How to determine the type of elements read from a JSON array?

[amwink]

The JSON files I need to read with my program have the form
{ "labels": [ 0, 0, 1, 1 ], "vectors": [ [ 1, 1 ], [ 2, 2 ], [ 2, 4 ], [ 4, 2 ] ] }
or
{ "labels": [ 0, 0, 1, 1 ], "vectors": [ "file1", "file2", "file3", "file4" ] }
to represent specimen that have a numerical label and numerical data, the second of which may be given in the JSONs themselves or read from files.

The labels are read in the line

std::vector <float> *y = 
     new std::vector <float> ( jdesign["labels"].get<std::vector<float>>() );

and for the first form the line

std::vector<std::vector<float>> 
     xtest1 ( jdesign["vectors"].get<std::vector<std::vector<intensity>>>() );

does the trick. But if I want to test for the second form (the values are given as file names) and use

std::vector<std::string> 
     xtest2 ( jdesign["vectors"].get<std::vector<std::string>>() );

I get:

terminate called after throwing an instance of 'nlohmann::detail::type_error'
what():  [json.exception.type_error.302] type must be string, but is array`

Is there a way to find out the type of the elements of vectors before assigning them to my program variables?

I am using Debian Linux (stretch, kernel 4.9.0-8-amd64) and GCC 8.2.0 and the json library as pulled from the GitHub repository.

[nlohmann]

You could call type() to get the type.

[amwink]

Thanks! but both examples above are json::value_t::array -- is it possible to test the types of the array elements in some way?

[nlohmann]

You would need to call it for each element. As the array has a begin/end iterator, you can also use std::all_of together with functions like is_string to check if all elements are strings, for example.

[amwink]

That makes sense, thanks again. But if I understand it correctly then I would always first need to do a get<T>() before I could check the contents of the array, which sounds like a catch-22? I think it is better then to add a variable to the JSON to find out what T should be before calling get<T>().

[nlohmann]

No, you can access the JSON vector before this:

if (std::all_of(jdesign["vectors"].begin(), jdesign["vectors"].end(), [](const json& el){ return el.is_string(); })
{
   // access as string vector, e.g. like
   std::vector<std::string> foo = jdesign["vectors"];
}

(untested code, but you should get the gist).