'inverting' a dict

**anton muhin** · Jul 18 '05, 07:11 AM

Re: 'inverting' a dict

Irmen de Jong wrote:[color=blue]
> Hi
> I have this dict that maps a name to a sequence of other names.
> I want to have it reversed, i.e., map the other names each to
> the key they belong to (yes, the other names are unique and
> they only occur once). Like this:
>
> { "key1": ("value1", "value2"), "key2": ("value3,) }
>
> -->
>
> { "value1": "key1", "value2": "key1", "value3": "key2" }
>
> What I'm doing is using a nested loop:
>
> dict2={}
> for (key,value) in dict1.items():
> for name in value:
> dict2[name] = key
>
> which is simple enough, but I'm hearing this little voice in
> the back of my head saying "there's a simpler solution".
> Is there? What is it? ;-)
>
> Thanks
> --Irmen.
>[/color]

It's seems like rather nice Python for me ;). Two more variants:

inverted = {}
for k, vs in d.iteritems():
inverted.update (dict([(v, k) for v in vs]))

map(
lambda (k, vs): inverted.update (dict([(v, k) for v in vs])),
d.iteritems()
)

happy new year,
anton.

**engsolnom@ipns.com** · Jul 18 '05, 07:11 AM

Re: 'inverting' a dict

On Tue, 30 Dec 2003 18:30:56 +0100, Irmen de Jong
<irmen@-NOSPAM-REMOVETHIS-xs4all.nl> wrote:
[color=blue]
>Hi
>I have this dict that maps a name to a sequence of other names.
>I want to have it reversed, i.e., map the other names each to
>the key they belong to (yes, the other names are unique and
>they only occur once). Like this:
>
>{ "key1": ("value1", "value2"), "key2": ("value3,) }
>
>-->
>
>{ "value1": "key1", "value2": "key1", "value3": "key2" }
>
>What I'm doing is using a nested loop:
>
>dict2={}
>for (key,value) in dict1.items():
> for name in value:
> dict2[name] = key
>
>which is simple enough, but I'm hearing this little voice in
>the back of my head saying "there's a simpler solution".
>Is there? What is it? ;-)
>
>Thanks
>--Irmen.[/color]

I had a need for a 'reverse' dictionary also. This is how I approached
it:

source_dict = {'A': 1,'B': 2,'C': 3,'D': 4}
target_dict = {}
for item in source_dict.ite ritems():
target_dict[item[1]] = item[0]

Or, a bit more contrived....

source_dict = {'A': [[1,2,3],100],'B': [[4,5,6],200],'C':
[[7,8,9],300]}
target_dict = {}
for item in source_dict.ite ritems():
target_dict[item[1][1]] = item[0]

Disclaimer: I'm a newbie
Norm

**Yermat** · Jul 18 '05, 07:11 AM

Re: 'inverting' a dict

or even shorter[color=blue][color=green][color=darkred]
>>> d = {'key1' : ('value1','valu e2'), 'key2': ('value3',) }[/color][/color][/color]
[color=blue][color=green][color=darkred]
>>> dict([(v,k) for k in d.iterkeys() for v in d[k]])[/color][/color][/color]
{'value3': 'key2', 'value2': 'key1', 'value1': 'key1'}

"anton muhin" <antonmuhin@ram bler.ru> a écrit dans le message de
news:bssgf2$10n t3$1@ID-217427.news.uni-berlin.de[color=blue]
> Irmen de Jong wrote:[color=green]
>> Hi
>> I have this dict that maps a name to a sequence of other names.
>> I want to have it reversed, i.e., map the other names each to
>> the key they belong to (yes, the other names are unique and
>> they only occur once). Like this:
>>
>> { "key1": ("value1", "value2"), "key2": ("value3,) }
>>
>> -->
>>
>> { "value1": "key1", "value2": "key1", "value3": "key2" }
>>
>> What I'm doing is using a nested loop:
>>
>> dict2={}
>> for (key,value) in dict1.items():
>> for name in value:
>> dict2[name] = key
>>
>> which is simple enough, but I'm hearing this little voice in
>> the back of my head saying "there's a simpler solution".
>> Is there? What is it? ;-)
>>
>> Thanks
>> --Irmen.
>>[/color]
>
> It's seems like rather nice Python for me ;). Two more variants:
>
> inverted = {}
> for k, vs in d.iteritems():
> inverted.update (dict([(v, k) for v in vs]))
>
> map(
> lambda (k, vs): inverted.update (dict([(v, k) for v in vs])),
> d.iteritems()
> )
>
> happy new year,
> anton.[/color]

**Peter Otten** · Jul 18 '05, 07:11 AM

Re: 'inverting' a dict

Irmen de Jong wrote:
[color=blue]
> I have this dict that maps a name to a sequence of other names.
> I want to have it reversed, i.e., map the other names each to
> the key they belong to (yes, the other names are unique and
> they only occur once). Like this:[/color]

[...]
[color=blue]
> What I'm doing is using a nested loop:
>
> dict2={}
> for (key,value) in dict1.items():
> for name in value:
> dict2[name] = key
>
> which is simple enough, but I'm hearing this little voice in
> the back of my head saying "there's a simpler solution".
> Is there? What is it? ;-)[/color]

Here's what I've come up with:

import itertools

original = {"key1": ("value1", "value2"), "key2": ("value3",)}

def forInv(original ):
result = {}
for (key, values) in original.iterit ems():
for val in values:
result[val] = key
return result

def updateInv(origi nal):
result = {}
for (key, values) in original.iterit ems():
result.update(d ict.fromkeys(va lues, key))
return result

def iterInv(origina l):
result = {}
for (key, values) in original.iterit ems():
result.update(d ict(itertools.i zip(values, itertools.repea t(key))))
return result

def iterInv2(origin al):
return dict(itertools. chain(*[itertools.izip( values,
itertools.repea t(key))
for key, values in original.iterit ems()]))

def compInv(origina l):
return dict([(val, key) for (key, values) in original.iterit ems() for
val in values])

wanted = { "value1": "key1", "value2": "key1", "value3": "key2" }

for inv in globals().value s():
if callable(inv):
print inv.__name__,
if inv(original) == wanted:
print "OK"
else:
print "FAILED"

Conclusion: my favourite toys, itertools and list comprehensions, lead to
clumsier code - well, me at least. So I would recommend that you don't
listen to that voice.

Peter

**Jeff Epler** · Jul 18 '05, 07:11 AM

Re: 'inverting' a dict

Does this listcomp give the right result?
dict([(v, k) for k, vs in original.iterit ems() for v in vs])

Jeff

**Wade Leftwich** · Jul 18 '05, 07:11 AM

Re: 'inverting' a dict

Irmen de Jong <irmen@-NOSPAM-REMOVETHIS-xs4all.nl> wrote in message news:<3ff1b688$ 0$319$e4fe514c@ news.xs4all.nl> ...[color=blue]
> Hi
> I have this dict that maps a name to a sequence of other names.
> I want to have it reversed, i.e., map the other names each to
> the key they belong to (yes, the other names are unique and
> they only occur once). Like this:
>
> { "key1": ("value1", "value2"), "key2": ("value3,) }
>
> -->
>
> { "value1": "key1", "value2": "key1", "value3": "key2" }
>
> What I'm doing is using a nested loop:
>
> dict2={}
> for (key,value) in dict1.items():
> for name in value:
> dict2[name] = key
>
> which is simple enough, but I'm hearing this little voice in
> the back of my head saying "there's a simpler solution".
> Is there? What is it? ;-)
>
> Thanks
> --Irmen.[/color]

def invert_dict(D):
return dict([(y,x) for (x,y) in D.items()])

-- Wade Leftwich
Ithaca, NY

**Irmen de Jong** · Jul 18 '05, 07:12 AM

Re: 'inverting' a dict

Yermat wrote:
[color=blue]
> or even shorter
>[color=green][color=darkred]
>>>>d = {'key1' : ('value1','valu e2'), 'key2': ('value3',) }[/color][/color]
>
>[color=green][color=darkred]
>>>>dict([(v,k) for k in d.iterkeys() for v in d[k]])[/color][/color]
>
> {'value3': 'key2', 'value2': 'key1', 'value1': 'key1'}[/color]

Yep, thanks, that must have been the solution that I was looking for!
I understand list comprehensions, but keep forgetting about the
multiple-loop-kind.

--Irmen.

**Irmen de Jong** · Jul 18 '05, 07:12 AM

Re: 'inverting' a dict

Jeff Epler wrote:[color=blue]
> Does this listcomp give the right result?
> dict([(v, k) for k, vs in original.iterit ems() for v in vs])[/color]

Yes it does, and it's even more concise than Yermat's solution

dict([(v,k) for k in d.iterkeys() for v in d[k]])

because it avoids a dict lookup every loop cycle. Thanks Jeff!

--Irmen

**Irmen de Jong** · Jul 18 '05, 07:12 AM

Re: 'inverting' a dict

Wade Leftwich wrote:
[color=blue]
> def invert_dict(D):
> return dict([(y,x) for (x,y) in D.items()])[/color]

No, this one only works for non-sequence-type values.
I wanted to map every item of a value (a sequence)
to the corresponding key.

--Irmen

**Elaine Jackson** · Jul 18 '05, 07:13 AM

Re: 'inverting' a dict

dict2=dict([(a,b) for b in dict1.keys() for a in dict1[b]])

HTH

"Irmen de Jong" <irmen@-NOSPAM-REMOVETHIS-xs4all.nl> wrote in message
news:3ff1b688$0 $319$e4fe514c@n ews.xs4all.nl.. .
| Hi
| I have this dict that maps a name to a sequence of other names.
| I want to have it reversed, i.e., map the other names each to
| the key they belong to (yes, the other names are unique and
| they only occur once). Like this:
|
| { "key1": ("value1", "value2"), "key2": ("value3,) }
|
| -->
|
| { "value1": "key1", "value2": "key1", "value3": "key2" }
|
| What I'm doing is using a nested loop:
|
| dict2={}
| for (key,value) in dict1.items():
| for name in value:
| dict2[name] = key
|
| which is simple enough, but I'm hearing this little voice in
| the back of my head saying "there's a simpler solution".
| Is there? What is it? ;-)
|
| Thanks
| --Irmen.
|

**Bob van der Poel** · Jul 18 '05, 07:13 AM

Re: 'inverting' a dict

This is sort of off topic for the thread, but I've got a similar
problem. In this case I have a dict like:

{ 'key1': 'value1', 'key2': value2}

and I sometimes need to find the key for the value. All values/keys are
unique. I just use a loop:

for a in dict:
if dict[a]== targ:
return a
return None

But it'd be nice to have something faster, etc.

Elaine Jackson wrote:[color=blue]
> dict2=dict([(a,b) for b in dict1.keys() for a in dict1[b]])
>
> HTH
>
>
> "Irmen de Jong" <irmen@-NOSPAM-REMOVETHIS-xs4all.nl> wrote in message
> news:3ff1b688$0 $319$e4fe514c@n ews.xs4all.nl.. .
> | Hi
> | I have this dict that maps a name to a sequence of other names.
> | I want to have it reversed, i.e., map the other names each to
> | the key they belong to (yes, the other names are unique and
> | they only occur once). Like this:
> |
> | { "key1": ("value1", "value2"), "key2": ("value3,) }
> |
> | -->
> |
> | { "value1": "key1", "value2": "key1", "value3": "key2" }
> |
> | What I'm doing is using a nested loop:
> |
> | dict2={}
> | for (key,value) in dict1.items():
> | for name in value:
> | dict2[name] = key
> |
> | which is simple enough, but I'm hearing this little voice in
> | the back of my head saying "there's a simpler solution".
> | Is there? What is it? ;-)
> |
> | Thanks
> | --Irmen.
> |
>
>[/color]

--
Bob van der Poel ** Wynndel, British Columbia, CANADA **
EMAIL: bvdpoel@kootena y.com
WWW: http://www.kootenay.com/~bvdpoel

**Raymond Hettinger** · Jul 18 '05, 07:13 AM

Re: 'inverting' a dict

Irmen de Jong <irmen@-NOSPAM-REMOVETHIS-xs4all.nl> wrote in message news:<3ff2cba6$ 0$316$e4fe514c@ news.xs4all.nl> ...[color=blue]
> Wade Leftwich wrote:
>[color=green]
> > def invert_dict(D):
> > return dict([(y,x) for (x,y) in D.items()])[/color]
>
> No, this one only works for non-sequence-type values.
> I wanted to map every item of a value (a sequence)
> to the corresponding key.
>
> --Irmen[/color]

Try a nested list comprehension:
[color=blue][color=green][color=darkred]
>>> data = { "key1": ("value1", "value2"), "key2": ("value3",) }
>>> dict([(v,k) for k,vlist in data.iteritems( ) for v in vlist])[/color][/color][/color]
{'value3': 'key2', 'value2': 'key1', 'value1': 'key1'}

Raymond Hettinger

P.S. In Py2.4, it will be possible to write this without the
brackets. The resulting generator expression is faster and more
memory friendly:
[color=blue][color=green][color=darkred]
>>> dict((v,k) for k,vlist in data.iteritems( ) for v in vlist)[/color][/color][/color]
{'value3': 'key2', 'value2': 'key1', 'value1': 'key1'}

**Peter Abel** · Jul 18 '05, 07:13 AM

Re: 'inverting' a dict

Irmen de Jong <irmen@-NOSPAM-REMOVETHIS-xs4all.nl> wrote in message news:<3ff1b688$ 0$319$e4fe514c@ news.xs4all.nl> ...[color=blue]
> Hi
> I have this dict that maps a name to a sequence of other names.
> I want to have it reversed, i.e., map the other names each to
> the key they belong to (yes, the other names are unique and
> they only occur once). Like this:
>
> { "key1": ("value1", "value2"), "key2": ("value3,) }
>
> -->
>
> { "value1": "key1", "value2": "key1", "value3": "key2" }
>
> What I'm doing is using a nested loop:
>
> dict2={}
> for (key,value) in dict1.items():
> for name in value:
> dict2[name] = key
>
> which is simple enough, but I'm hearing this little voice in
> the back of my head saying "there's a simpler solution".
> Is there? What is it? ;-)
>
> Thanks
> --Irmen.[/color]

.... and after all there is always a hacky one-liner:[color=blue][color=green][color=darkred]
>>> org = {"key1": ("value1", "value2"), "key2": ("value3",)}
>>> dict(reduce(lam bda l,(k,v):l.exten d(zip(v,(k,)*le n(v))) or l,org.items(),[]))[/color][/color][/color]
{'value3': 'key2', 'value2': 'key1', 'value1': 'key1'}[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Happy new year
Peter

**sdd** · Jul 18 '05, 07:13 AM

Re: 'inverting' a dict

Bob van der Poel wrote:[color=blue]
> This is sort of off topic for the thread, but I've got a similar
> problem. In this case I have a dict like:
>
> { 'key1': 'value1', 'key2': value2}
>
> and I sometimes need to find the key for the value. All values/keys are
> unique. I just use a loop:
>
> for a in dict:
> if dict[a]== targ:
> return a
> return None[/color]
For a few hunts, I'd do:
for key,value in dictionary.iter items():
if value == target:
return key
return None

Of course, if it probed a lot between changes, it's better to

reversedictiona ry = dict([(v,k) for k,v in dictionary.item s()])

-Scott David Daniels
Scott.Daniels@A cm.Org

'inverting' a dict

'inverting' a dict

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