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From: Malcolm McLean <malcolm.arthur.mclean@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Which newsgroup for json parsing?
Date: Fri, 17 May 2024 05:24:31 +0100
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On 16/05/2024 20:21, bart wrote:
> On 16/05/2024 19:53, Scott Lurndal wrote:
>> =?UTF-8?Q?Josef_M=C3=B6llers?= <josef@invalid.invalid> writes:
>>> On 16.05.24 18:27, Mikko wrote:
>>>> On 2024-05-08 13:04:56 +0000, Josef Möllers said:
>>>>
>>>>> Hi all,
>>>>>
>>>>> I am trying to parse a json string received through MQTT from a
>>>>> "Shelly Plug S", e.g.
>>>>> {"id":0, "source":"button", "output":true, "apower":0.0,
>>>>> "voltage":237.9, "current":0.000,
>>>>> "aenergy":{"total":0.000,"by_minute":[0.000,0.000,0.000],"minute_ts":1715172119},"temperature":{"tC":41.1, "tF":106.0}}
>>>>>
>>>>> I am trying to use libjson-glib but I can't figure out what to use as
>>>>> the first argument to json_gobject_from_data()!
>>>>> I am also looking at libjson-c but cannot find any examples that could
>>>>> guide me.
>>>>>
>>>>> Thanks in advance,
>>>>
>>>> Sometimes it is easier to make a parser than to use an existing one.
>>>> In this case I might try LEX.
>>>>
>>>
>>> Sorry to be nitpicking, but lex is a "lexical analyzer" aka a "scanner",
>>> it can only recognize tokens, eg the brackets, quotes, identifiers etc,
>>> but not structures.
>>> "yacc" (or its "bison" equivalent" would be a parser but definitely too
>>> heavy.
>>>
>>> What one could do would be to use LEX to recognize the tokens and the
>>> write a recursive descent parser in plain C.
>>
>> I've done full expression parsing and subsequent evaluation with
>> lex (or flex).  No parser needed.
>>
> 
> You've done 'full expression PARSING', then you say 'No PARSER needed'.
> 
> A bit contradictory?
> 
> Clearly parsing IS needed, so either you've used LEX from code that acts 
> as a parser, or it does more than just recognise tokens.
> 
Yes.

I took Ben's advice and completely rewrote my XML parser wirh a formal 
lexer and recursive descent grammar. It was good idea, even though XML 
is simple enough to get away with a ad hoc approach. Similarly, with 
JSON. once you've lexed into identifiers, values, and curly and square 
btackets, the grammar is so simple that you almost don't notice that you 
are writing a parser.
   --
Check out Basic Algorithms and my other books:
https://www.lulu.com/spotlight/bgy1mm