eval parse is not working properly in my function - r

Question

I Have the following dataset which includes 2 variables:
dt4<-structure(list(a1 = c(4L, 4L, 3L, 4L, 4L), a2 = c(1L,
3L, 4L, 5L, 4L)), .Names = c("a1", "a2"
), row.names = c(NA, -5L), class = c("tbl_df", "tbl", "data.frame"
))
I Have the following function that add labels and levels to an existing dataset:
Add_Labels_Level_To_Dataset <- function(df, df_name,levels_list,labels_list) {
df[] <- lapply( df, ordered)
for (i in 1:length(colnames(df))) {
arg0<-paste0(df_name,"[i]", "<-ordered(", df_name, "$'", colnames(df)[i], "', levels=c(", levels_list[[i]], "), labels = c(", labels_list[[i]],"))" )
eval(parse(text=arg0))
}
df
}
which is run by that R command:
Add_Labels_Level_To_Dataset(dt4, "dt4", level_list, labels_list)
The lists supplied in the R command are the following ones which represents the ordered levels of each variable in the dataset, respectively:
label_list=list("'S','SA','SB','SC,'SD'", "'S','SA','SB','SC,'SD'")
level_list=list("5,4,3,2,1", "5,4,3,2,1")
Why my function is not working properly?
I dont know what is wrong with that!
When I run the R commands outside R function, they tie levels/ labels to the dataset given. However, when I run my R function, this does not happen!
df_name="dt4"
df=dt4
levels_list=level_list
labels_list=label_list
i=3
df[] <- lapply( df, ordered)
arg0<-paste0(df_name,"[i]", "<-ordered(", df_name, "$'", colnames(df)[i], "', levels=c(", levels_list[[i]], "), labels = c(", labels_list[[i]],"))" )
eval(parse(text=arg0))
Can you help?

Answer 1

This is a xy problem. I agree with #MrFlick that parse should be avoided.
On the original post the main issue is the function should be returning dt4 and not df. There are some missing ' (single quote) when defining label_list.
We could use mapply and avoid the single quote:
label_list=list(c('S','SA','SB','SC','SD'), c('S','SA','SB','SC','SD'))
level_list=list(c(5,4,3,2,1), c(5,4,3,2,1))
as.data.frame(mapply(function(x, labels,levels ) {ordered(x, labels,levels)}, dt4, level_list, label_list, SIMPLIFY = F))
# a1 a2
#1 SA SD
#2 SA SB
#3 SB SA
#4 SA S
#5 SA SA

Answer 2

Using eval/parse should be avoided. There are tpyically much easier ways to do what you want in R. For example, with this code, we can just write
Add_Labels_Level_To_Dataset <- function(df, levels_list, labels_list) {
df[] <- Map(function(data, levels, labels) {
ordered(data, levels=strsplit(levels,",")[[1]], labels=strsplit(labels, ",")[[1]])
}, df, levels_list, labels_list)
df
}
And we can call it like
dt4 <- Add_Labels_Level_To_Dataset(dt4, level_list, label_list)
Note that it returns a new data.frame which you can reassign to dt4 or some other variable. Functions in R should never modify objects outside their own scope which is one of the other reasons you were running into problems with your function.