Dictionary bidirectional

[Kless]

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!

[WDC]

Re: Dictionary bidirectional

On Jul 13, 4:21 pm, Kless <jonas....@goog lemail.comwrote :

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?

[bukzor]

Re: Dictionary bidirectional

On Jul 13, 4:21 pm, Kless <jonas....@goog lemail.comwrote :

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

[bearophileHUGS@lycos.com]

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

[Larry Bates]

Re: Dictionary bidirectional

bearophileHUGS@ lycos.com 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

[bukzor]

Re: Dictionary bidirectional

On Jul 13, 6:53 pm, Larry Bates <larry.ba...@we bsafe.com`wrote :

bearophileH...@ lycos.com 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

[Kless]

Re: Dictionary bidirectional

But in my dictionary both keys and values are unique.

On Jul 14, 7:34 am, Dennis Lee Bieber <wlfr...@ix.net com.comwrote:

On Sun, 13 Jul 2008 16:21:11 -0700 (PDT), Kless
<jonas....@goog lemail.comdecla imed the following in comp.lang.pytho n:
>

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.

[Ken Starks]

Re: Dictionary bidirectional

Dennis Lee Bieber wrote:

On Sun, 13 Jul 2008 16:21:11 -0700 (PDT), Kless
<jonas.esp@goog lemail.comdecla imed the following in comp.lang.pytho n:
>

>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.

[bearophileHUGS@lycos.com]

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 = 1000000
M = 2 * N

data = dict.fromkeys(x range(N), 0)
to_delete = range(M)
shuffle(to_dele te)

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

[Kless]

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 MyCoolDictionar y(dict):
def __init__(self, *args, **kargs):
dict.__init__(s elf, *args, **kargs)

def __getitem__(sel f, 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=MyCoolDi ctionary({"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
-------------

[Impotent Verse]

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("VI II") ]

--------------

.... 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.

[Gerry]

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 <simon.pever... @gmail.comwrote :

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("VI II") ]
>
--------------
>
... 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.