[Solved] Parsing a JSon reply issue

roseicollis

Hi,

I'm doing a request to an url which gives me the next Json reply:

@
"{"data":"[{"Name":"One","Code":""},{"Name":"Two","Code":"Alpha"}]"}" @

I am using Qt 8.4.5 so I have to do it with QNetwork and QScript. So I did the next:

@
QNetworkRequest request(_Url);
QNetworkReply *reply = nam->get(request);

QString InfoByteArray = (QString)reply->readAll();

if ( reply->error()== QNetworkReply::NoError)
{
//------------ JSON Parser --------------//

QScriptEngine engine;
QScriptValue result = engine.evaluate("("+InfoByteArray+")");

QScriptValue InfoData= result.property("data");
qDebug() << "InfoData" << InfoData.toString();
}
@

With that I get all the part on the right of "data":

@
[{"Name":"One","Code":""},{"Name":"Two","Code":"Alpha"}]"
@

Now I want to take the Name en Code values ofc but I can't get them, I try doing this but never goes into the while:

@
QScriptValueIterator it(InfoData);
while (it.hasNext())
{
it.next();
QScriptValue entry = it.value();

QString name= entry.property("Name").toString(); //this doesnt works
QString name2= entry.property("data").property("Name").toString(); //this either

}
@

And if I try doing that out of the while, I just get empty strings :
@
QString name= result.property("Name").toString(); // qDebug() << name; returns ""
QString name2= entry.property("data").property("Name").toString();
@

So...Any Idea of what am I doing wrong here?

Thank you!

andre

You don't "have to use" Qt script. There are plenty of other parsers out there. "This one":https://github.com/nlohmann/json for instance. Or "this":http://libqxt.bitbucket.org/doc/tip/qxtjson.html one if you want to stay closer to Qt.

roseicollis

Hi Andre,

What I don't understand is why I was able to get a part but not all I need... I mean, I was following this example: "Example":http://www.makina-corpus.org/blog/access-json-webservice-qt-c

So the logical thing is that if I got the right-side part of "data", I had to be able to get the next info, right? :P

Btw...Thank you, I'll take a look with the second one (dunno how to include the first one on my project still). One quick question about the second one:

To use that library I understand that you have to include the Qxt module to the pro:

CONFIG += qxt
QXT = core

and then: #include <Qxt> But seems tahtmy project still doesnt have the Qxt library (I've run qmake and build it again)

andre

You can usually just pick up single classes too. AFAIK, the project as such is no longer maintained, and the recommended use is now to use classes directly.

roseicollis

So I add json.cc and json.h from the first link to my project...

I got some errors that I solved but now it says:

error: 'using object_t = class std::map<std::basic_string<char>, nlohmann::json>' has no member named 'emplace'

In a line in middle of the *.cc.... Isn't there any other way????
Hate when things aren't easy -.-" hehe

andre

Sounds like you are not using C++/11, which is needed for this code to work.

roseicollis

Yes, that was the first error i solved. I put it on the .pro file as

QMAKE_CXXFLAGS += -std=c++11
and tried too with
QMAKE_CXXFLAGS += -std=gnu++11

And the compile output gaves something like:

@
g++ -c -pipe -std=c++11 -g -Wall -W -D_REENTRANT -DQT_SCRIPT_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/lib/qt4/mkspecs/linux-g++[...]@

andre

Did you try with simply putting
@
CONFIG += c++11
@
in your .pro file?

roseicollis

Then I get the same error I had first:

/usr/lib/gcc/i686-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support is currently experimental, and must be enabled with the -std=c++11 or -std=gnu++11 compiler options.

Which I solved with
@
QMAKE_CXXFLAGS = -std=c++11

(and even with the next line instead of the other the error dissapears)
QMAKE_CXXFLAGS = -std=gnu+11
@

The error I was saying is in line 197:
@
for (auto& element : a)
{
value_.object->emplace(std::make_pair(std::move(element[0]), std::move(element[1]))); // HERE IS THE ERROR
}
@

roseicollis

I've found this other solution: "JSon":https://github.com/gaudecker/qt-json

It seems that it works but now I can't get the name and code values, I'm trying to do:

@
bool ok;

QtJson::JsonObject result = QtJson::parse(myJson, ok).toMap();
if(!ok) {
qFatal("An error occurred during parsing");
}else
{
qDebug() << "data" << result["data"].toString();
qDebug() << " try1" << result["Name"].toString();

QtJson::JsonObject nested = result["data"].toMap();
qDebug() << "try2:" << nested["Name"].toString();

@

But try1 and try2 are empy. Any Idea of how to make that work?

Thanks!

JKSH

Hi,

[quote author="roseicollis" date="1423496917"]But try1 and try2 are empy. Any Idea of how to make that work?[/quote]Your data is a JSON array, not a JSON object.

roseicollis

Oh I see...

But if I put:
@
QtJson::JsonArray nested = result["data"].toMap();
qDebug() << "try2:" << nested["Name"].toString();
@

It says:
@
conversion from 'QMap<QString, QVariant>' to non-scalar type 'QtJson::JsonArray {aka QList<QVariant>}' requested
@

I don't really know how to parse that array... I've tried this too:

@
QtJson::JsonObject bbb = QtJson::parse(result["data"].toString(), ok).toMap();
qDebug() << bbb;
@
But it only prints: QMap()

So... I'm really stuck there... -.-''

JKSH

Are you familiar with JSON syntax?

represents an array.
{ } represents an object.

You need to understand your structure first: You have 1 array. That array contains 2 objects. Each object contains 2 strings

Extract the objects from the array like this:
@
auto object1 = result[0];
auto object2 = result[1];
@

After that, you can extract the "Name" and "data" fields from the objects.

Remember, arrays are indexed by integers, while objects/maps are indexed by strings.

roseicollis

Hi JKSH,

Ermmm noup, I wasn't familiar with it but now I understand it better :) Thanks!

One thing: what is auto for you? O.o a JsonObject?

@
QtJson::JsonObject object1 = result[0].toMap(); //object1 returns: QMap()
QVariant object1 = result[0]; //object1 returns: QVariant(, )
QVariant object1 = result[0].toString();//object1 returns: QVariant(QString, )
@

EDIT: Didn't know there were something called 'auto' O.o but trying that:

@
QtJson::JsonObject result = QtJson::parse(myJson, ok).toMap();
auto object1 = result[0];
QtJson::JsonObject trying = QtJson::parse(object1, ok).toMap();
qDebug() <<" trying msg" << trying;
@

it says: error: no matching function for call to 'parse(QVariant&, bool&)'
And if I try with
@
QtJson::JsonObject trying = QtJson::parse(object1["Name"], ok).toMap();
@
It says: error: no match for 'operator[]' in 'object1["Name"]'

JKSH

You're welcome. :)

[quote author="roseicollis" date="1423567075"]
@
QtJson::JsonObject result = QtJson::parse(myJson, ok).toMap();
@
[/quote]2 issues:

Like I said before, your data is an array, not an object. So, you should make result a JsonArray, not a JsonObject. Also, toMap() produces an object. You want to use toList() instead, which produces an array.

What type is myJson? Your compiler complained because you passed a QVariant into QtJson::parse(). What type does it actually accept? (hint: look inside json.h)

[quote author="roseicollis" date="1423567075"]EDIT: Didn't know there were something called 'auto' O.o[/quote]'auto' is a "C++11 keyword":http://geekswithblogs.net/BlackRabbitCoder/archive/2012/03/15/c-little-wonders-the-c11-auto-keyword-redux.aspx. When you use it, the compiler will try to figure out the variable's type automatically.

roseicollis

Hi again ^^

[quote author="JKSH" date="1423568300"]
Like I said before, your data is an array, not an object. So, you should make result a JsonArray, not a JsonObject. Also, toMap() produces an object. You want to use toList() instead, which produces an array.
[/quote]
Didn't know that it creates an object too... (anyway i tried before with strings and bytearray hopeing something works hahaha but not obviously)

[quote author="JKSH" date="1423568300"]
What type is myJson? Your compiler complained because you passed a QVariant into QtJson::parse(). What type does it actually accept? (hint: look inside json.h)
[/quote]

myJson is a QString:
@
QNetworkRequest request(_Url);
QNetworkReply *reply = nam->get(request);
QString myJson = (QString)reply->readAll();
@

I know that this error is because of the types but I was thinking that it takes Variants and now saw that not. The definition of parse is:
@
QVariant parse(const QString &json, bool &success);
@

If I try with:
@
QtJson::JsonArray result = QtJson::parse(myJson, ok).toList();
qDebug() << " a " << result;
auto object1 = result[0];
qDebug() << " a1 " << object1;
@

Then it prints:
a()
ASSERT failure in QList<T>::operator[]: "index out of range",

Which is normal if a() is empty O.o

I'll have to inquire about what c++11 is, really thank you so much! Now I have new weekend reading :D

JKSH

Hmm... what do you get if you do this?

@
qDebug() << myJson;
qDebug() << QtJson::parse(myJson, ok);
qDebug() << ok;
@

C++11 is the updated version of C++ which came out in 2011 :) It offers lots of new language features to make C++ more powerful and easier to use. C++14 is in the works, but it's no longer 2014 now so the update will probably be called C++15 instead.

roseicollis

This returns the next:

@
"{"data":"[{"Name":"One","Code":""},{"Name":"Two","Code":"Alpha"}]"}"

QVariant(QVariantMap, QMap(("data", QVariant(QString, "[{"Name":"One","Code":""},{"Name":"Two","Code":"Alpha"}]") ) ) )

true
@

wooow I thought there were only C, C# and C++ .... what an inonce girl I am XDDDD I will definitely look more about it at the weekend, it sounds really interesting!

JKSH

[quote author="roseicollis" date="1423572240"]
@
"{"data":"[{"Name":"One","Code":""},{"Name":"Two","Code":"Alpha"}]"}"
@

[/quote]I'm so sorry... I though myJson was
@
[{"Name":"One","Code":""},{"Name":"Two","Code":"Alpha"}]
@

...that's why I thought your data was an array of 2 objects. :( Sorry for sending you on a wild goose chase!

Anyway, your actual JSON data is: 1 object which contains 1 string value. I must say your data is formatted quite strangely (where did you get it from?).

So, in the first run, you can only extract one string:
@
QtJson::JsonObject outerObject = QtJson::parse(myJson, ok).toMap();
QString innerString = outerObject.toString();

qDebug() << myJson;
qDebug() << innerString;
@

You will see that myJson contains lots of '' characters, but innerString doesn't. That's because the data in outerObject is escaped -- the JSON parser treats it as a single string instead of a JSON array.

Anyway, after parsing it the first time, you now have un-escaped data in innerString, so you can send it through the JSON parser again to extract the array:

@
QtJson::JsonArray outerArray = QtJson::parse(innerString, ok).toList();

qDebug() << ok;
qDebug() << outerArray;
@

Now, you can apply my previous posts.

Good luck!

[quote author="roseicollis" date="1423572240"]wooow I thought there were only C, C# and C++ .... what an inonce girl I am XDDDD I will definitely look more about it at the weekend, it sounds really interesting![/quote]There are lots and lots of programming languages out there. :) C, C# and C++ are different languages.

However, C++03, C++11, and C++14 are different versions of the same language.

Happy reading!

roseicollis

[quote author="JKSH" date="1423574139"]
. :( Sorry for sending you on a wild goose chase!
[/quote]
No problem! I learnt a lot with that too :D

[quote author="JKSH" date="1423574139"]
I must say your data is formatted quite strangely (where did you get it from?)
[/quote]

I don't know which should be its structure but this is what I will receive from a webservice which is made by a work mate and the only thing I know is: I have to parse it and show it on a widget (Still dunno if combobox or what will be the best option)

[quote author="JKSH" date="1423574139"]
@
QtJson::JsonObject outerObject = QtJson::parse(myJson, ok).toMap();
QString innerString = outerObject.toString();

qDebug() << myJson;
qDebug() << innerString;
@
[/quote]

I had to make some changes to make it work:

@
QtJson::JsonObject outerObject = QtJson::parse(myJson, ok).toMap();
QString innerString = outerObject["data"].toString();
qDebug() << myJson;
qDebug() << innerString;
QtJson::JsonArray outerArray = QtJson::parse(innerString, ok).toList();
qDebug() << ok;
qDebug() << outerArray;
auto object1 = outerArray[0].toMap();
qDebug() << " DD" << object1["Name"];
@

So:

DD returns: DD QVariant(QString, "One")

Now I'm trying to get that "One" string alone but at least I can see it!! I'm almost there, so cloose!

[quote author="JKSH" date="1423574139"]
There are lots and lots of programming languages out there. :) C, C# and C++ are different languages.
However, C++03, C++11, and C++14 are different versions of the same language.
[/quote]

Yes, I know there are soo many programming languages but didn't think about versions.. Never though they exist for the languages too haha