Using lists inside data.table columns

by

Using :=:

dt = data.table(id = 1:2, comment = vector("list", 2L))

# assign value 1 to just the first column of 'comment'
dt[1L, comment := 1L]

# assign value of 1 and "a" to rows 1 and 2
dt[, comment := list(1, "a")]

# assign value of "a","b" to row 1, and 1 to row 2 for 'comment'
dt[, comment := list(c("a", "b"), 1)]

# assign list(1, "a") to just 1 row of 'comment'
dt[1L, comment := list(list(list(1, "a")))]

For the last case, you’ll need one more list because data.table uses list(.) to look for values to assign to columns by reference.

Using set:

dt = data.table(id = 1:2, comment = vector("list", 2L))

# assign value 1 to just the first column of 'comment'
set(dt, i=1L, j="comment", value=1L)

# assign value of 1 and "a" to rows 1 and 2
set(dt, j="comment", value=list(1, "a"))

# assign value of "a","b" to row 1, and 1 to row 2 for 'comment'
set(dt, j="comment", value=list(c("a", "b"), 1))

# assign list(1, "a") to just 1 row of 'comment'
set(dt, i=1L, j="comment", value=list(list(list(1, "a"))))

HTH

I’m using the current development version 1.9.3, but should just work fine on any other version.

> sessionInfo()
R version 3.0.3 (2014-03-06)
Platform: x86_64-apple-darwin10.8.0 (64-bit)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.9.3

loaded via a namespace (and not attached):
[1] plyr_1.8.0.99  reshape2_1.2.2 stringr_0.6.2  tools_3.0.3

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