For a dfm object, returns a (weighted) document frequency for each term. The default is a simple count of the number of documents in which a feature occurs more than a given frequency threshold. (The default threshold is zero, meaning that any feature occurring at least once in a document will be counted.)
docfreq(
x,
scheme = c("count", "inverse", "inversemax", "inverseprob", "unary"),
base = 10,
smoothing = 0,
k = 0,
threshold = 0
)a dfm
type of document frequency weighting, computed as follows, where \(N\) is defined as the number of documents in the dfm and \(s\) is the smoothing constant:
count\(df_j\), the number of documents for which \(n_{ij} > threshold\)
inverse$$\textrm{log}_{base}\left(s + \frac{N}{k + df_j}\right)$$
inversemax$$\textrm{log}_{base}\left(s + \frac{\textrm{max}(df_j)}{k + df_j}\right)$$
inverseprob$$\textrm{log}_{base}\left(\frac{N - df_j}{k + df_j}\right)$$
unary1 for each feature
the base with respect to which logarithms in the inverse document frequency weightings are computed; default is 10 (see Manning, Raghavan, and Schütze 2008, p123).
added to the quotient before taking the logarithm
added to the denominator in the "inverse" weighting types, to prevent a zero document count for a term
numeric value of the threshold above which a feature will considered in the computation of document frequency. The default is 0, meaning that a feature's document frequency will be the number of documents in which it occurs greater than zero times.
a numeric vector of document frequencies for each feature
Manning, C. D., Raghavan, P., & Schütze, H. (2008). Introduction to Information Retrieval. Cambridge: Cambridge University Press. https://nlp.stanford.edu/IR-book/pdf/irbookonlinereading.pdf
dfmat1 <- dfm(tokens(data_corpus_inaugural))
docfreq(dfmat1[, 1:20])
#> fellow-citizens of the senate and
#> 19 59 59 9 59
#> house representatives : among vicissitudes
#> 8 14 37 43 5
#> incident to life no event
#> 6 59 49 57 9
#> could have filled me with
#> 34 59 5 46 58
# replication of worked example from
# https://en.wikipedia.org/wiki/Tf-idf#Example_of_tf.E2.80.93idf
dfmat2 <-
matrix(c(1,1,2,1,0,0, 1,1,0,0,2,3),
byrow = TRUE, nrow = 2,
dimnames = list(docs = c("document1", "document2"),
features = c("this", "is", "a", "sample",
"another", "example"))) %>%
as.dfm()
dfmat2
#> Document-feature matrix of: 2 documents, 6 features (33.33% sparse) and 0 docvars.
#> features
#> docs this is a sample another example
#> document1 1 1 2 1 0 0
#> document2 1 1 0 0 2 3
docfreq(dfmat2)
#> this is a sample another example
#> 2 2 1 1 1 1
docfreq(dfmat2, scheme = "inverse")
#> this is a sample another example
#> 0.00000 0.00000 0.30103 0.30103 0.30103 0.30103
docfreq(dfmat2, scheme = "inverse", k = 1, smoothing = 1)
#> this is a sample another example
#> 0.2218487 0.2218487 0.3010300 0.3010300 0.3010300 0.3010300
docfreq(dfmat2, scheme = "unary")
#> this is a sample another example
#> 1 1 1 1 1 1
docfreq(dfmat2, scheme = "inversemax")
#> this is a sample another example
#> 0.00000 0.00000 0.30103 0.30103 0.30103 0.30103
docfreq(dfmat2, scheme = "inverseprob")
#> this is a sample another example
#> 0 0 0 0 0 0