It's almost always better to use the key= argument.

STeVe