Hi Andrejs,The calculations are a bit different to what I've come across in
Mining Massive Datasets (2nd Ed. Ullman et. al., Cambridge Press) available
here:http://www.mmds.org/
Their calculation of IDF is as follows:
IDFi = log2(N / ni)
where N is the number of documents and ni is the number of documents in which
the word appears. This looks different to your IDF function.
For TF, they use
TFij = fij / maxk fkj
That is:
For document j, the term frequency of the term i in j is the number of
times i appears in j divided by the maximum number of times any term appears in
j. Stop words are usually excluded when considering the maximum).
So, in your case, the
TFa1 = 2 / 2 = 1
TFb1 = 1 / 2 = 0.5TFc1 = 1/2 = 0.5TFm1 = 2/2 = 1...
IDFa = log2(3 / 2) = 0.585
So, TFa1 * IDFa = 0.585
Wikipedia mentions an adjustment to overcome biases for long documents, by
calculating TFij = 0.5 + {(0.5*fij)/maxk fkj}, but that doesn't change anything
for TFa1, as the value remains 1.
In other words, my calculations don't agree with yours, and neither seem to
agree with Spark :)
Regards,Ashic.
Date: Thu, 30 Oct 2014 22:13:49 +0000
Subject: how idf is calculated
From: andr...@sindicetech.com
To: u...@spark.incubator.apache.org
Hi,I'm writing a paper and I need to calculate tf-idf. Whit your help I managed
to get results, I needed, but the problem is that I need to be able to explain
how each number was gotten. So I tried to understand how idf was calculated and
the numbers i get don't correspond to those I should get .
I have 3 documents (each line a document)a a b c m me a c d e ed j k l m m c
When I calculate tf, I get this
(1048576,[99,100,106,107,108,109],[1.0,1.0,1.0,1.0,1.0,2.0])(1048576,[97,98,99,109],[2.0,1.0,1.0,2.0])(1048576,[97,99,100,101],[1.0,1.0,1.0,3.0]
idf is supposedly calculated idf = log((m + 1) / (d(t) + 1))m -number of
documents (3 in my case).d(t) - in how many documents is term presenta:
log(4/3) =0.1249387366b: log(4/2) =0.3010299957c: log(4/4) =0d: log(4/3)
=0.1249387366e: log(4/2) =0.3010299957l: log(4/2) =0.3010299957m: log(4/3)
=0.1249387366
When I output idf vector `
idf.idf.toArray.filter(_.>(0)).distinct.foreach(println(_)) `I get
:1.38629436111989060.287682072451780850.6931471805599453
I understand why there are only 3 numbers, because only 3 are unique :
log(4/2), log(4/3), log(4/4), but I don't understand how numbers in idf where
calculated
Best regards,Andrejs
