bvstone

Parsing JSON using RPG with Scott Klement's Port of YAJL (Yet Another JSON Library)

Posted:

Parsing JSON using RPG with Scott Klement's Port of YAJL (Yet Another JSON Library)

In a previous article we showed an example of how you can build JSON very easily using Scott Klement's port of the YAJL JSON Library.

In this three part article we will focus on parsing the JSON data.

The data we will be working with in the two examples will be the same data that we built in the previous article on building JSON.

The first example was JSON data that represented our customer records.  The data was presented in a JSON array object with each array element representing one customer's information:

{
    "customerList": [
        {
            "email": "customer1@email.com",
            "lname": "LastName",
            "fname": "FirstName",
            "comp": "Company, Inc.",
            "add1": "Address1",
            "add2": "Address2",
            "city": "Eugene",
            "state": "OR",
            "zip": "97401",
            "country": "USA"
         },
         {
            "email": "customer2@email.com",
            "lname": "LastName",
            "fname": "FirstName",
            "comp": "Company, Inc.",
            "add1": "Address1",
            "add2": "Address2",
            "city": "Fargo",
            "state": "ND",
            "zip": "58201",
            "country": "USA"
         },
        ....
    ]
}

To parse this data we will need to loop through each element in the array and process each array element.  

The following program was what we came up with (using the samples provided as a starting point):

     H DFTACTGRP(*NO) BNDDIR('YAJL')
      ****************************************************************
      * Imports
      ****************************************************************
      /include yajl_h
      ****************************************************************
      * Global Definitions
      ****************************************************************
     D CSTMSTDS      E DS                  EXTNAME(CSTMSTPF)
      ****************************************************************
      * Work Variables
      ****************************************************************
     D docNode         s                   like(yajl_val)
     D custList        s                   like(yajl_val)
     D node            s                   like(yajl_val)
     D val             s                   like(yajl_val)
      *
     D i               s             10i 0
     D errMsg          s            500a   varying inz('')
      ****************************************************************
      /free

       docNode = yajl_stmf_load_tree( '/tmp/makejson1.json' : errMsg );

       if errMsg <> '';
          // handle error
       endif;

       exsr $Header;

       yajl_tree_free(docNode);

       *inlr = *on;
       //***************************************************************
       //* Read Header
       //***************************************************************
       begsr $Header;

         custList = YAJL_object_find(docNode: 'customerList');

         i = 0;

         dow YAJL_ARRAY_LOOP(custList:i:node );
            val = YAJL_object_find(node:'email');
            CMEMAIL = yajl_get_string(val);

            val = YAJL_object_find(node: 'lname');
            CMLNAME = yajl_get_string(val);

            val = YAJL_object_find(node: 'fname');
            CMFNAME = yajl_get_string(val);

            val = YAJL_object_find(node: 'comp');
            CMFCOMP = yajl_get_string(val);

            val = YAJL_object_find(node: 'add1');
            CMFADD1 = yajl_get_string(val);

            val = YAJL_object_find(node: 'add2');
            CMFADD2 = yajl_get_string(val);

            val = YAJL_object_find(node: 'city');
            CMFCITY = yajl_get_string(val);

            val = YAJL_object_find(node: 'state');
            CMFSTATE = yajl_get_string(val);

            val = YAJL_object_find(node: 'zip');
            CMFZIP = yajl_get_string(val);

            val = YAJL_object_find(node: 'country');
            CMFCNT = yajl_get_string(val);
         enddo;

       endsr;

In the D-Specs we first see a few variables defined "like" yajl_val.  These are pointers that will be used to point to the beginning of each JSON object that we process.  Think of these pointers as 
"holding places" so we know where we are in the JSON structure.  Similar to when you're reading a book and you put a finger on the first paragraph, then as you read each word use your other finger to follow the words in the paragraph.

The first variable, docNode, is used to point to the very beginning of the JSON data.  Once we load the JSON data using yajl_stmf_load_tree() the docNode pointer will point to the start of the data.  

Next, we need to find the beginning of the first element in the data.  In this case it's an array named customerList and will point to the beginning of the array of customer data JSON objects.  We find the start using the YAJL_object_find() procedure.

The YAJL_object_find() procedure accepts 2 parameters

  1. An input parameter that should be a pointer to the parent object (in this case, the entire JSON data set)
  2. The name of the node we want to find (in this case, customerList)

If found, a pointer to the object will be returned.  If not found, the pointer value will be *NULL.

Once we find the customer array we need to loop through each item and process them.  This is done with a very interesting procedure named YAJL_ARRAY_LOOP().  This procedure takes 3 parameters:

  1. An input parameter that is a pointer to the beginning of the array to process.  In this case, it should be the pointer we found earlier that points to the beginning of the customer array, customerList.
  2. An input/output parameter that is the current index being processed (that should be set to zero before processing).  This index is incremented automatically on each iteration of the loop by the procedure itself.  In our case this is a variable named i.
  3. A output parameter that holds a pointer to the current array element that is being processed.  This is important when we want to find information for the specific array element.  In our case this pointer will be named node.

For each item in the array we process them by using the YAJL_object_find() procedure and the name of each item in the customer object we want to get the data for.  It's important to note that the name of the node we are using for each piece of customer data is node.  This is the parameter returned from the YAJL_ARRAY_LOOP() call that was executed previously.

Take a look at this figure for a visual representation of what is happening:

So, right now we have 3 pointers.  docNode and customerList are reference points in the JSON data for the entire document and the beginning of the customer list, respectively.  

The third pointer, node travels down each array element of the customer list as we loop through.  This means we will have access to the data in each node and the data can be processed normally.

This example is a very simple one, and in the Part 2 of this series we will be processing another set of data that includes embedded arrays more than one level deep.

Related Articles:


Last edited 07/12/2019 at 10:52:09



Latest Posts:

GreenTools for G Suite (G4G) Now Includes Send Mail Functionality (G4GSMAIL Addon) GreenTools for G Suite (G4G) Now Includes Send Mail Functionality (G4GSMAIL Addon)
Posted by August 27, 2019
BVSTools >> BVSTools Announcements >> GreenTools for G Suite (Google Apps) (G4G) Specific Announcements
GreenTools For G Suite (G4G) v12.00 Released With Base OAuth 2.0 Functionality GreenTools For G Suite (G4G) v12.00 Released With Base OAuth 2.0 Functionality
Posted by July 28, 2019
BVSTools >> BVSTools Announcements >> GreenTools for G Suite (Google Apps) (G4G) Specific Announcements
BVSTools Small Price Increase in 2020 BVSTools Small Price Increase in 2020
Posted by July 26, 2019
BVSTools >> BVSTools Announcements
GreenTools for Vertex Cloud (VTXCLOUD) Now Available GreenTools for Vertex Cloud (VTXCLOUD) Now Available
Posted by July 22, 2019
BVSTools >> BVSTools Announcements >> GreenTools for Vertex Cloud (VTXCLOUD) Specific Announcements
GreenTools for Google Apps (G4G) - Drive Addon Successfully Verified by Google GreenTools for Google Apps (G4G) - Drive Addon Successfully Verified by Google
Posted by July 22, 2019
BVSTools >> BVSTools Announcements >> GreenTools for G Suite (Google Apps) (G4G) Specific Announcements
Why I Cancelled my DynDNS Service and How I Replaced It with an IBM i Application Why I Cancelled my DynDNS Service and How I Replaced It with an IBM i Application
Posted by July 17, 2019
IBM Power Systems >> (QGPL) IBM i
Green Tools for G Suite (G4G) Product Updates (Licensing, Functionality, Base Product) Green Tools for G Suite (G4G) Product Updates (Licensing, Functionality, Base Product)
Posted by July 13, 2019
BVSTools >> BVSTools Announcements >> GreenTools for G Suite (Google Apps) (G4G) Specific Announcements
Reading JSON Data from Standard Input With YAJL and RPG Reading JSON Data from Standard Input With YAJL and RPG
Posted by July 12, 2019
Programming >> Proof of Concept (POC)
MAILTOOL Updated to Allow Use of IBM Global Security Kit (GSKIT) for SSL/TLS Communications MAILTOOL Updated to Allow Use of IBM Global Security Kit (GSKIT) for SSL/TLS Communications
Posted by June 19, 2019
BVSTools >> BVSTools Announcements >> eMail Tool (MAILTOOL) Specific Announcements
GETURI v10.00 Released Supporting IBM Global Security Kit (GSKIT) and Server Name Indication (SNI) GETURI v10.00 Released Supporting IBM Global Security Kit (GSKIT) and Server Name Indication (SNI)
Posted by June 11, 2019
BVSTools >> BVSTools Announcements >> Get URI (GETURI) Specific Announcements
BVSTools Now Offers Vertex Cloud Interface BVSTools Now Offers Vertex Cloud Interface
Posted by April 15, 2019
BVSTools >> BVSTools Announcements
Token Has an Invalid Signature Error for Office 365 Email Token Has an Invalid Signature Error for Office 365 Email
Posted by March 22, 2019
BVSTools >> BVSTools Software Discussion >> GreenTools for Microsoft Apps (G4MS) Specific Discussion
Resending Emails that have Errored Out with Updated Router or Authentication Information Resending Emails that have Errored Out with Updated Router or Authentication Information
Posted by March 1, 2019
BVSTools >> BVSTools Software Discussion >> Email Tools (MAILTOOL) Specific Discussion
BVSTools Offers Toolset to Work With HubSpot OAuth 2.0 APIs On Your IBM i BVSTools Offers Toolset to Work With HubSpot OAuth 2.0 APIs On Your IBM i
Posted by January 27, 2019
BVSTools >> BVSTools Announcements
G4MSDRV Currently Not Supported G4MSDRV Currently Not Supported
Posted by January 17, 2019
BVSTools >> BVSTools Announcements >> GreenTools for Microsoft Apps (G4MS) Specific Announcements

Reply




Copyright 1983-2019 BVSTools
GreenBoard(v3) Powered by the eRPG SDK, MAILTOOL Plus!, GreenTools for Google Apps, jQuery, jQuery UI, BlockUI, CKEditor and running on the IBM i (AKA AS/400, iSeries, System i).