R convert json to display in data.table

Question

I have a data table where one of the columns contains JSON. I am trying to extract the contents so that each variable is a column.

library(jsonlite)
library(data.table)

df<-data.table(a=c('{"tag_id":"34","response_id":2}',
                   '{"tag_id":"4","response_id":1,"other":4}',
                   '{"tag_id":"34"}'),stringsAsFactors=F)

The desired result, which does not apply to the "other" variable:

   tag_id response_id 
1     "34"      2 
2     "4"       1 
3     "34"      NA

I tried several versions:

parseLog <- function(x){
  if (is.na(x))
    e=c(tag_id=NA,response_id=NA)
  else{
    j=fromJSON(x)
    e=c(tag_id=as.integer(j$tag_id),response_id=j$response_id)
  }
  e
}

which seems to work well to get a list of vectors (or lists, if c is replaced by a list), but when I try to convert the list to data.table, something doesn't work properly.

   parsed<-lapply(df$a,parseLog)
   rparsed<-do.call(rbind.data.frame,parsed)
   colnames(rparsed)<-c("tag_id","response_id")

Due to a missing value in the third row. How can I solve this problem in pure R-ish? How can I make the parse method return NA for a missing value. Alternative. Is there a "fill" parameter, for example, for rbind, which can be used in rbind.data.frame or a similar method?

, , 11M , .

, rbind.data.frame . ? , rbindlist, , , ( do.call ):

rparsed<-do.call(rbindlist,fill=T,parsed)

: , , , 11M :

df<-data.table(a=c('{"tag_id":"34","response_id":2}',
                   '{"trash":"34","useless":2}',                          
                   '{"tag_id":"4","response_id":1,"other":4}',
                   NA,
                   '{"response_id":"34"}', 
                   '{"tag_id":"34"}'),stringsAsFactors=F)

tag_id response_id.

Answer 1

, , , , :

library(data.table)
library(jsonlite)
df[, json := sapply(a, fromJSON)][, rbindlist(lapply(json, data.frame), fill=TRUE)]

#or if you need all the columns :
#df[, json := sapply(a, fromJSON)][,
#   c('tag_id', 'response_id') := rbindlist(lapply(json, data.frame), fill=TRUE)]

:

> df[, json := sapply(a, fromJSON)][, rbindlist(lapply(json, data.frame), fill=TRUE)]
   tag_id response_id
1:     34           2
2:      4           1
3:     34          NA

EDIT:

.

, , data.frame :

df[, json := sapply(a, fromJSON)][, 
   rbindlist(lapply(json, function(x) data.frame(x)[-3]), fill=TRUE)]

#   tag_id response_id
#1:     34           2
#2:      4           1
#3:     34          NA