Fortran 95 JSON Parser
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.

Fortran 95 JSON Parser

Getting Started

program example1

    ! Typical usage should only require an explicit use of the fson module.
    ! The other modules will be used privately by fson as required.  
    use fson

    ! declare a pointer variable.  Always use a pointer with fson_value.
    type(fson_value), pointer :: value

    ! parse the json file
    value => fson_parse("test1.json")

    ! print the parsed data to the console
    call fson_print(value)    

    ! extract data from the parsed value        

    ! clean up
    call fson_destroy(value)

end program example1

Example JSON

This JSON will serve as a reference for the following examples.
If you are not already familiar with JSON you can read more at: and

     "name"       : {"first": "John", "last" : "Smith"},
     "age"        : 25,
     "address"    :
       { "streetAddress": "21 2nd Street",
         "city"         : "New York",
         "state"        : "NY",
         "postalCode"   : "10021"},
       [ {"type"  : "home", "number": "212 555-1234"},
         {"type"  : "fax",  "number": "646 555-4567"} ]

Extracting Data

Getting the data from the parsed fson_value to your variable is easy. All extraction is performed through a call to fson_get().
This subroutine is overloaded for different target value types.

program example1

! The fson mudule has the basic parser and lookup 
use fson

! Functions for accessing data as an array
use fson_value_m, only: fson_value_count, fson_value_get
character(len=1024) :: strval, strval2
integer i

! Declare a pointer variables.  Always use a pointer with fson_value.
type(fson_value), pointer :: json_data, array, item

! Parse the json file
json_data => fson_parse("test1.json")

! Get the first and last name and print them
call fson_get(json_data, "name.first", strval)
call fson_get(json_data, "name.last",  strval2)
print *, "name.first = ", trim(strval)
print *, "name.last  = ", trim(strval2)

! Use a lookup string to get the first phone number
call fson_get(json_data, "PhoneNumber[1].number", strval)     
print *, "PhoneNumber[1].number = ", trim(strval)
print *, ""

! Get the phone numbers as an array
call fson_get(json_data, "PhoneNumber", array)

! Loop through each array item
do i = 1, fson_value_count(array)
  ! Get the array item (this is an associative array)
  item => fson_value_get(array, i)
  ! Lookup the values from the array
  call fson_get(item, "type", strval)
  call fson_get(item, "number", strval2)
  ! Print out the values
  print *, "Phone Number:"
  print *, "type = ", trim(strval), ", number = ", trim(strval2)
end do

! clean up
call fson_destroy(json_data)

end program example1

The program output is the following:

name.first = John
name.last  = Smith
PhoneNumber[1].number = 212555-1234

Phone Number:
type = home, number = 212555-1234
Phone Number:
type = fax, number = 646555-4567

Extracting arrays

You can also extract entire arrays, as Fortran allocatable arrays, using fson_get(). This assumes the array's elements are all of the same type (integer, real, double precision, logical or character). Rank-1 (vector) or rank-2 (matrix) arrays are supported.

The following example parses a JSON file containing an vector of integers called "v" and a matrix of real numbers called "m", and prints them:

program extract_array

  use fson
  implicit none
  type(fson_value), pointer :: data
  integer, allocatable :: vec(:)
  real, allocatable :: mat(:,:)
  data => fson_parse("data.json")

  call fson_get(data, "v", vec)
  call fson_get(data, "m", mat)
  print *, vec
  print *, mat

  call fson_destroy(data)
  deallocate(vec, mat)

end program extract_array

When extracting character arrays, it is necessary to specify a string length when the array is declared, e.g.:

integer, parameter :: str_len = 8
character(len = str_len), allocatable :: m(:)

data => fson_parse("data.json")
call fson_get(data, "char_array", m)


| Operator | Description              | 
|    $     | Root object/value        | 
|    .     | Child operator           |
|    []    | Array element            |