Thanks Mehmet! I tried that and it seems to work on a small test case. I'm also
experimenting now with your other suggestion, a UDF.
I will probably use something like this, which seems less tricky and does not
rely on a sort:
#!/usr/bin/python
import random
@outputSchema('id_bag: {items: (item: chararray)}')
def random_subset(bag, n):
# return bag if it has <= n elements or n=-1, else return n random elements
from it
if n == -1 or len(bag) <= n:
return bag
else:
return random.sample(bag, n)
Thanks again,
Will
William F Dowling
Senior Technologist
Thomson Reuters
-----Original Message-----
From: Mehmet Tepedelenlioglu [mailto:mehmets...@yahoo.com]
Sent: Tuesday, May 27, 2014 5:09 PM
To: user@pig.apache.org user@pig.apache.org
Subject: Re: How to sample an inner bag?
If you know how many items you want from each inner bag exactly, you can hack
it like this:
x = foreach x {
y = foreach x generate RANDOM() as rnd, *;
y = order y by rnd;
y = limit y $SAMPLE_NUM;
y = foreach y generate $1 ..;
generate group, y;
}
Basically randomize the inner bag, sort it wrt the random number and limit it
to the sample size you want. No reducers needed.
If the inner bags are huge, ordering will obviously be expensive. If you don’t
like this, you might have to write your own udf.
Mehmet
On May 27, 2014, at 10:03 AM, <william.dowl...@thomsonreuters.com>
<william.dowl...@thomsonreuters.com> wrote:
> Hi Pig users,
>
> Is there an easy/efficient way to sample an inner bag? For example, with
> input in a relation like
>
> (id1,att1,{(a,0.01),(b,0.02),(x,0.999749968742)})
> (id1,att2,{(a,0.03),(b,0.04),(x,0.998749217772)})
> (id2,att1,{(b,0.05),(c,0.06),(x,0.996945334509)})
>
> I’d like to sample 1/3 the elements of the bags, and get something like
> (ignoring the non-determinism)
> (id1,att1,{(x,0.999749968742)})
> (id1,att2,{(b,0.04)})
> (id2,att1,{(b,0.05)})
>
> I have a circumlocution that seems to work using flatten+ group but that
> looks ugly to me:
>
> tfidf1 = load '$tfidf' as (id: chararray,
> att: chararray,
> pairs: {pair: (word: chararray, value: double)});
>
> flat_tfidf = foreach tfidf1 generate id, att, FLATTEN(pairs);
> sample_flat_tfidf = sample flat_tfidf 0.33;
> tfidf2 = group sample_flat_tfidf by (id, att);
>
> tfidf = foreach tfidf2 {
> pairs = foreach sample_flat_tfidf generate pairs::word, pairs::value;
> generate group.id, group.att, pairs;
> };
>
> Can someone suggest a better way to do this? Many thanks!
>
> William F Dowling
> Senior Technologist
>
> Thomson Reuters
>
>
>
