Apply varieties of term frequency weightings to a dfm.
tf(x, scheme = c("count", "prop", "propmax", "boolean", "log", "augmented", "logave"), base = 10, K = 0.5)
x  object for which idf or tfidf will be computed (a documentfeature matrix) 

scheme  divisor for the normalization of feature frequencies by document. Valid types include:

base  base for the logarithm when 
K  the K for the augmentation when 
A document feature matrix to which the weighting scheme has been applied.
tf(x, scheme = "prop")
is equivalent to weight(x, "relFreq")
).
Manning, C. D., Raghavan, P., & Schutze, H. (2008). Introduction to Information Retrieval. Cambridge University Press. https://en.wikipedia.org/wiki/Tfidf#Term_frequency_2