在R中的data.table中使用eval，parse和as.character

Question

I am having some trouble running a combination of eval, parse and as.character for a data.table. I basically want to convert a given column of the data table to as.character output of the same column.

library(data.table)
options(datatable.WhenJisSymbolThenCallingScope=TRUE) 
# an options that I heard may solve the problem
iris2 <- data.table(iris)
VARS <- colnames(iris)
j <- 1

iris2[,eval(parse(text = paste0(VARS[j])))] # this works fine

iris2[,eval(parse(text = paste0(VARS[j]))) := as.character(eval(parse(text = paste0(VARS[j]))))] 
#but this fails

From the looks of it, it appears the eval and parse functions work fine but when it comes to updating the column with := it seems to break. Could someone tell me what the issue is?

Answer 1

We can use the data.table methods to transform the variables. Specify the 'VARS' or subset of 'VARS' ie 'VARS[j]' in .SDcols , loop through the columns (in case we want to loop for multiple columns) and assign ( := ) to the columns specified in 'VARS[j]`

iris2[, VARS[j] := lapply(.SD, as.character) , .SDcols = VARS[j]]
str(iris2)
#Classes ‘data.table’ and 'data.frame':  150 obs. of  5 variables:
#$ Sepal.Length: chr  "5.1" "4.9" "4.7" "4.6" ...
#$ Sepal.Width : num  3.5 3 3.2 3.1 3.6 3.9 3.4 3.4 2.9 3.1 ...
#$ Petal.Length: num  1.4 1.4 1.3 1.5 1.4 1.7 1.4 1.5 1.4 1.5 ...
#$ Petal.Width : num  0.2 0.2 0.2 0.2 0.2 0.4 0.3 0.2 0.2 0.1 ...
#$ Species     : Factor w/ 3 levels "setosa","versicolor",..: 1 1 1 1 1 1 1 1 1