Что обозначают в Redis аббревиатуры AOF и RDB?

OK для обоих data.table и data.frame

# toy data
df <- structure(list(x = structure(c(2L, 1L), .Label = c("This actually is not", 
"This is interesting"), class = "factor")), .Names = "x", row.names = c(NA, 
-2L), class = "data.frame")

#                      x
# 1  This is interesting
# 2 This actually is not

# the code
split_result <- strsplit(as.character(df$x), " ")
length_n <- sapply(split_result, length)
length_max <- seq_len(max(length_n))
as.data.frame(t(sapply(split_result, "[", i = length_max))) # Or as.data.table(...)

#     V1       V2          V3   V4
# 1 This       is interesting <NA>
# 2 This actually          is  not