Please help on this sorted function

[fl]

Hi,

I try to learn sorted(). With the tutorial example:




>>> ff=sorted({1: 'D', 2: 'B', 3: 'B', 4: 'E', 5: 'A'})
>>> ff
[1, 2, 3, 4, 5]



I don't see what sorted does in this dictionary, i.e. the sequence of
1..5 is unchanged. Could you explain it to me?


Thanks,

[fl]

On Tuesday, June 2, 2015 at 1:20:40 PM UTC-7, fl wrote:

> Hi,
>
> I try to learn sorted(). With the tutorial example:
>
>
>
>
> >>> ff=sorted({1: 'D', 2: 'B', 3: 'B', 4: 'E', 5: 'A'})
> >>> ff
> [1, 2, 3, 4, 5]
>
>
>
> I don't see what sorted does in this dictionary, i.e. the sequence of
> 1..5 is unchanged. Could you explain it to me?
>
>
> Thanks,

Excuse me. After a small modification, it can see the effect.


>>> ff=sorted({1: 'D', 2: 'B', 5: 'B', 4: 'E', 3: 'A'})
>>> ff
[1, 2, 3, 4, 5]


I am still new to Python. How to get the sorted dictionary output:

{1: 'D', 2: 'B', 3: 'A', 4: 'E', 5: 'B'}

[Joonas Liik]

>>> ff=sorted({1: 'D', 2: 'B', 3: 'B', 4: 'E', 5: 'A'})
>>> ff
[1, 2, 3, 4, 5]

sorted({1: 'D', 2: 'B', 3: 'B', 4: 'E', 5: 'A'}) is equivalent to
sorted(iter({1: 'D', 2: 'B', 3: 'B', 4: 'E', 5: 'A'}))

and iter(dict) iterates over the dict keys, so when you do
iter({1: 'D', 2: 'B', 3: 'B', 4: 'E', 5: 'A'}) you essentially get
[1,2,3,4,5]
and sorted([1,2,3,4,5]) returns [1,2,3,4,5]
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[Joonas Liik]

my_dict = {1: 'D', 2: 'B', 3: 'A', 4: 'E', 5: 'B'}

# dict.items() returns an iterator that returns pairs of (key, value) pairs
# the key argument to sorted tells sorted what to sort by,
operator.itemgetter is a factory function , itemgetter(1)== lambda
iterable: iterable[1]
sorted_dict = sorted(my_dict.items(), key=itemgetter(1))

# at this moment sorted dict is a generator of key-value tuples in the
right order
sorted_dict = OrderedDict(sorted_dict) # turn the generator in to an actual
dict.

# notice: regular dicts are NOT ORDERED, you need a special type of dict to
preserve the order, hence OrderedDict
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[Gary Herron-2]

On 06/02/2015 01:20 PM, fl wrote:

> Hi,
>
> I try to learn sorted(). With the tutorial example:
>
>
>
>
>>>> ff=sorted({1: 'D', 2: 'B', 3: 'B', 4: 'E', 5: 'A'})
>>>> ff
> [1, 2, 3, 4, 5]
>
>
>
> I don't see what sorted does in this dictionary, i.e. the sequence of
> 1..5 is unchanged. Could you explain it to me?
>
>
> Thanks,

It's best to think of dictionaries as unordered collections of key/value
pairs.  Dictionaries are not sequences, do not have any particular
ordering, and in full generality *can't* be sorted in any sensible way.

For instance, this slightly odd (but perfectly legal) dictionary
     >>> d = {'a':123, 456:'b'}
can't be sorted
     >>> sorted(d)
     Traceback (most recent call last):
       File "<stdin>", line 1, in <module>
     TypeError: unorderable types: int() < str()
because it doesn't make sense to order/compare the two keys 'a' and 456.

If your dictionary is a little better behaved, say
     >>> d = {'a':123, 'b':456}
you may be able to sort the keys
     >>> sorted(d)
     ['a', 'b']
or the values
     >>> sorted(d.values())
     [123, 456]
or the key/value tuples (called items)
     >>> sorted(d.items())
     [('a', 123), ('b', 456)]
but each of those attempts to sort could fail on a general dictionary if
the individual keys or values are not sortable.

There is also an implementation of a type of dictionary that remembers
the order in which the items are *inserted*.  It's in the collections
module and called OrderedDict.


Gary Herron






--
Dr. Gary Herron
Department of Computer Science
DigiPen Institute of Technology
[phone]

[Chris Angelico]

On Wed, Jun 3, 2015 at 6:25 AM, fl <rxjwg98 at gmail.com> wrote:
> I am still new to Python. How to get the sorted dictionary output:
>
> {1: 'D', 2: 'B', 3: 'A', 4: 'E', 5: 'B'}

Since dictionaries don't actually have any sort of order to them, the
best thing to do is usually to simply display it in order. And there's
a very handy function for doing that: a pretty-printer.

>>> import pprint
>>> pprint.pprint({1: 'D', 2: 'B', 5: 'B', 4: 'E', 3: 'A'})
{1: 'D', 2: 'B', 3: 'A', 4: 'E', 5: 'B'}

This one comes with Python, so you can use it as easily as that above
example. Or you could do it this way:

>>> from pprint import pprint
>>> pprint({1: 'D', 2: 'B', 5: 'B', 4: 'E', 3: 'A'})
{1: 'D', 2: 'B', 3: 'A', 4: 'E', 5: 'B'}

For a lot of Python data structures, this will give you a tidy and
human-readable display.

ChrisA

[Steven D'Aprano-11]

On Wednesday 03 June 2015 06:42, Joonas Liik wrote:

> my_dict = {1: 'D', 2: 'B', 3: 'A', 4: 'E', 5: 'B'}
>
> # dict.items() returns an iterator that returns pairs of (key, value)
> # pairs the key argument to sorted tells sorted what to sort by,
> operator.itemgetter is a factory function , itemgetter(1)== lambda
> iterable: iterable[1]
> sorted_dict = sorted(my_dict.items(), key=itemgetter(1))
>
> # at this moment sorted dict is a generator of key-value tuples in the
> right order
> sorted_dict = OrderedDict(sorted_dict) # turn the generator in to an
> actual dict.
>
> # notice: regular dicts are NOT ORDERED, you need a special type of dict
> # to
> preserve the order, hence OrderedDict

OrderedDicts preserve the *insertion order*, they don't sort the keys.


Ordinary dicts are unordered. The order you see is arbitrary and
unpredictable:

py> d = {}
py> d['C'] = 1; d['A'] = 2; d['B'] = 3
py> d
{'A': 2, 'C': 1, 'B': 3}


Ordered dicts are ordered by insertion order:

py> from collections import OrderedDict
py> d = OrderedDict()
py> d['C'] = 1; d['A'] = 2; d['B'] = 3
py> d
OrderedDict([('C', 1), ('A', 2), ('B', 3)])


Python doesn't have a SortedDict, where the keys are kept in sorted order.



--
Steve