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intersection, union, difference, symmetric difference for dictionaries

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  intersection, union, difference, symmetric difference for dictionaries mauro <mauro@gmail.com> - 2014-02-25 20:32 +0000
    Re: intersection, union, difference, symmetric difference for dictionaries Nick Timkovich <prometheus235@gmail.com> - 2014-02-25 14:37 -0600
    Re: intersection, union, difference, symmetric difference for dictionaries Skip Montanaro <skip@pobox.com> - 2014-02-25 14:40 -0600
    Re: intersection, union, difference, symmetric difference for dictionaries Peter Otten <__peter__@web.de> - 2014-02-25 21:53 +0100
    Re: intersection, union, difference, symmetric difference for dictionaries Peter Otten <__peter__@web.de> - 2014-02-25 21:58 +0100
    Re: intersection, union, difference, symmetric difference for dictionaries Tim Chase <python.list@tim.thechases.com> - 2014-02-25 15:03 -0600
      Re: intersection, union, difference, symmetric difference for dictionaries Duncan Booth <duncan.booth@invalid.invalid> - 2014-02-25 22:21 +0000
        Re: intersection, union, difference, symmetric difference for dictionaries Tim Chase <tim@thechases.com> - 2014-02-25 16:35 -0600
        Re: intersection, union, difference, symmetric difference for dictionaries Tim Chase <python.list@tim.thechases.com> - 2014-02-25 16:36 -0600
        Re: intersection, union, difference, symmetric difference for dictionaries Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2014-02-25 22:58 +0000
      Re: intersection, union, difference, symmetric difference for dictionaries Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-25 23:10 +0000
        Re: intersection, union, difference, symmetric difference for dictionaries Tim Chase <python.list@tim.thechases.com> - 2014-02-25 20:21 -0600
    Re: intersection, union, difference, symmetric difference for dictionaries Ben Finney <ben+python@benfinney.id.au> - 2014-02-26 08:27 +1100
    Re:intersection, union, difference, symmetric difference for dictionaries Dave Angel <davea@davea.name> - 2014-02-25 16:35 -0500
    Re: intersection, union, difference, symmetric difference for dictionaries Peter Otten <__peter__@web.de> - 2014-02-25 22:54 +0100
    Re: intersection, union, difference, symmetric difference for dictionaries mauro <mauro@gmail.com> - 2014-02-25 22:02 +0000
    Re: intersection, union, difference, symmetric difference for dictionaries mauro <mauro@gmail.com> - 2014-02-25 22:03 +0000
    Re: intersection, union, difference, symmetric difference for dictionaries Tim Chase <python.list@tim.thechases.com> - 2014-02-25 16:10 -0600
    Re: intersection, union, difference, symmetric difference for dictionaries mauro <mauro@gmail.com> - 2014-02-25 22:11 +0000
      Re: intersection, union, difference, symmetric difference for dictionaries Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-02-25 22:35 +0000
    Re: intersection, union, difference, symmetric difference for dictionaries MRAB <python@mrabarnett.plus.com> - 2014-02-25 23:07 +0000
      Re: intersection, union, difference, symmetric difference for dictionaries Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-02-26 00:37 +0000
    Re: intersection, union, difference, symmetric difference for dictionaries Ben Finney <ben+python@benfinney.id.au> - 2014-02-26 10:14 +1100
    Re: intersection, union, difference, symmetric difference for dictionaries MRAB <python@mrabarnett.plus.com> - 2014-02-25 23:25 +0000
    Re: intersection, union, difference, symmetric difference for dictionaries Grant Edwards <invalid@invalid.invalid> - 2014-02-25 20:44 +0000
    Re: intersection, union, difference, symmetric difference for dictionaries John Gordon <gordon@panix.com> - 2014-02-25 20:44 +0000
      Re: intersection, union, difference, symmetric difference for dictionaries Chris Angelico <rosuav@gmail.com> - 2014-03-02 09:29 +1100

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#67074

On 2014-02-25 21:27, Ben Finney wrote:
> Peter Otten <__peter__@web.de> writes:
>
>> mauro wrote:
>>
>> > - Dictionaries and sets are both accessed by key
>>
>> but sets have no values
>
> Or rather, sets *only* have values. Dictionaries have keys, sets do not
> have keys.
>
But a dictionary can have duplicate values, a set cannot.

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#67079

On Tue, 25 Feb 2014 23:07:28 +0000, MRAB wrote:

> On 2014-02-25 21:27, Ben Finney wrote:

>> Or rather, sets *only* have values. Dictionaries have keys, sets do not
>> have keys.
>>
> But a dictionary can have duplicate values, a set cannot.

It is usual to talk about the things stored in dicts and sets using 
slightly different terminology:

  - mappings such as dicts have *key/value pairs*

  - sets have *elements*

The elements of a set are most closely related to the keys of a mapping, 
not the values. Set elements are unique, just like mapping keys. 
Specifically, the keys in a mapping make up a set; the values in a 
mapping do not.

When talking specifically about Python dicts and sets, there is an 
additional restriction: both set elements and dict keys have to be 
hashable.

English being a little sloppy at times, sometimes we also talk about the 
*values of a set*, but here "value" is being used as a synonym for 
"element" and not as a technical term for the thing which mappings 
associate with a key.



-- 
Steven

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#67076

MRAB <python@mrabarnett.plus.com> writes:

> On 2014-02-25 21:27, Ben Finney wrote:
> > Peter Otten <__peter__@web.de> writes:
> >
> >> mauro wrote:
> >>
> >> > - Dictionaries and sets are both accessed by key
> >>
> >> but sets have no values
> >
> > Or rather, sets *only* have values. Dictionaries have keys, sets do
> > not have keys.
> >
> But a dictionary can have duplicate values, a set cannot.

Yes. Your “but” implies you think that contradicts my statement; it
doesn't. So I'm not sure what point you're making.

-- 
 \         “Alternative explanations are always welcome in science, if |
  `\   they are better and explain more. Alternative explanations that |
_o__) explain nothing are not welcome.” —Victor J. Stenger, 2001-11-05 |
Ben Finney

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#67077

On 2014-02-25 23:14, Ben Finney wrote:
> MRAB <python@mrabarnett.plus.com> writes:
>
>> On 2014-02-25 21:27, Ben Finney wrote:
>> > Peter Otten <__peter__@web.de> writes:
>> >
>> >> mauro wrote:
>> >>
>> >> > - Dictionaries and sets are both accessed by key
>> >>
>> >> but sets have no values
>> >
>> > Or rather, sets *only* have values. Dictionaries have keys, sets do
>> > not have keys.
>> >
>> But a dictionary can have duplicate values, a set cannot.
>
> Yes. Your “but” implies you think that contradicts my statement; it
> doesn't. So I'm not sure what point you're making.
>
The keys of a dictionary must be unique, like a set. The values of a
dictionary don't have to be unique, unlike a set.

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#67359

On 2014-02-25, mauro <mauro@gmail.com> wrote:
>  Dictionaries and sets share a few properties:
> - Dictionaries keys are unique as well as sets items
> - Dictionaries and sets are both unordered
> - Dictionaries and sets are both accessed by key
> - Dictionaries and sets are both mutables
>
> So I wonder why operations such us intersection, union, difference, 
> symmetric difference that are available for sets and are not available 
> for dictionaries without going via key dictviews. 

What would be the definition of the union, intersection, and
difference of these two dicts?

  {1:'blue', 2:'red'}
  {1:'green', 3:'yellow'}

-- 
Grant Edwards               grant.b.edwards        Yow! I smell like a wet
                                  at               reducing clinic on Columbus
                              gmail.com            Day!

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#67360

In <G57Pu.24239$Th2.4990@tornado.fastwebnet.it> mauro <mauro@gmail.com> writes:

> - Dictionaries and sets are both accessed by key

As far as I have used sets, they are not accessed by key.

>>> x = set([1, 2, 'buckle my shoe'])
>>> x
set([1, 2, 'buckle my shoe'])
>>> 1 in x
True
>>> 5 in x
False

-- 
John Gordon         Imagine what it must be like for a real medical doctor to
gordon@panix.com    watch 'House', or a real serial killer to watch 'Dexter'.

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#67382

On Wed, Feb 26, 2014 at 7:44 AM, John Gordon <gordon@panix.com> wrote:
> In <G57Pu.24239$Th2.4990@tornado.fastwebnet.it> mauro <mauro@gmail.com> writes:
>
>> - Dictionaries and sets are both accessed by key
>
> As far as I have used sets, they are not accessed by key.
>
>>>> x = set([1, 2, 'buckle my shoe'])
>>>> x
> set([1, 2, 'buckle my shoe'])
>>>> 1 in x
> True
>>>> 5 in x
> False

>>> x = {1:3, 2:4, 'buckle my shoe':'spamming the door'}
>>> x
{1: 3, 2: 4, 'buckle my shoe': 'spamming the door'}
>>> 1 in x
True
>>> 5 in x
False

A dict does the exact same thing with its keys as a set does with its
elements. Actually, one of the things that periodically trips me up
when I switch from Pike to Python is that a Python set can't be used
like this:

>>> x = set()
>>> x["test"] = 1
>>> x["foo"] = 1
>>> assert x == {"test","foo"}

Instead, I have to use x.add("test"). In Pike, I can treat a set as if
it were a mapping where every value is simply the integer 1 (Python
could use True instead). Setting it to any truthy value would add it,
setting to any falsy value would remove it. This wouldn't be a huge
change; it certainly wouldn't fundamentally change the set type -
because it *is* working with keys.

Hmm. Actually, it shouldn't be too hard to subclass and add that. Lessee.

class set(set):
    def __setitem__(self, key, state):
        if state: self.add(key)
        else: self.remove(key)
    def __getitem__(self, key):
        return key in self

I might need to toss that into site.py or something. It'd work as long
as I don't use set literal notation anywhere.

ChrisA

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