How to access elements of an array within a TMDB json data? - c++

Question

all!
I request a JSON from TMDB and save it as a file on the local harddisk. Then I read in the file and decode it with nlohmann::json. The next step is to iterate the json data and extract a few parts of the info. While it is no problem getting types „string“, „boolean“ etc. I’m struggling with an „array“ type. (Later on the type „object“ might show same problems…) Goal is to transform the json data into some „ini“ style type like
[tt1234567]
title = abcdefghij
runtime = 123
...
I iterate through the root of the decoded json by:
using json = nlohmann::json;
{
auto jsonData = json::parse( jsonText );
// std::cout << jsonData.dump( 1 ) << "\n";
for ( const auto &jsonItem : jsonData.items() )
{
jsonKey = jsonItem.key();
jsonValue = "";
if ( jsonItem.value().is_null() ) { jsonValue = "(null)"; }
else if ( jsonItem.value().is_boolean() ) { if ( jsonItem.value() ) { jsonValue = "(boolean) yes"; } else { jsonValue = "(boolean) no"; } }
else if ( jsonItem.value().is_string() ) { jsonValue = "(string) '" + string_left( jsonItem.value(), 25 ) + "'"; }
[ . . . ]
std::cout << jsonKey << ": " << jsonValue << "\n";
Screen output is like:
adult: (boolean) no
belongs_to_collection: (null)
budget: (unsigned) 45000000
credits: (object)
genres: (array) [ . . . ]
[ et al ]
My problem is that I don’t know the correct syntax to handle the „array“ type and, in fact, I’m not quite sure if it is really an array despite the fact it is enclosed in []. The code block
else if ( jsonItem.key() == "genres" ) // array
{
std::cout << " jsonItem: " << jsonItem << "\n"; // {"genres":[{"id":12,"name":"Abenteuer"},{"id":28,"name":"Action"}]}
jsonKey = jsonItem.key();
std::cout << " jsonKey: " << jsonKey << "\n"; // genres
// jsonValue = jsonItem.value(); // <-- returns array, but jsonValue expects string
// std::cout << " jsonValue: " << jsonValue << "\n";
auto jsonValueArray = jsonItem.value().array();
std::cout << " jsonValueArray: " << jsonValueArray << " (" << sizeof( jsonValueArray ) << ")\n"; // [] (16)
auto jsonValueFlat = jsonItem.value().flatten();
std::cout << " jsonValueFlat: " << jsonValueFlat << "\n"; // {"/0/id":12,"/0/name":"Abenteuer","/1/id":28,"/1/name":"Action"}
std::cout << " " << jsonKey << " elements: " << jsonValueArray.size() << "\n"; // 0
i = 0;
// for ( const auto &jsonValue : jsonValueArray )
// for ( i = jsonValueArray.begin(); i < jsonValueArray.end(); i++ )
for ( i = 0; i < jsonValueArray.size(); i++ )
{
std::cout << jsonValue << "\n";
iniKey = "Genre" + std::to_string( i );
iniValue = "";
iniValue = jsonValue;
iniText.append( iniKey );
iniText.append( " = " );
iniText.append( iniValue );
iniText.append( "\n" );
// i++;
}
}
produces
jsonItem:       {"genres":[{"id":12,"name":"Abenteuer"},{"id":28,"name":"Action"}]}
jsonKey: genres
jsonValueArray: [] (16)
jsonValueFlat:  {"/0/id":12,"/0/name":"Abenteuer","/1/id":28,"/1/name":"Action"}
genres elements: 0
So I see a jsonItem with „genres: [xxx]“ content, thus it is identified as an array. The „sizeof“ returns 16 and I interpret it as 4 pointer with 4 bytes each (or 2 with 8 bytes?). On the other hand the array() function seems to return an empty array [] with 0 elements. And now I’m stuck…
What I want to achieve: Extracting the „genres“ list from the json and concatenate the elements with „;“ like
genres = Abenteuer;Action
in the above exampe.
Michael

Answer 1

Ok, I figured it out. The central point was the misunderstanding of .array() from the JSON object. After fiddling around a bit the following branch
else if ( jsonItem.key() == "genres" ) // array
{
std::cout << " jsonItem: " << jsonItem << "\n"; // {"genres":[{"id":12,"name":"Abenteuer"},{"id":28,"name":"Action"}]}
jsonKey = jsonItem.key();
std::cout << " jsonKey: " << jsonKey << "\n"; // genres
auto jsonValueArray = jsonItem.value(); // [{"id":12,"name":"Abenteuer"},{"id":28,"name":"Action"}] (16)
std::cout << " jsonValueArray: " << jsonValueArray << " (" << sizeof( jsonValueArray ) << ")\n"; // [] (16)
i = 0;
for ( const auto &jsonValue : jsonValueArray )
{
iniKey = "Genres";
iniValue = jsonValue["name"];
std::cout << " " << jsonValue << " --> key:'" << iniKey << "', value:'" << iniValue << "'\n";
if ( i == 0 )
{
iniText.append( iniKey );
iniText.append( " = " );
}
else
{
iniText.append( ";" );
}
iniText.append( iniValue );
i++;
}
iniText.append( "\n" );
std::cout << " genres added: " << std::to_string( i ) << "\n";
}
now produces
genres: (array) [ . . . ]
jsonItem: {"genres":[{"id":12,"name":"Abenteuer"},{"id":28,"name":"Action"}]}
jsonKey: genres
jsonValueArray: [{"id":12,"name":"Abenteuer"},{"id":28,"name":"Action"}] (16)
{"id":12,"name":"Abenteuer"} --> key:'Genres', value:'Abenteuer'
{"id":28,"name":"Action"} --> key:'Genres', value:'Action'
genres added: 2
resp.
[tt0446013]
adult = no
Genres = Abenteuer;Action
ID_TMDB = 1534
ID_IMDB = tt0446013
Title_Original = Pathfinder
Overview = An sich stammt der junge
Release_Date = 2007-01-11
Runtime = 99
Tagline = Zwei Welten , ein Krieger
Title = Pathfinder - Fährte des
and that's the output I wanted to have.