Formula in JSON

[NotYourFan]

Hey i have a JSON like:

.
.
.
"math": "(2.0 * value_x) + 5 * 25.0"
.
.

Now i want to define the value in my JSON ("value_x") in my Qt-Programm.

in Qt try this:

double value_x = 12.4;
double function = value.at(0)->getValue().at(0)->math() // HERE I READ MY JSON AND I GET A STRING LIKE: "(2.0 * value_x) + 5 * 25.0"

double result = HERE I WANT TO GET 2*12.4 +5 * 25.0

Any Idea how i can define a value in JSON and get the result from the formula (also from JSON) ?

[jsulm]

@NotYourFan You can replace value_x with the number in the JSON string before evaluating it.

[SGaist]

Hi,

Isn't that the same question as your other thread here ?

[NotYourFan]

@jsulm
i should define the "value_x" in Qt.
"value_x" is an Input in my GUI ... so i dont know what is value_x before i defined in my GUI ....
The next Point is that the "unknow value" (in this case value_x) has every time an other name.

[J.Hilk]

@SGaist
also very similar to his other 2 topics

https://forum.qt.io/topic/101564/mathematische-funktion-aus-einer-json-datei-lesen-und-berechnen
and
https://forum.qt.io/topic/100609/berechnungen-in-qt

@NotYourFan
like I said before your best bet
is a Scriptinglanguage binding
JavaScript, Python, QML, Lua, Swift .....

[jsulm]

@NotYourFan That's why I suggested to replace value_x with the number, didn't I?
See https://doc.qt.io/qt-5/qstring.html#replace-5
"has every time an other name" - then how do you know what you have to define?

[NotYourFan]

Thanks @all

i solve my problem like this:

#include <QScriptEngine> 
.
.

QScriptEngine engine;
engine.globalObject().setProperty(value.at(0)->getValue.at(0).name(), value.at(0)->getValue.at(0).math());
qDebug() << "foo times two is:" << engine.evaluate("foo * 2").toNumber();

https://doc.qt.io/qt-5/qtscript-index.html

THANKS specially @J-Hilk for the bullet point "script" :)

[VRonin]

You are using a bazooka to kill a fly: https://en.wikipedia.org/wiki/Shunting-yard_algorithm

[NotYourFan]

@VRonin

Thank you for your Feedback.

I do this way because my JSON-File is every time diffrent.

So with this solution my formula works fine.