Re: [BangPypers] Dictionary : An elementary question

2010-08-19 Thread Shashwat Anand 
On Thu, Aug 19, 2010 at 10:31 AM, Sahasranaman MS sah...@naman.ms wrote:

  On Thursday 19 August 2010 07:08 AM, Kenneth Gonsalves wrote:

 On Wed, 2010-08-18 at 10:28 -0700, Anand Shankar wrote:

 I have no clues. Any inputs??

 sort order of dictionary keys is not guaranteed. Only a list will return
 items in the same order as entered.

 Python has an OrderedDict class from Python 3.1
 http://docs.python.org/dev/whatsnew/3.1.html#pep-372-ordered-dictionaries
 For older versions of python,  there are several other implementations that
 you could use.


OrderedDict = BST
Dict = HashTable



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Re: [BangPypers] Dictionary : An elementary question

2010-08-19 Thread Jeffrey Jose 
On Thu, Aug 19, 2010 at 10:31 AM, Sahasranaman MS sah...@naman.ms wrote:

  On Thursday 19 August 2010 07:08 AM, Kenneth Gonsalves wrote:

 On Wed, 2010-08-18 at 10:28 -0700, Anand Shankar wrote:

 I have no clues. Any inputs??

 sort order of dictionary keys is not guaranteed. Only a list will return
 items in the same order as entered.

 Python has an OrderedDict class from Python 3.1
 http://docs.python.org/dev/whatsnew/3.1.html#pep-372-ordered-dictionaries
 For older versions of python,  there are several other implementations that
 you could use.



When I was writing my reply I was thinking  - wasnt there something added
recently which is a dictionary and was ordered. Unfortunately I'm stuck on
2.5, so havent used this one.
Thanks for this.

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Re: [BangPypers] Dictionary : An elementary question

2010-08-19 Thread Anand Shankar 




Thanks a Ton to all. I got a mucher deeper insight than my question deserved!!!

Thanks once again

anand



  
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Re: [BangPypers] Dictionary : An elementary question

2010-08-19 Thread Dipo Elegbede 
Hi All,
There really shouldn't be so much debate on the question asked.
Someone actually gave a direct and clear answer. I'm new at python and
his explanations were quite understandable.
As far as dictionaries are concerned, when you retrieve keys, there is
no guarantee of a particular order.
If you call the d.keys() consecutively, you might get two different orders.
However, to get it in an alphabetical order, there is a sort() method
to use. I am not too sure of the syntax but it should be something
like this: sort(d.keys()).
Please if I'm wrong point it out immediately to avoid transmitting wrong codes.
I hope it helps.
Regards,

On 8/19/10, Shashwat Anand anand.shash...@gmail.com wrote:
 On Wed, Aug 18, 2010 at 10:58 PM, Anand Shankar
 anand_shan...@yahoo.comwrote:

 During a tutorial python session with my colleagues I was presented with a
 basic
 question

  d = {'apple':2,'banana':5, 'coke': 6}
  print d.keys()
 ['coke', 'apple', 'banana']


 Question is why does it not return

 ['apple','banana','coke']

 Similarly:

  d = {'a':2,'b':4,'c':5,'d':4,'e':3}
  print d.keys()
 ['a', 'c', 'b', 'e', 'd']

 why not

 ['a', 'b', 'c', 'd', 'e']

 I have no clues. Any inputs??


 You will most likely want to read about how Hash-tables work.


 anand




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Re: [BangPypers] Dictionary : An elementary question

2010-08-19 Thread Praveen Kumar 
Here is a small script to sort the dictionary based on key/choice

a = {key3: 5 , key2: 8, key1: 2}
b = {key2: 7 , key1: 4, key3: 9}
c = {key1: 6 , key3: 1, key2: 1}
undecorated = [a, b, c] # how do you sort this list?

sort_on = key3
decorated = [(dict_[sort_on], dict_) for dict_ in [b]]
decorated.sort()
print decorated
print ---
result = [dict_ for (key, dict_) in decorated]
print result


Praveen Kumar
+91 9620621342
http://praveensunsetpoint.wordpress.com
Bangalore


On Thu, Aug 19, 2010 at 10:26 AM, Dipo Elegbede delegb...@dudupay.comwrote:

 Hi All,
 There really shouldn't be so much debate on the question asked.
 Someone actually gave a direct and clear answer. I'm new at python and
 his explanations were quite understandable.
 As far as dictionaries are concerned, when you retrieve keys, there is
 no guarantee of a particular order.
 If you call the d.keys() consecutively, you might get two different orders.
 However, to get it in an alphabetical order, there is a sort() method
 to use. I am not too sure of the syntax but it should be something
 like this: sort(d.keys()).
 Please if I'm wrong point it out immediately to avoid transmitting wrong
 codes.
 I hope it helps.
 Regards,

 On 8/19/10, Shashwat Anand anand.shash...@gmail.com wrote:
  On Wed, Aug 18, 2010 at 10:58 PM, Anand Shankar
  anand_shan...@yahoo.comwrote:
 
  During a tutorial python session with my colleagues I was presented with
 a
  basic
  question
 
   d = {'apple':2,'banana':5, 'coke': 6}
   print d.keys()
  ['coke', 'apple', 'banana']
 
 
  Question is why does it not return
 
  ['apple','banana','coke']
 
  Similarly:
 
   d = {'a':2,'b':4,'c':5,'d':4,'e':3}
   print d.keys()
  ['a', 'c', 'b', 'e', 'd']
 
  why not
 
  ['a', 'b', 'c', 'd', 'e']
 
  I have no clues. Any inputs??
 
 
  You will most likely want to read about how Hash-tables work.
 
 
  anand
 
 
 
 
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Re: [BangPypers] Dictionary : An elementary question

2010-08-19 Thread Jeffrey Jose 
On Thu, Aug 19, 2010 at 10:26 AM, Dipo Elegbede delegb...@dudupay.comwrote:

 Hi All,
 There really shouldn't be so much debate on the question asked.
 Someone actually gave a direct and clear answer. I'm new at python and
 his explanations were quite understandable.


I'm sorry that you sensed a debate in what happened to the question.
Personally I find it very refreshing when people contribute with different
ideas.
In this particular case, Shaswat Anand's reply of what OrdererdDicts are
(BSTs) was actually a new information to me. Had we closed the question the
moment we sense a right answer, I wouldnt have known that.


 If you call the d.keys() consecutively, you might get two different orders.


I dont think  it works the way you say it. d.keys() *should* return the same
way no matter how many times you call it. If the dictionary (d) changes in
between - because of the way HashTables/HashMaps are implemented - you are
bound to get in a different order.

*Long story short. *d.keys() will return the same thing if d doesnt change.


/jeff
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Re: [BangPypers] Dictionary : An elementary question

2010-08-19 Thread Dhananjay Nene 
Lots of good answers.

Warning: *this answer is ultra simplistic one to explain the implementation
succinctly*.

A little more from an implementation perspective dict operations usually
involve converting a dict into a hash. This hash is then converted into a
bucket. And sometimes when one gets into a collision, an alternative bucket
may need to get selected. In a simplistic example lets say we have 100
buckets, the hash is computed by adding the ascii values of the bytes
comprising the key.

Now lets assume the the key is the character 'AB'
Since the ascii code of 'A' is 65, and 'B' is 66 and there are no other
characters, then given our hash algorithm the hash will work out to 65 + 66
= 131

A simple hash to bucket mapping model is to use mod / modulo. Since 131 mod
100 which leads us to 31.

Assuming there are no key collisions and the buckets are stored as an array,
the key will now be stored in bucket no. 31 (zero based index). If there is
a collision - the key will be stored in some other bucket based on the
resolution provided by the collision management algorithm

Usually when one invokes a .keys() method, the most efficient way is to
run through the buckets array and spit out the keys stored in each non empty
bucket.

That should hopefully clarify why .keys() generally does not return keys
ordered by the keys themselves, it often will return keys ordered by the
buckets they are stored into.

Also since changing the internal logic of the hash table with regards to key
- hash - bucket mapping and collision management, will change the internal
order for the same set of keys.
Similarly even if the same keys are inserted into the hash table in
different orders, the resultant order of keys can change if some of the keys
collide.

As a result hash tables will generally not issue any guarantee any ordering
on the keys since that would require additional efforts and the implied
performance loss - something thats often unnecessary for most typical dict
operations.

Dhananjay

On Wed, Aug 18, 2010 at 10:58 PM, Anand Shankar anand_shan...@yahoo.comwrote:

 During a tutorial python session with my colleagues I was presented with a
 basic
 question

  d = {'apple':2,'banana':5, 'coke': 6}
  print d.keys()
 ['coke', 'apple', 'banana']


 Question is why does it not return

 ['apple','banana','coke']

 Similarly:

  d = {'a':2,'b':4,'c':5,'d':4,'e':3}
  print d.keys()
 ['a', 'c', 'b', 'e', 'd']

 why not

 ['a', 'b', 'c', 'd', 'e']

 I have no clues. Any inputs??

 anand




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Re: [BangPypers] Dictionary : An elementary question

2010-08-18 Thread Vivek Khurana 
On Wed, Aug 18, 2010 at 10:58 PM, Anand Shankar anand_shan...@yahoo.com wrote:
 During a tutorial python session with my colleagues I was presented with a 
 basic
 question

 d = {'apple':2,'banana':5, 'coke': 6}
 print d.keys()
 ['coke', 'apple', 'banana']


 Question is why does it not return

 ['apple','banana','coke']

 Similarly:

 d = {'a':2,'b':4,'c':5,'d':4,'e':3}
 print d.keys()
 ['a', 'c', 'b', 'e', 'd']

 why not

 ['a', 'b', 'c', 'd', 'e']

 I have no clues. Any inputs??

 Quoting from python docs[1]

  The keys() method of a dictionary object returns a list of all the
keys used in the dictionary, in arbitrary order (if you want it
sorted, just apply the sort() method to the list of keys). 

[1]: http://docs.python.org/tutorial/datastructures.html#dictionaries

regards
Vivek
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Re: [BangPypers] Dictionary : An elementary question

2010-08-18 Thread Vinay Shastry 
On 18 August 2010 22:58, Anand Shankar anand_shan...@yahoo.com wrote:
 During a tutorial python session with my colleagues I was presented with a 
 basic
 question

 d = {'apple':2,'banana':5, 'coke': 6}
 print d.keys()
 ['coke', 'apple', 'banana']


 Question is why does it not return

 ['apple','banana','coke']
snip
 I have no clues. Any inputs??


For this reason:
It is best to think of a dictionary as an unordered set of key: value pairs

http://docs.python.org/tutorial/datastructures.html#dictionaries

 d = {'apple':2,'banana':5, 'coke': 6}
 d
{'coke': 6, 'apple': 2, 'banana': 5}
 print d.keys()
['coke', 'apple', 'banana']

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Re: [BangPypers] Dictionary : An elementary question

2010-08-18 Thread Jeffrey Jose 
Excellent responses so far.

Dictionaries are optimized for retrieving key/value pairs. And to achieve
that, it compromises on the order in which stuff is stored.

This and more is very nicely presented in the Pycon 2010 talk - The Mighty
Dictionary. Highly recommended.

http://us.pycon.org/2010/conference/schedule/event/12/

http://us.pycon.org/2010/conference/schedule/event/12//jeff

On Wed, Aug 18, 2010 at 11:04 PM, Vinay Shastry vinayshas...@gmail.comwrote:

 On 18 August 2010 22:58, Anand Shankar anand_shan...@yahoo.com wrote:
  During a tutorial python session with my colleagues I was presented with
 a basic
  question
 
  d = {'apple':2,'banana':5, 'coke': 6}
  print d.keys()
  ['coke', 'apple', 'banana']
 
 
  Question is why does it not return
 
  ['apple','banana','coke']
 snip
  I have no clues. Any inputs??
 

 For this reason:
 It is best to think of a dictionary as an unordered set of key: value
 pairs

 http://docs.python.org/tutorial/datastructures.html#dictionaries

  d = {'apple':2,'banana':5, 'coke': 6}
  d
 {'coke': 6, 'apple': 2, 'banana': 5}
  print d.keys()
 ['coke', 'apple', 'banana']

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Re: [BangPypers] Dictionary : An elementary question

2010-08-18 Thread Shashwat Anand 
On Wed, Aug 18, 2010 at 10:58 PM, Anand Shankar anand_shan...@yahoo.comwrote:

 During a tutorial python session with my colleagues I was presented with a
 basic
 question

  d = {'apple':2,'banana':5, 'coke': 6}
  print d.keys()
 ['coke', 'apple', 'banana']


 Question is why does it not return

 ['apple','banana','coke']

 Similarly:

  d = {'a':2,'b':4,'c':5,'d':4,'e':3}
  print d.keys()
 ['a', 'c', 'b', 'e', 'd']

 why not

 ['a', 'b', 'c', 'd', 'e']

 I have no clues. Any inputs??


You will most likely want to read about how Hash-tables work.


 anand




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Re: [BangPypers] Dictionary : An elementary question

2010-08-18 Thread Sahasranaman MS 

 On Thursday 19 August 2010 07:08 AM, Kenneth Gonsalves wrote:

On Wed, 2010-08-18 at 10:28 -0700, Anand Shankar wrote:

I have no clues. Any inputs??

sort order of dictionary keys is not guaranteed. Only a list will return
items in the same order as entered.
Python has an OrderedDict class from Python 3.1  
http://docs.python.org/dev/whatsnew/3.1.html#pep-372-ordered-dictionaries
For older versions of python,  there are several other implementations 
that you could use.

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