Different tf-idf values in R and hand calculation

问题

I am playing around in R to find the tf-idf values.

I have a set of documents like:

D1 = "The sky is blue."
D2 = "The sun is bright."
D3 = "The sun in the sky is bright."

I want to create a matrix like this:

   Docs      blue    bright       sky       sun
   D1 tf-idf 0.0000000 tf-idf 0.0000000
   D2 0.0000000 tf-idf 0.0000000 tf-idf
   D3 0.0000000 tf-idf tf-idf tf-idf

So, my code in R:

library(tm)
docs <- c(D1 = "The sky is blue.", D2 = "The sun is bright.", D3 = "The sun in the sky is bright.")

dd <- Corpus(VectorSource(docs)) #Make a corpus object from a text vector
#Clean the text
dd <- tm_map(dd, stripWhitespace)
dd <- tm_map(dd, tolower)
dd <- tm_map(dd, removePunctuation)
dd <- tm_map(dd, removeWords, stopwords("english"))
dd <- tm_map(dd, stemDocument)
dd <- tm_map(dd, removeNumbers)
 inspect(dd)
    A corpus with 3 text documents

    The metadata consists of 2 tag-value pairs and a data frame
    Available tags are:
    create_date creator 
    Available variables in the data frame are:
    MetaID 

    $D1
    sky blue

    $D2
     sun bright

    $D3
      sun sky bright

    > dtm <- DocumentTermMatrix(dd, control = list(weighting = weightTfIdf))
    > as.matrix(dtm)
      Terms
            Docs      blue    bright       sky       sun
            D1 0.7924813 0.0000000 0.2924813 0.0000000
            D2 0.0000000 0.2924813 0.0000000 0.2924813
            D3 0.0000000 0.1949875 0.1949875 0.1949875

If I do a hand calculation then the matrix should be:

            Docs  blue      bright       sky       sun
            D1    0.2385     0.0000000 0.3521    0.0000000
            D2    0.0000000 0.3521    0.0000000 0.3521
            D3    0.0000000 0.1949875 0.058     0.058

I am calculating like say blue as tf = 1/2 = 0.5 and idf as log(3/1) = 0.477121255. Therefore tf-idf = tf*idf = 0.5*0.477 = 0.2385. In this way, I am calculating the other tf-idf values. Now, I am wondering, why I am getting different results in the matrix of hand calculation and in the matrix of R? Which gives the correct results? Am I doing something wrong in hand calculation or is there something wrong in my R code?

回答1:

The reason your hand calculation doesn't agree with the DocumentTermMatrix calculation is you are using a different log base. When you say log(3/1) = 0.477121255 you must be using log base 10. In R, that would be log10(3). The default log in R is natural log so if you type log(3) in R you get ~1.10. But the weightTfIdf uses log base 2 for its calculations. Thus when calculating tf-idf for "blue" you get

(1/2)*log2(3/1) = 0.7924813

I hope that clears things up.

来源：https://stackoverflow.com/questions/24011395/different-tf-idf-values-in-r-and-hand-calculation

标签

matrix

tf-idf