parsing3.md

Parsing a Json object - part 3: JSON strings

At last, we have all the tools needed to parse a JSON file. Yet, the most common usage of json is to request the data from a website, download the data, and store it in a string. Parsing, processing, and displaying the data are the last things we do. For such usage, we wouldn't use the json_object_from_file function. Admittedly we could: download the file and store it somewhere and then load the file into a JSON object. All this is unnecessary, wasteful, and slow. Json-c has a function to create a json_object from a C string:

• json_object* json_tokener_parse(const char *str)

The usage of this function is rather simple.

Consider json-str00.c:

#include <stdio.h>
#include <json-c/json.h>

int 
main(void)
{
   char * str = "{\"firstName\" : \"John\", \"lastName\" :  \"Smith\", \"email\":[{\"type\": \"personal\",\"url\": \"[email protected]\"},{\"type\": \"office\",\"url\": \"[email protected]\"} ] }";
   json_object *root = json_tokener_parse(str);
   printf("The json string:\n\n%s\n\n", json_object_to_json_string(root));
   
   printf("The json object to string:\n\n%s\n", json_object_to_json_string_ext(root, JSON_C_TO_STRING_PRETTY));
   json_object_put(root);
   return 0;
}

Once compiled and executed, the results:

$ ./json-str00 
The json string:

{ "firstName": "John", "lastName": "Smith", "email": [ { "type": "personal", "url": "[email protected]" }, { "type": "office", "url": "[email protected]" } ] }

The json object to string:

{
  "firstName":"John",
  "lastName":"Smith",
  "email":[
    {
      "type":"personal",
      "url":"[email protected]"
    },
    {
      "type":"office",
      "url":"[email protected]"
    }
  ]
}

Not much to say here, other than it is a hassle to escape all the quote characters in a JSON representation for usage in a C program. However, this is not a problem for downloading and storing JSON from the internet.

Problem:

1. Take json-parse04.c previously discussed and add the macro below and use json_tokener_parse(SAMPLE_JSON) instead of json_object_from_file. Save the file as json-parse05.c.

Verify everything works as before:

#define SAMPLE_JSON                            \
   "{"                                         \
   "  \"firstName\": \"John\","                \
   "  \"lastName\": \"Smith\","                \
   "  \"isAlive\": true,"                      \
   "  \"age\": 27,"                            \
   "  \"address\": {"                          \
   "    \"streetAddress\": \"21 2nd Street\"," \
   "    \"city\": \"New York\","               \
   "    \"state\": \"NY\","                    \
   "    \"postalCode\": \"10021-3100\""        \
   "  },"                                      \
   "  \"phoneNumbers\": ["                     \
   "    {"                                     \
   "      \"type\": \"home\","                 \
   "      \"number\": \"212 555-1234\""        \
   "    },"                                    \
   "    {"                                     \
   "      \"type\": \"office\","               \
   "      \"number\": \"646 555-4567\""        \
   "    }"                                     \
   "  ],"                                      \
   "  \"children\": ["                         \
   "    {"                                     \
   "      \"name\": \"Tammy Smith\","          \
   "      \"age\": 3"                          \
   "    },"                                    \
   "    {"                                     \
   "      \"name\": \"Judah Smith\","          \
   "      \"age\": 7"                          \
   "    }"                                     \
   "  ],"                                      \
   "  \"spouse\": {"                           \
   "      \"name\": \"Amanda Smith\","         \
   "      \"age\": 23"                         \
   "  }"                                       \
   "}"