Overriding default values?

[simulationcloud]

I have a json file that I read in using nlohmann:json. It contains nested dicts/lists that are configurations specified by users.

Only when a user specifies a value in this file (read in to a json object) should it override the default value, which is specified by another json object.

My question is: what's the easiest way to replace all the items in one json object with that of another json object, if that item exists in the other json object?

[nlohmann]

There is the update function that could be used for this. You could load the default values in a value v and then call v.update(u) where u are the values given by the user.

[simulationcloud]

Hi nlohmann, thanks for the help. When I call update, I get this error:
terminate called after throwing an instance of 'nlohmann::detail::type_error'
what(): [json.exception.type_error.312] cannot use update() with array

My default object looks like:
json jsn =
{
"geometry" ,
{
"endianness" , "little"
},
"nsSolver" ,
{
"tol" , 1.0e-8,
"CFL0" , 1.0,
"CFL1" , 200.0,
"CFLsteps" , 100,
"JacobiSweeps" , 30
},
"ransSolver" ,
{
"tol" , 1.0e-6,
"JacobiSweeps" , 30
},
"freestream" ,
{
"Re" , 1000.0,
"M" , 0.5,
"Tref" , 300,
"alpha" , 0.0,
"beta" , 0.0
},
{
"boundaries",
{

        }
   }
};

It seems it may be complaining about the hierarchy?

[nlohmann]

It says that you can only call update on an object. Can you provide the code where you call update?

[simulationcloud]

Here's the whole code:

#include "ParseInput.h"
#include "nlohmann/json.hpp"
#include <fstream>
#include <iostream>

json getDefaultConfig(void){
    json jsn =
    {
        "geometry" ,
        {
            "endianness" , "little"
        },
        "nsSolver" ,
        {
            "tol" , 1.0e-8,
            "CFL0" , 1.0,
            "CFL1" , 200.0,
            "CFLsteps" , 100,
            "JacobiSweeps" , 30
        },
        "ransSolver" ,
        {
            "tol" , 1.0e-6,
            "JacobiSweeps" , 30
        },
        "freestream" ,
        {
            "Re" , 1000.0,
            "M" , 0.5,
            "Tref" , 300,
            "alpha" , 0.0,
            "beta" , 0.0
        },
        {
            "boundaries",
            {

            }
       }
    };
    return jsn;
}

json readConfig(std::string file){
    std::ifstream i(file);
    json j;
    i >> j;
    i.close();

    json dflt = getDefaultConfig();
    //dflt.update(j.begin(), j.end());
    for(auto it= j.begin(); it != j.end(); ++it){
        dflt[it.key()].update(it.value());
    }
    return dflt;
}

[nlohmann]

What would be the content of an input file?

[simulationcloud]

The example content of an input file is:

{
    "geometry" :
    {
        "meshName" : "wing.ugrid",
        "endianness" : "little"
    },
    "nsSolver" :
    {
        "tol" : 1.0e-8,
        "CFL0" : 0.1,
        "CFL1" : 500.0,
        "CFLsteps" : 100,
        "JacobiSweeps" : 50
    },
    "ransSolver" :
    {
        "tol" : 1.0e-6,
        "CFL0" : 0.1,
        "CFL1" : 5.0,
        "CFLsteps" : 100,
        "JacobiSweeps" : 50
    },
    "freestream" :
    {
        "Re" : 10000.0,
        "M" : 0.25,
        "Tref" : 300,
        "alpha" : 0.0,
        "beta" : 0.0
    },
    "boundaries" : {
        "1" : {
            "type" : "NoSlipWall"
        },
        "2" : {
            "type" : "SlipWall"
        },
        "3" : {
            "type" : "Freestream"
        }
    }
}

Thanks for the help!

[nlohmann]

I think the issue is the JSON value returned by getDefaultConfig. I think you want to create an object, but you actually create this:

[
    "geometry",
    [
        "endianness",
        "little"
    ],
    "nsSolver",
    [
        "tol",
        "1e-08",
        "CFL0",
        1.0,
        "CFL1",
        200.0,
        "CFLsteps",
        100,
        "JacobiSweeps",
        30
    ],
    "ransSolver",
    [
        "tol",
        "1e-06",
        "JacobiSweeps",
        30
    ],
    "freestream",
    [
        "Re",
        1000.0,
        "M",
        0.5,
        "Tref",
        300,
        "alpha",
        0.0,
        "beta",
        0.0
    ],
    [
        "boundaries",
        {}
    ]
]

You need to take care about the braces required to create an object:

json j = { {"key", "value"} };

That is, you need to have an extra pair of braces around each key/value pair.

[nlohmann]

I assumed how your JSON value should look like.

#include <iostream>
#include <fstream>
#include "json.hpp"

using json = nlohmann::json;

json getDefaultConfig(void){
    json jsn = R"(
    {
        "geometry": {
            "endianness": "little"
        },
        "nsSolver": {
            "tol": 1e-08,
            "CFL0": 1.0,
            "CFL1": 200.0,
            "CFLsteps": 100,
            "JacobiSweeps": 30
        },
        "ransSolver": {
            "tol": 1e-06,
            "JacobiSweeps": 30
        },
        "freestream": {
            "Re": 1000.0,
            "M": 0.5,
            "Tref": 300,
            "alpha": 0.0,
            "beta": 0.0
        },
        "boundaries": {}
    })"_json;
    return jsn;
}

json readConfig(std::string file){
    std::ifstream i(file);
    json j;
    i >> j;
    i.close();
    
    json dflt = getDefaultConfig();
    
    //dflt.update(j.begin(), j.end());
    for(auto it= j.begin(); it != j.end(); ++it) {
        dflt[it.key()].update(it.value());
    }
    return dflt;
}

int main() {
    auto j = readConfig("file.json");
    std::cout << std::setw(2) << j << std::endl;
}

Output:

{
  "boundaries": {
    "1": {
      "type": "NoSlipWall"
    },
    "2": {
      "type": "SlipWall"
    },
    "3": {
      "type": "Freestream"
    }
  },
  "freestream": {
    "M": 0.25,
    "Re": 10000.0,
    "Tref": 300,
    "alpha": 0.0,
    "beta": 0.0
  },
  "geometry": {
    "endianness": "little",
    "meshName": "wing.ugrid"
  },
  "nsSolver": {
    "CFL0": 0.1,
    "CFL1": 500.0,
    "CFLsteps": 100,
    "JacobiSweeps": 50,
    "tol": 1e-08
  },
  "ransSolver": {
    "CFL0": 0.1,
    "CFL1": 5.0,
    "CFLsteps": 100,
    "JacobiSweeps": 50,
    "tol": 1e-06
  }
}