set.seed(45L)
X = data.frame(A=sample(3, 10, TRUE),
B=sample(letters[1:3], 10, TRUE),
C=sample(10), stringsAsFactors=FALSE)
# get the frequency of each "A,B" combination
setDT(X)[, .N, by="A,B"][]
# convert list to data.table
# autofill names
X = list(1:4, letters[1:4])
setDT(X)
# don't provide names
X = list(a=1:4, letters[1:4])
setDT(X, FALSE)
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