How to write to json with children from R

Question

I want to turn an R data.frame into a JSON object in order to use it for preparing data visualizations with d3.js. I found a lot of questions that asked how to get JSON into R, but very few on how to write data from R to JSON.

A particular problem is that the JSON file needs to be nested by using factors, i.e. columns of the data.frame. I think that writing from nested lists could be a solution, but I already failed to create a nested list from a data.frame :(

I have preprared an example:

this represents my data.frame (called "MyData").

ID  Location Station   Size Percentage
1     Alpha    Zeta    Big       0.63
2     Alpha    Zeta Medium       0.43
3     Alpha    Zeta  small       0.47
4     Alpha    Yota    Big       0.85
5     Alpha    Yota Medium       0.19
6     Alpha    Yota  small       0.89
7      Beta   Theta    Big       0.09
8      Beta   Theta Medium       0.33
9      Beta   Theta  small       0.79
10     Beta    Meta    Big       0.89
11     Beta    Meta Medium       0.71
12     Beta    Meta  small       0.59

now, I want to turn it into something like this valid json format, including the children nodes:

   {
 "name":"MyData",
 "children":[
   {
     "name":"Alpha",
     "children":[
        {
           "name":"Zeta",
           "children":[
              {
                 "name":"Big",
                 "Percentage":0.63
              },
              {
                 "name":"Medium",
                 "Percentage":0.43
              },
              {
                 "name":"Small",
                 "Percentage":0.47
              }
           ]
        },
        {
           "name":"Yota",
           "children":[
              {
                 "name":"Big",
                 "Percentage":0.85
              },
              {
                 "name":"Medium",
                 "Percentage":0.19
              },
              {
                 "name":"Small",
                 "Percentage":0.89
              }
           ]
        }
    ]   
},
    {
     "name":"Zeta",
     "children":[
        {
           "name":"Big",
           "Percentage":0.63
        },
        {
           "name":"Medium",
           "Percentage":0.43
        },
        {
           "name":"Small",
           "Percentage":0.47
        }
     ]
  },
  {
     "name":"Yota",
     "children":[
        {
           "name":"Big",
           "Percentage":0.85
        },
        {
           "name":"Medium",
           "Percentage":0.19
        },
        {
           "name":"Small",
           "Percentage":0.89
        }
     ]
  }
  ]
 }

If anyone could help me out I would be very much grateful! thank you

Answer 1

This is a recursive approach which is cleaner:

require(RJSONIO)

makeList<-function(x){
  if(ncol(x)>2){
    listSplit<-split(x[-1],x[1],drop=T)
    lapply(names(listSplit),function(y){list(name=y,children=makeList(listSplit[[y]]))})
  }else{
    lapply(seq(nrow(x[1])),function(y){list(name=x[,1][y],Percentage=x[,2][y])})
  }
}


jsonOut<-toJSON(list(name="MyData",children=makeList(MyData[-1])))
cat(jsonOut)