I was wondering if there is a cleaner one-liner to make a list of multiple copies of the same data.table object not by reference, avoiding the need for a loop.
The use case is that I want to create m multiple data.tables that will differ only by the values of a single column. Hence I would make a list with equal objects, then iterate over every data.table and modify the values of one column, ending up with m different data.tables.
Currently my clunky inelegant approach is:
library(data.table)
# Load data and create data.table
data("airquality")
dt <- data.table(airquality)
# Number of copies
m = 3
# Pre-allocate
dt_list <- list()
# Populate the list with `m` copies of the original data.table
for (i in 1:m) {
dt_list[[i]] <- copy(dt)
}
# Assign by reference within each data.table
for (i in 1:m) {
dt_list[[i]][, Ozone := rep(i)]
}
dt_list
# [[1]]
# Ozone Solar.R Wind Temp Month Day
# 1: 1 190 7.4 67 5 1
# 2: 1 118 8.0 72 5 2
# 3: 1 149 12.6 74 5 3
# 4: 1 313 11.5 62 5 4
# 5: 1 NA 14.3 56 5 5
# [[2]]
# Ozone Solar.R Wind Temp Month Day
# 1: 2 190 7.4 67 5 1
# 2: 2 118 8.0 72 5 2
# 3: 2 149 12.6 74 5 3
# 4: 2 313 11.5 62 5 4
# 5: 2 NA 14.3 56 5 5
# [[3]]
# Ozone Solar.R Wind Temp Month Day
# 1: 3 190 7.4 67 5 1
# 2: 3 118 8.0 72 5 2
# 3: 3 149 12.6 74 5 3
# 4: 3 313 11.5 62 5 4
# 5: 3 NA 14.3 56 5 5
Other similar questions consider different data.table objects or they assign by reference, so that all copies are also changed when one is changed. For example by doing dt_list <- rep(list(dt), m), all Ozone columns are filled with 3s.
One way to solve your problem:
dt_list = lapply(1:3, \(i) copy(dt)[, Ozone := i])
# or
dt_list = lapply(1:3, \(i) set(copy(dt), j="Ozone", value=i))
[[1]]
Ozone Solar.R Wind Temp Month Day
<int> <int> <num> <int> <int> <int>
1: 1 190 7.4 67 5 1
2: 1 118 8.0 72 5 2
3: 1 149 12.6 74 5 3
4: 1 313 11.5 62 5 4
5: 1 NA 14.3 56 5 5
---
149: 1 193 6.9 70 9 26
150: 1 145 13.2 77 9 27
151: 1 191 14.3 75 9 28
152: 1 131 8.0 76 9 29
153: 1 223 11.5 68 9 30
[[2]]
Ozone Solar.R Wind Temp Month Day
<int> <int> <num> <int> <int> <int>
1: 2 190 7.4 67 5 1
2: 2 118 8.0 72 5 2
3: 2 149 12.6 74 5 3
4: 2 313 11.5 62 5 4
5: 2 NA 14.3 56 5 5
---
149: 2 193 6.9 70 9 26
150: 2 145 13.2 77 9 27
151: 2 191 14.3 75 9 28
152: 2 131 8.0 76 9 29
153: 2 223 11.5 68 9 30
[[3]]
Ozone Solar.R Wind Temp Month Day
<int> <int> <num> <int> <int> <int>
1: 3 190 7.4 67 5 1
2: 3 118 8.0 72 5 2
3: 3 149 12.6 74 5 3
4: 3 313 11.5 62 5 4
5: 3 NA 14.3 56 5 5
---
149: 3 193 6.9 70 9 26
150: 3 145 13.2 77 9 27
151: 3 191 14.3 75 9 28
152: 3 131 8.0 76 9 29
153: 3 223 11.5 68 9 30