Object not found for lapply data.table within function

Question

Suppose I have the following R code:

library(data.table)
L <- list(a=data.table(x=c(1,2,3),y=c(10,20,30)),
          b=data.table(x=c(4,5,6),y=c(40,50,60)),
          c=data.table(x=c(7,8,9),y=c(70,80,90)))


columnName <- "x"         
r <- lapply(L,"[",i=get(columnName) %in% c(1,4))

f <- function(L1) {
     columnName1 <- "x"
     r1 <- lapply(L1,"[",i=get(columnName1) %in% c(1,4))
     return(r1)
}

r1 <- f(L)

My question is: Why does the assignment to r1 at the bottom fail inside the function with

Error in get(columnName1) : object 'columnName1' not found

The assigment to r further up works fine. Likewise, if, inside the function I change it to a global assignment for columnName1 via <<-, but then I have created a global variable which I don't really want.... How can I cleanly rewrite this such that data.table finds columnName1 within its scope? And what am I missing about scoping? I would have thought that if it can't find columnName1 within the "[" function it will look one environment "up" and find it there? It must look in the global environment though, but not in the parent?

Answer 1

I'm guessing slight what you're looking to return from the data.table. If you're looking to return rows where column x equals 1 and 4 it's usually easier to get it to work with a single value from the list and then get it to work with lapply

library(data.table)
columnName1 <- "x"
L$a[get(columnName1) %in% c(1,4)]

to iterate through the list

lapply(L, function(x) x[get(columnName1) %in% c(1,4)])

if you want a function that can specify the column name and row numbers

f <- function(list, col, row) {lapply(list, function(x, lcol, lrow) x[get(lcol) %in% lrow], lcol=col, lrow=row)}
f(L, "x", c(1,4))