Reversing a dict?

[krumblebunk]

Hi - further to my earlier query regarding partial matches (which with
all your replies enabled me to advance my understanding, thanks), I
now need to reverse a dict.

I know how to reverse a list (with the reverse method - very handy),
but it doesn't seem possible to reverse a dict.

I suspect what I need to do is somehow go from:

thelist=list(thedict)
thelist.reverse()
thedict=dict(thelist)

Does anyone know how to convert / or reverse a dict?

thanks

kb.

 

[cokofreedom]

Hi - further to my earlier query regarding partial matches (which with
all your replies enabled me to advance my understanding, thanks), I
now need to reverse a dict.

I know how to reverse a list (with the reverse method - very handy),
but it doesn't seem possible to reverse a dict.

I suspect what I need to do is somehow go from:

thelist=list(thedict)
thelist.reverse()
thedict=dict(thelist)

Does anyone know how to convert / or reverse a dict?

thanks

kb.

Issue 1: A dictionary is not ordered so cannot be reversed, as is.

Saw something like this though:

info = {"PHP":"17th May",
"Perl":"15th June",
"Java":"7th June",
"Python":"26th May",
"Tcl":"12th July",
"MySQL":"24th May"}

topics = info.keys()
topics.sort()
topics.reverse()

for topic in topics:
print "Next",topic,"course starts",info[topic]

 

[Jeremy Sanders]

Hi - further to my earlier query regarding partial matches (which with
all your replies enabled me to advance my understanding, thanks), I
now need to reverse a dict.

There is no guaranteed order to the items stored in a dictionary. They can
and will move around as the dict is modified. Have a look at
diveintopython:

http://www.diveintopython.org/getting_to_know_python/dictionaries.html

You'll have to store your keys in a list or tuple to keep them ordered.

Jeremy

 

[krumblebunk]

Thanks all!!

kb.

 

[Ian Kelly]

There is no guaranteed order to the items stored in a dictionary. They can
and will move around as the dict is modified. Have a look at
diveintopython:

http://www.diveintopython.org/getting_to_know_python/dictionaries.html

You'll have to store your keys in a list or tuple to keep them ordered.

You might also take a look at the odict[1] or ordereddict[2] modules.

[1] http://www.voidspace.org.uk/python/odict.html
[2] http://www.xs4all.nl/~anthon/Python/ordereddict/

 

[Paul Hankin]

Hi - further to my earlier query regarding partial matches (which with
all your replies enabled me to advance my understanding, thanks), I
now need to reverse a dict.

I know how to reverse a list (with the reverse method - very handy),
but it doesn't seem possible to reverse a dict.

I suspect what I need to do is somehow go from:

Does anyone know how to convert / or reverse a dict?

kb.

Issue 1: A dictionary is not ordered so cannot be reversed, as is.

Saw something like this though:

info = {"PHP":"17th May",
       "Perl":"15th June",
       "Java":"7th June",
       "Python":"26th May",
       "Tcl":"12th July",
       "MySQL":"24th May"}

topics = info.keys()
topics.sort()
topics.reverse()

for topic in topics:
   print "Next",topic,"course starts",info[topic]

Better:

for topic, when in sorted(topics.iteritems(), reverse=True):
print 'Next %s course starts %s' % (topic, when)

 

[castironpi]

Hi - further to my earlier query regarding partial matches (which with
all your replies enabled me to advance my understanding, thanks), I
now need to reverse a dict.

I know how to reverse a list (with the reverse method - very handy),
but it doesn't seem possible to reverse a dict.

It is not. Dictionaries provide constant time add and remove, -not-
order. Are you sorting based on key or value?

If you want a sorted state and sorted remove, use the 'bisect' module.