counting occurrences

[Grant Griffin]

Hi Gang,

I have long used dictionaries to counting occurrences of something (usually
strings).  It goes something like this:

    counter = {}
    for s in strings:
        counter[s] = counter.get(s, 0) + 1

(I used to use "has_key" until I caught on to "get".)

This then usually leads to a need to "sort keys by value"--something like this:

   itemlist = [(value, key) for (key, value) in counter.items()]
   itemlist.sort()
   itemlist.reverse()

(Which demonstrates yet _another_ valuable use for list comprehensions<wink>.)

Then, "itemlist" contains sorted (value, key) pairs--usually to be printed or
something.

For me, this sort of thing comes up so often that it qualifies as an "idiom". 
However, the sorting thing is a pain (at least a first--then you get so you like
to impress people by having finally found a use for list comprehensions<wink>),
so I mentioned to a friend that it would sure be nice if Python dictionaries had
an "items_sorted_by_value" method.  But, among other things, my friend (who has
been using Python for a *long* time) said that he has never wanted such a thing,
because he doesn't see counting occurrences "as a dictionary operation".

So how do you think he does it?

tim:-help-me-out-here<wink>-ly y'rs,

=g2

_____________________________________________________________________

Grant R. Griffin                                       g2 at dspguru.com
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