Re: Dictionary bidirectional
If keys and values are unique, maybe just store both in the same
dictionary:
mydict[a] = b
mydict[b] = a
...
Gerry
On Jul 14, 8:31 am, Impotent Verse <[EMAIL PROTECTED]> wrote:
> If keys and values are unique you could do this...
>
> --
>
> # Left : Right
> roman = { "One" : "I",
> "Two" : "II",
> "Three" : "III",
> "Four" : "IV",
> "Five" : "V",
> "Six" : "VI",
> "Seven" : "VII",
> "Eight" : "VIII",
> "Nine" : "IX",
> "Ten" : "X" }
>
> left, right = zip( *roman.items() )
> left = list(left)
> right = list(right)
>
> print left[ right.index("VIII") ]
>
> --
>
> ... result is "Eight".
>
> Hmmm! zip returns tuples, which need to be turned into lists to do
> much with. Maybe not the best solution in this case.
>
> Verse.
--
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Re: Dictionary bidirectional
This is a better version of the class that I sen't you, now it raises a KeyError when you try to insert a value that is already in class MyCoolDictionary(dict): def __init__(self, *args, **kargs): dict.__init__(self, *args, **kargs) def __setitem__(self, *args, **kargs): for i in dictio.values(): if args[0]==i or args[1]==i: raise KeyError, 'Value already exists' dict.__setitem__(self, *args, **kargs) def __getitem__(self, item): try: return dict.__getitem__(self, item) except KeyError: keys=[] for i in dictio.keys(): if dictio[i]==item: keys.append(i) if not keys: raise KeyError, 'Can\'t found key or value "' + str(item) + '"' return keys Feel free to change anything you want or need Regards, Akathorn Greyhat 2008/7/14 Kless <[EMAIL PROTECTED]>: > I need a dictionary where get the result from a 'key' (on left), but > also from a 'value' (on right), how to get it? > > I know that dictionaries aren't bidirectional, but is there any way > without use two dictionaries? > > > Thanks in advance! > -- > http://mail.python.org/mailman/listinfo/python-list > -- http://mail.python.org/mailman/listinfo/python-list
Re: Dictionary bidirectional
If keys and values are unique you could do this...
--
# Left : Right
roman = { "One" : "I",
"Two" : "II",
"Three" : "III",
"Four": "IV",
"Five": "V",
"Six" : "VI",
"Seven" : "VII",
"Eight" : "VIII",
"Nine": "IX",
"Ten" : "X" }
left, right = zip( *roman.items() )
left = list(left)
right = list(right)
print left[ right.index("VIII") ]
--
... result is "Eight".
Hmmm! zip returns tuples, which need to be turned into lists to do
much with. Maybe not the best solution in this case.
Verse.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Dictionary bidirectional
Akathorn Greyhat sent me by email the next solution, which althought
isn't generic it works well:
-
You could make your own class for that, maybe something like
#
class MyCoolDictionary(dict):
def __init__(self, *args, **kargs):
dict.__init__(self, *args, **kargs)
def __getitem__(self, item):
try:
return dict.__getitem__(self, item)
except KeyError:
keys=[]
for i in dictio.keys():
if dictio[i]==item:
keys.append(i)
return keys
dictio=MyCoolDictionary({"a" : 1, "b" : 2, "c" : 2})
print dictio["a"]
print dictio["b"]
print dictio[1]
print dictio[2]
#
The output of this code is:
1
2
['a']
['c', 'b']
Note that it isn't finish, maybe you'll need to make some kind of test
before adding a new value because with this code one value can have
multiple keys, and I have fixed it by returning a list of keys instead
a single value. It's up to you =)
I'm sorry of my poor english =(
Regards,
Akathorn Greyhat
-
--
http://mail.python.org/mailman/listinfo/python-list
Re: Dictionary bidirectional
Larry Bates: > The only case where it would be faster would be if most of the keys were NOT > in > the dictionary (rather odd use case). Otherwise I believe you will find the > first way quicker as the exceptions are infrequent. I have written a small benchmark: from random import shuffle def test1(data, to_delete): for item in to_delete: try: del data[item] except KeyError: pass def test2(data, to_delete): for item in to_delete: if item in data: del data[item] N = 100 M = 2 * N data = dict.fromkeys(xrange(N), 0) to_delete = range(M) shuffle(to_delete) from timeit import default_timer as clock t = clock() #test1(data, to_delete) # 2.4 s test2(data, to_delete) # 0.8 s print round(clock() - t, 2), "s" It creates a dictionary of the first million integers, and then tries to delete the first two million of integers. So about 1/2 numbers are present to be deleted. In this situation the version with the try- except seems about 3 times slower than the other. Bye, bearophile -- http://mail.python.org/mailman/listinfo/python-list
Re: Dictionary bidirectional
Dennis Lee Bieber wrote:
On Sun, 13 Jul 2008 16:21:11 -0700 (PDT), Kless
<[EMAIL PROTECTED]> declaimed the following in comp.lang.python:
I need a dictionary where get the result from a 'key' (on left), but
also from a 'value' (on right), how to get it?
I know that dictionaries aren't bidirectional, but is there any way
without use two dictionaries?
Just out of curiosity... What do you expect to have returned from...
aDict = { "one" : "two",
"three" : "four",
"What?" : "two" }
when looking for the value "two"?
In a dictionary, the /keys/ are unique... but the /values/ can be
duplicates.
I wonder if anyone has implemented an 'equivalence class' class (for
finite sets) based on this.
Obviously the relation defined by
k1~k2 iff D[k1] == D[k2]
does partition the set of all keys as an equivalence class.
So ... as a kind of inverse you could return a set, a subset of the
keys. How you would get a canonical representative of that set is
a different matter, of course. Unless, as in the OP's case, it is
a singleton set.
It would seem more efficient to do this when a key-value pair is
added or removed from the original dictionary rather than iterating
over all the keys each time you used it.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Dictionary bidirectional
But in my dictionary both keys and values are unique.
On Jul 14, 7:34 am, Dennis Lee Bieber <[EMAIL PROTECTED]> wrote:
> On Sun, 13 Jul 2008 16:21:11 -0700 (PDT), Kless
> <[EMAIL PROTECTED]> declaimed the following in comp.lang.python:
>
> > I need a dictionary where get the result from a 'key' (on left), but
> > also from a 'value' (on right), how to get it?
>
> > I know that dictionaries aren't bidirectional, but is there any way
> > without use two dictionaries?
>
> Just out of curiosity... What do you expect to have returned from...
>
> aDict = { "one" : "two",
> "three" : "four",
> "What?" : "two" }
>
> when looking for the value "two"?
>
> In a dictionary, the /keys/ are unique... but the /values/ can be
> duplicates.
--
http://mail.python.org/mailman/listinfo/python-list
Re: Dictionary bidirectional
On Jul 13, 6:53 pm, Larry Bates <[EMAIL PROTECTED]> wrote: > [EMAIL PROTECTED] wrote: > > bukzor: > >> You need to use two dictionaries. Here's a class that someone's > >> written that wraps it up into a single dict-like object for you: > >>http://www.faqts.com/knowledge_base/view.phtml/aid/4376 > > > It contains code like: > > > try: > > del self.data[item] > > except KeyError: > > pass > > > Exceptions are useful in python, but with dictionaries this is > > probably faster (and shorter), even if it may perform two lookups: > > > if item in self.data: > > del self.data[item] > > > Bye, > > bearophile > > The only case where it would be faster would be if most of the keys were NOT > in > the dictionary (rather odd use case). Otherwise I believe you will find the > first way quicker as the exceptions are infrequent. > > -Larry /agree -- http://mail.python.org/mailman/listinfo/python-list
Re: Dictionary bidirectional
-- Forwarded message --
From: Akathorn Greyhat <[EMAIL PROTECTED]>
Date: 2008/7/14
Subject: Re: Dictionary bidirectional
To: Kless <[EMAIL PROTECTED]>
2008/7/14 Kless <[EMAIL PROTECTED]>:
I need a dictionary where get the result from a 'key' (on left), but
> also from a 'value' (on right), how to get it?
>
> I know that dictionaries aren't bidirectional, but is there any way
> without use two dictionaries?
>
>
> Thanks in advance!
> --
> http://mail.python.org/mailman/listinfo/python-list
>
You could make your own class for that, maybe something like
#
class MyCoolDictionary(dict):
def __init__(self, *args, **kargs):
dict.__init__(self, *args, **kargs)
def __getitem__(self, item):
try:
return dict.__getitem__(self, item)
except KeyError:
keys=[]
for i in dictio.keys():
if dictio[i]==item:
keys.append(i)
return keys
dictio=MyCoolDictionary({"a" : 1, "b" : 2, "c" : 2})
print dictio["a"]
print dictio["b"]
print dictio[1]
print dictio[2]
#
The output of this code is:
1
2
['a']
['c', 'b']
Note that it isn't finish, maybe you'll need to make some kind of test
before adding a new value because with this code one value can have multiple
keys, and I have fixed it by returning a list of keys instead a single
value. It's up to you =)
I'm sorry of my poor english =(
Regards,
Akathorn Greyhat
--
http://mail.python.org/mailman/listinfo/python-list
Re: Dictionary bidirectional
[EMAIL PROTECTED] wrote: bukzor: You need to use two dictionaries. Here's a class that someone's written that wraps it up into a single dict-like object for you: http://www.faqts.com/knowledge_base/view.phtml/aid/4376 It contains code like: try: del self.data[item] except KeyError: pass Exceptions are useful in python, but with dictionaries this is probably faster (and shorter), even if it may perform two lookups: if item in self.data: del self.data[item] Bye, bearophile The only case where it would be faster would be if most of the keys were NOT in the dictionary (rather odd use case). Otherwise I believe you will find the first way quicker as the exceptions are infrequent. -Larry -- http://mail.python.org/mailman/listinfo/python-list
Re: Dictionary bidirectional
bukzor: > You need to use two dictionaries. Here's a class that someone's > written that wraps it up into a single dict-like object for you: > http://www.faqts.com/knowledge_base/view.phtml/aid/4376 It contains code like: try: del self.data[item] except KeyError: pass Exceptions are useful in python, but with dictionaries this is probably faster (and shorter), even if it may perform two lookups: if item in self.data: del self.data[item] Bye, bearophile -- http://mail.python.org/mailman/listinfo/python-list
Re: Dictionary bidirectional
On Jul 13, 4:21 pm, Kless <[EMAIL PROTECTED]> wrote: > I need a dictionary where get the result from a 'key' (on left), but > also from a 'value' (on right), how to get it? > > I know that dictionaries aren't bidirectional, but is there any way > without use two dictionaries? > > Thanks in advance! You need to use two dictionaries. Here's a class that someone's written that wraps it up into a single dict-like object for you: http://www.faqts.com/knowledge_base/view.phtml/aid/4376 -- http://mail.python.org/mailman/listinfo/python-list
Re: Dictionary bidirectional
On Jul 13, 4:21 pm, Kless <[EMAIL PROTECTED]> wrote: > I need a dictionary where get the result from a 'key' (on left), but > also from a 'value' (on right), how to get it? > > I know that dictionaries aren't bidirectional, but is there any way > without use two dictionaries? > > Thanks in advance! You want to print the key as well as the value? -- http://mail.python.org/mailman/listinfo/python-list
Dictionary bidirectional
I need a dictionary where get the result from a 'key' (on left), but also from a 'value' (on right), how to get it? I know that dictionaries aren't bidirectional, but is there any way without use two dictionaries? Thanks in advance! -- http://mail.python.org/mailman/listinfo/python-list
