R convert data.frame to json - arrays

Question

I'm trying to convert a data.frame into json format
my data.frame has the following structure
a <- rep(c("Mario", "Luigi"), each = 3)
b <- sample(34:57, size = length(a))
df <- data.frame(a,b)
> df
a b
1 Mario 43
2 Mario 34
3 Mario 36
4 Luigi 45
5 Luigi 52
6 Luigi 35
What I want to create is something like this (to finally print it to a .json file)
[
{
"a": "Mario",
"b": [43, 34, 36]
},
{
"a": "Luigi",
"b": [45, 52, 35]
}
]
I've tried different packages handling json format but so far failed to produce this kind of output. I usually end up with something like this
[
{
"a":"Mario",
"b":43
},
{
"a":"Mario",
"b":34
},
{
"a":"Mario",
"b":36
},
{
"a":"Luigi",
"b":45
},
{
"a":"Luigi",
"b":52
},
{
"a":"Luigi",
"b":35
}
]

Answer 1

If you nest b as a list column, it will convert correctly:
library(jsonlite)
# converts b to nested list column
df2 <- aggregate(b ~ a, df, list)
df2
## a b
## 1 Luigi 49, 42, 37
## 2 Mario 46, 50, 45
toJSON(df2, pretty = TRUE)
## [
## {
## "a": "Luigi",
## "b": [49, 42, 37]
## },
## {
## "a": "Mario",
## "b": [46, 50, 45]
## }
## ]
or if you prefer dplyr:
library(dplyr)
df %>% group_by(a) %>%
summarise(b = list(b)) %>%
toJSON(pretty = TRUE)
or data.table:
library(data.table)
toJSON(setDT(df)[, .(b = list(b)), by = a], pretty = TRUE)
which both return the same thing.

Answer 2

To get the required JSON structure you will want your data in a list, something like:
l <- list(list(a = "Mario",
b = c(43,34,36)),
list(a = "Luigi",
b = c(45,52,35)))
## then can use the library(jsonlite) to convert to JSON
library(jsonlite)
toJSON(l, pretty = T)
[
{
"a": ["Mario"],
"b": [43, 34, 36]
},
{
"a": ["Luigi"],
"b": [45, 52, 35]
}
]
So to split your data into this format, you can do
l <- lapply(unique(df$a), function(x) list(a = x, b = df[a == x,"b"]) )
## and then the conversion works
toJSON(l, pretty = T)
[
{
"a": ["Mario"],
"b": [44, 49, 50]
},
{
"a": ["Luigi"],
"b": [39, 57, 35]
}
]
This works for the simple case, but if it gets more complex it might be better to re-design how you create your data.frame, and instead create a list(s) to begin with.
Reference
The jsonlite vignette is a very good resource.