conflictcoder
Reputation: 393
data.table: How do I pass a character vector to a function get data.table to treat its contents as column names?
Here is a data.table:
library(data.table)
DT <- data.table(airquality)
This example produces the output I want:
DT[, `:=`(New_Ozone= log(Ozone), New_Wind=log(Wind))]
How can I write a function log_those_columns such that the following code snippet outputs the same result?
old_names <- c("Ozone", "Wind")
new_names <- c("New_Ozone", "New_Wind")
log_those_columns(DT, old_names, new_names)
Note that I need old_names and new_names to be flexible enough to contain any number of columns.
(I see from the similar StackOverflow questions on this topic that the answer probably involves some combination of .SD, with=F, parse(), eval(), and/or substitute(), but I can't seem to nail which of those to use and where).
Upvotes: 4
Views: 614
Answers (2)
Uwe
Reputation: 42544
Picking up MichaelChirico's comment, the function definition can be written as:
log_those_columns <- function(DT, cols_in, cols_new) {
DT[, (cols_new) := lapply(.SD, log), .SDcols = cols_in]
}
which returns:
log_those_columns(DT, old_names, new_names)
DT
Ozone Solar.R Wind Temp Month Day New_Ozone New_Wind 1: 41 190 7.4 67 5 1 3.713572 2.001480 2: 36 118 8.0 72 5 2 3.583519 2.079442 3: 12 149 12.6 74 5 3 2.484907 2.533697 4: 18 313 11.5 62 5 4 2.890372 2.442347 5: NA NA 14.3 56 5 5 NA 2.660260 --- 149: 30 193 6.9 70 9 26 3.401197 1.931521 150: NA 145 13.2 77 9 27 NA 2.580217 151: 14 191 14.3 75 9 28 2.639057 2.660260 152: 18 131 8.0 76 9 29 2.890372 2.079442 153: 20 223 11.5 68 9 30 2.995732 2.442347
as expected.
A more flexible approach
The function used to transform the data can be passed as a parameter as well:
fct_those_columns <- function(DT, cols_in, cols_new, fct) {
DT[, (cols_new) := lapply(.SD, fct), .SDcols = cols_in]
}
The call:
fct_those_columns(DT, old_names, new_names, log)
head(DT)
works as expected:
Ozone Solar.R Wind Temp Month Day New_Ozone New_Wind 1: 41 190 7.4 67 5 1 3.713572 2.001480 2: 36 118 8.0 72 5 2 3.583519 2.079442 3: 12 149 12.6 74 5 3 2.484907 2.533697 4: 18 313 11.5 62 5 4 2.890372 2.442347 5: NA NA 14.3 56 5 5 NA 2.660260 6: 28 NA 14.9 66 5 6 3.332205 2.701361
The function name can be passed as character:
fct_those_columns(DT, old_names, new_names, "sqrt")
head(DT)
Ozone Solar.R Wind Temp Month Day New_Ozone New_Wind 1: 41 190 7.4 67 5 1 6.403124 2.720294 2: 36 118 8.0 72 5 2 6.000000 2.828427 3: 12 149 12.6 74 5 3 3.464102 3.549648 4: 18 313 11.5 62 5 4 4.242641 3.391165 5: NA NA 14.3 56 5 5 NA 3.781534 6: 28 NA 14.9 66 5 6 5.291503 3.860052
or as an anonymous function:
fct_those_columns(DT, old_names, new_names, function(x) x^(1/2))
head(DT)
Ozone Solar.R Wind Temp Month Day New_Ozone New_Wind 1: 41 190 7.4 67 5 1 6.403124 2.720294 2: 36 118 8.0 72 5 2 6.000000 2.828427 3: 12 149 12.6 74 5 3 3.464102 3.549648 4: 18 313 11.5 62 5 4 4.242641 3.391165 5: NA NA 14.3 56 5 5 NA 3.781534 6: 28 NA 14.9 66 5 6 5.291503 3.860052
An even more flexible approach
The function below derives the names of the new columns by prepending the names of the input columns with the name of the function automatically:
fct_those_columns <- function(DT, cols_in, fct) {
fct_name <- substitute(fct)
cols_new <- paste(if (class(fct_name) == "name") fct_name else fct_name[3], cols_in, sep = "_")
DT[, (cols_new) := lapply(.SD, fct), .SDcols = cols_in]
}
DT <- data.table(airquality)
fct_those_columns(DT, old_names, sqrt)
fct_those_columns(DT, old_names, data.table::as.IDate)
fct_those_columns(DT, old_names, function(x) x^(1/2))
DT
Ozone Solar.R Wind Temp Month Day sqrt_Ozone sqrt_Wind as.IDate_Ozone as.IDate_Wind x^(1/2)_Ozone x^(1/2)_Wind 1: 41 190 7.4 67 5 1 6.403124 2.720294 1970-02-11 1970-01-08 6.403124 2.720294 2: 36 118 8.0 72 5 2 6.000000 2.828427 1970-02-06 1970-01-09 6.000000 2.828427 3: 12 149 12.6 74 5 3 3.464102 3.549648 1970-01-13 1970-01-13 3.464102 3.549648 4: 18 313 11.5 62 5 4 4.242641 3.391165 1970-01-19 1970-01-12 4.242641 3.391165 5: NA NA 14.3 56 5 5 NA 3.781534 <NA> 1970-01-15 NA 3.781534 --- 149: 30 193 6.9 70 9 26 5.477226 2.626785 1970-01-31 1970-01-07 5.477226 2.626785 150: NA 145 13.2 77 9 27 NA 3.633180 <NA> 1970-01-14 NA 3.633180 151: 14 191 14.3 75 9 28 3.741657 3.781534 1970-01-15 1970-01-15 3.741657 3.781534 152: 18 131 8.0 76 9 29 4.242641 2.828427 1970-01-19 1970-01-09 4.242641 2.828427 153: 20 223 11.5 68 9 30 4.472136 3.391165 1970-01-21 1970-01-12 4.472136 3.391165
Note that x^(1/2)_Ozone is not a syntactically valid name in R and needs to be put in backquotes:
DT$`x^(1/2)_Ozone`
Upvotes: 5
Onyambu
Reputation: 79208
you only need to write a function:
log_those_columns <- function(D,old_names,new_names)
DT[,(new_names) := lapply(mget(old_names),log)]
log_those_columns(DT,old_names,new_names)
DT
Ozone Solar.R Wind Temp Month Day New_Ozone New_Wind
1: 41 190 7.4 67 5 1 3.713572 2.001480
2: 36 118 8.0 72 5 2 3.583519 2.079442
3: 12 149 12.6 74 5 3 2.484907 2.533697
4: 18 313 11.5 62 5 4 2.890372 2.442347
5: NA NA 14.3 56 5 5 NA 2.660260
---
Upvotes: 1
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