reversing a dictionary (newbie)

[Steve Purcell]

Christopher Brewster wrote:
> I have written a program to extract pairs of words from a corpus (the BNC).
> It is my first Python program
> Once it has processed the files I "reverse" the dictionary so that the keys
> are frequencies and the values lists of word pairs of that frequency.

What if two words have the same frequency?  You probably want the 'reversing'
process to generate a list of (frequency, word) tuples.

But anyway... to reverse a dictionary, you can do:

>>> dict = {'a':1, 'b':2, 'c':3}
>>> reversed = {}
>>> for k, v in dict.items():
...   reversed[v] = k
>>> reversed
{3: 'c', 2: 'b', 1: 'a'}

Expect total memory usage to be triple that of 'dict' alone towards the end
of the loop, because of the 'items()' call.

-Steve

-- 
Steve Purcell, Pythangelist
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