create a named list element, if it doesnt exist already in R

Question

I have a list, masterList, to which i want to append another lists:, contentList and contentList2. the content lists are the result belonging to "01-response" - the first element of the masterList.

masterList <- list("01-response" = NULL, "02-response" = NULL,"03-response" = NULL,"04-response" = NULL,"05-response" = NULL)
contentList <- list(item1 = "text", item2 = "text2")
contentList2 <- list(item1 = "moretext", item2 = "moretext2")

I want to append contentList and contentList2 to masterList[["01-response"]]. However, i want all the contents to be stored inside a named list inside the masterList: `masterList[["01-response"]][["contents"]], like:

masterList
$`01-response`
$`01-response`$content
$`01-response`$content$item1
[1] "text"

$`01-response`$content$item2
[1] "text2"

$`01-response`$content$item1
[1] "moretext"

$`01-response`$content$item2
[1] "moretext2"



$`02-response`
NULL

$`03-response`
NULL

$`04-response`
NULL

$`05-response`
NULL 

The problem is with the appending. I need to check if `masterList[["01-response"]][["contents"]] exists, before i append. If it exists, i simply append. If it doesnt exist, i need to create it first.

Lets specify the element as a variable, such: listElement <- "01-response". If i were to add a third list, contentList3 <- list(item1 = "moretext3", item2 = "moretext4"), i would simply run:

`listElement <- "01-response"`
    if(exists("content", where = masterList[[listElement]])){
      masterList[[listElement]][["content"]] <- append(masterList[[listElement]][["content"]],
                                                                         contentList3) 
    }else{
      masterList[[listElement]] <- append(masterList[[listElement]],
                                                      list(content = contentList3)) 
    }

However, this code breakes if the masterList is empty:

masterList
$`01-response`
NULL

$`02-response`
NULL

$`03-response`
NULL

$`04-response`
NULL

$`05-response`
NULL

exists("content", where = masterList[[listElement]])
Error in as.environment(where) : using 'as.environment(NULL)' is defunct

How can i check if "content" exists at the level of masterList[[listElement]]?

Note: this happens inside an function, threfore i want to remain flexible and avoid using masterList[["01-response"]]. I use masterList[[listElement]] instead, where listElement <- "01-response"

Answer 1

I think you can work without knowing if content exists already.

masterList[["01-response"]]$content <- c(masterList[["01-response"]]$content, contentList, contentList2)
str(masterList)
# List of 5
#  $ 01-response:List of 1
#   ..$ content:List of 4
#   .. ..$ item1: chr "text"
#   .. ..$ item2: chr "text2"
#   .. ..$ item1: chr "moretext"
#   .. ..$ item2: chr "moretext2"
#  $ 02-response: NULL
#  $ 03-response: NULL
#  $ 04-response: NULL
#  $ 05-response: NULL

The trick of this is that c(x, y) will create if x is NULL, and append if x is a pre-existing list.

To answer one of your questions:

How can i check if "content" exists at the level of masterList[[listElement]]?

"content" %in% names(masterList[["01-response"]])