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Type Mapping

Source: vignettes/type-mapping.Rmd
type-mapping.Rmd

JSON and R have similar but not identical type systems. This article summarizes the common mappings used by fromJSON() and toJSON().

JSON to R 

#>         JSON                                            R
#> 1     object        named list or simplified named vector
#> 2      array             list or simplified vector/matrix
#> 3     string                                    character
#> 4     number                                      numeric
#> 5 true/false                                      logical
#> 6       null NULL or the value supplied through nullValue
str(fromJSON('{"a": 1, "b": 2}', simplify = FALSE))
#> List of 2
#>  $ a: num 1
#>  $ b: num 2
str(fromJSON("[1, 2, 3]"))
#>  num [1:3] 1 2 3
str(fromJSON('["a", "b", "c"]'))
#>  chr [1:3] "a" "b" "c"
str(fromJSON("[true, false]"))
#>  logi [1:2] TRUE FALSE
str(fromJSON("[1, null, 3]", nullValue = NA, simplify = TRUE))
#>  num [1:3] 1 NA 3

R to JSON 

#>                     R                              JSON
#> 1          named list                            object
#> 2        unnamed list                             array
#> 3       atomic vector                             array
#> 4 named atomic vector                            object
#> 5              matrix                      nested array
#> 6          data.frame object by column, or array by row
#> 7                  NA                              null
#> 8                NULL                              null
toJSON(list(a = 1, b = TRUE))
#> [1] "{\n \"a\":        1,\n\"b\": true \n}"
toJSON(list(1, TRUE, "x"))
#> [1] "[\n        1,\ntrue,\n\"x\" \n]"
toJSON(c(1, 2, 3))
#> [1] "[        1,        2,        3 ]"
toJSON(c(a = 1, b = 2))
#> [1] "{\n \"a\":        1,\n\"b\":        2 \n}"
toJSON(matrix(1:4, nrow = 2))
#> [1] "[ [ 1, 3 ],\n[ 2, 4 ] ]"
toJSON(data.frame(a = 1:2, b = c("x", "y")))
#> [1] "{\n \"a\": [ 1, 2 ],\n\"b\": [ \"x\", \"y\" ] \n}"
toJSON(NA)
#> [1] "[ null ]"
toJSON(NULL)
#> [1] "[ null ] "

Empty containers

Empty containers can carry different meaning depending on names.

toJSON(list())
#> [1] "[]"
toJSON(emptyNamedList)
#> [1] "{}"

fromJSON("[]")
#> list()
fromJSON("{}")
#> named list()

Simplification constants

The exported constants can be combined to control simplification.

Strict
#> [1] 14
StrictNumeric
#> [1] 4
StrictCharacter
#> [1] 8
StrictLogical
#> [1] 2

fromJSON('[1, true]', simplify = Strict)
#> [[1]]
#> [1] 1
#> 
#> [[2]]
#> [1] TRUE
fromJSON('[1, true]', simplify = TRUE)
#> [1] 1 1