Boris Borcic wrote:
> Bruza wrote:
>> No. That does not solve the problem. What I want is a function
>>
>> def randomPick(n, the_items):
>>
>> which will return n DISTINCT items from "the_items" such that
>> the n items returned are according to their probabilities specified
>> in the (item, pro) elements inside "the_items".
>
> and in the initial post you wrote :
>
> > But how about the general case, for N > 1 and N < len(items)?
>
> The problem is you need to make "the n items returned are according
> to their probabilities" more precise. "According to their probabilities"
> implies
> n INDEPENDENT picks, but this is contradictory with the n picks having to
> provide DISTINCT results (what clearly constrains picks relative to each
> other).
>
> Of course there are obvious ways to combine the results of random choices of
> single items to obtain a set like you want, but it is not obvious that they
> are
> equivalent.
>
> This is the most simple-minded :
>
> def randomPick(n, the_items) :
> assert n<len(the_items)
> result = set()
> while len(result)<n :
> result.add(singlePick(the_items))
> return sorted(result)
>
> This is another (but it won't work with your version of singlePick as it is,
> although it would with that provided by the other posters) :
>
> def randomPick(n, the_items) :
> result = []
> items = dict(the_items)
> for k in range(n) :
> pick = singlePick(items.items())
> result.append(pick)
> del items[pick]
> return result
>
> I would be surprised if they had exactly the same statistical properties,
> IOW,
> if they did fit the same exact interpretation of "according to their
> probabilities".
>
>
yet another one, constructing a list of n-sets first, and then picking one;
like the other solutions, it doesn't optimize for repeated use.
def pickn(items,n) :
"yields all n-sublists of (destructed) items"
if n<=len(items) :
if n :
item = items.pop()
for res in pickn(items[:],n) :
yield res
for res in pickn(items,n-1) :
res.append(item)
yield res
else :
yield []
def randomPick(n,the_items) :
"randomly pick n distinct members of the_items"
the_sets = list(pickn(the_items[:],n))
divisor = len(the_sets)*float(n)/len(the_items)
for k,s in enumerate(the_sets) :
the_sets[k] = (sorted(who for who,_ in s),
int(1+sum(p for _,p in s)/divisor)) # mhh...
return singlePick(the_sets)
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