[datatable-help] R function

[Douglas Clark]

A data.table way to do this could be:

library(data.table)
x = c(10, 25, 4, 10, 9, 4, 4)
DT <- data.table(x)
DT
#     x
# 1: 10
# 2: 25
# 3:  4
# 4: 10
# 5:  9
# 6:  4
# 7:  4

# create a new column y as the count in each group defined by x
DT[, y := .N, by = x]
DT
#     x y
# 1: 10 2
# 2: 25 1
# 3:  4 3
# 4: 10 2
# 5:  9 1
# 6:  4 3

# you can then either directly use DT$y
DT$y
# [1] 2 1 3 2 1 3 3
# or extract the y column to a separate vector y
y <- DT[, y]
y
# [1] 2 1 3 2 1 3 3

# or all of the above in one line:
y <- data.table(x = c(10, 25, 4, 10, 9, 4, 4) )[, y := .N, by = x][, y]
y
# [1] 2 1 3 2 1 3 3




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