R: data.table- Adding a variable to a list of data tables that holds the name of each table in the list

Question

I'm currently working in R with data.table and am trying to add a variable "data_set" to a list of data tables that holds the name of that item in the list.

For example - The list of tables is med, symp, and care and I would like to add the variable data_set to each table where in data table med, "data_set" would equal "med" and in data table symp, "data_set" would equal "symp". I know I can use lapply for this, but I'm stuck on how to store the name of each table in the "data_set" variable. I know it would go something like this....

   tables<-c("med", "symp", "care")
   tables_1<-lapply(mget(tables),function(x)x[, data_set:=…...])

How can I reference and store the name of each data table in the "data_set" variable?

r2evans · Accepted Answer · 2020-05-04 23:37:30Z

3

I suggest you can use Map instead of lapply here.

med <- data.table(a=1); symp <- data.table(b=1); care <- data.table(d=1)
tables <- c("med", "symp", "care")
tables_1 <- Map(function(x, nm) copy(x)[, data_set := nm], mget(tables), tables)
tables_1
# $med
#    a data_set
# 1: 1      med
# $symp
#    b data_set
# 1: 1     symp
# $care
#    d data_set
# 1: 1     care

I use copy inferring that your use of lapply was not needing side-effect. Without copy, your global-env variables (med, symp, care) would have had the column added as well. This might be desired and/or acceptable, in which case remove copy to reduce the memory footprint.

answered May 4, 2020 at 23:37

r2evans

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9 Comments

bziggy Over a year ago

This should work, but I get the error- Error in mget(tables) : invalid first argument

r2evans Over a year ago

If tables is a character vector and all of those names exist in the current environment, then it will work. (I can reproduce that error if I mis-type mget(table), which is using the function table and not the variable tables.)

r2evans Over a year ago

It is likely referencing data.table::tables and failing.

bziggy Over a year ago

I was able to get this to work. My mistake. I was using it on a list that resulted from a previous lapply by accident. If I use it on the original list (as listed in my question) it works! But is there a way to get it to work on a list of data tables that results from a previous lapply? Would I need to alter the list prior to plugging it in?

chinsoon12 Over a year ago

and also lapply(tables, function(x) get(x)[, data_set := x])

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