[Tutor] How to get a key from dictionary?

[Lloyd Kvam]

Depending upon the values you are dealing with, and how often you need to
do the reverse lookup, you could build a reverse dictionary.  Borrowing from
dman's example:
b_dict = {}
for item in a_dict.items()
	[b_dict[item[1]] = item[0]
 >>> b_dict
{2: 'bb', 1: 'aa'}

dman wrote:

> On Mon, Mar 25, 2002 at 11:16:28PM +0100, A wrote:
> | Hi,
> | Is there a possibility to get, from a dictionary, a key according to a 
> | value ?
> | For example
> | I have a dictionary
> | 
> | dict={'aa':1,'bb':2}
> | 
> | and 
> | dict['aa']
> | is 1
> | 
> | But how can I for value 1 find out  key? (That is here  'aa')
> 
> That is not how dictionaries are intended to be used.  It is possible,
> with a little work and a few CPU cycles, to work backwards like that.
> If you give more details regarding why you want to do this then
> perhaps we can suggest a more efficient solution.  In the meantime,
> 
> a_dict = { 'aa' : 1 , 'bb' : 2 }
> items = a_dict.items()
> VALUE = 1 # the value you want to find
> t = filter( lambda x : x[1] == VALUE , items )[0]
> key = t[0]
> print key
> 
> 
> What this does it get the list of (key,value) tuples from the
> dictionary, then iterate over it finding all the pairs that have the
> value you want to find.  It then takes the first of those (bear in mind
> dicts can have duplicate values but not duplicate keys) and prints the
> key for that pair.  This algorithm requires linear time.  As the
> dictionary grows larger the time for this code to execute grows at the
> same rate.
> 
> Also note that using 'dict' as a variable name is not recommended
> since it is a built-in name in python >= 2.2.
> 
> HTH,
> -D
> 
> 


-- 
Lloyd Kvam
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