Finding a value in an array

[pboettch]

I started using nlohmann::json today (thanks for it) and I wanted to check whether a certain value is contained in an array: I used find(). It compiled but it does not work.

#include <json.hpp>
using json = nlohmann::json;
int main(void)
{
	json j({"hello", "world"});
	std::cerr << j << "\n";
	std::cerr << (j.find("hello") != j.end()) << " expecting 1\n";
	for (const auto &i: j)
		if (i == "hello")
			std::cerr << "found it while iterating\n";
    return 0;
}

it gives

["hello","world"]
0 expecting 1
found it while iterating

Reading json.hpp I saw that find() is often done with a key. What is the expected behavior of finding on an array?

[pboettch]

Just looking a bit more - I think I found it ;-)

from json.hpp:

 iterator find(typename object_t::key_type key)
 {
     auto result = end();

     if (is_object())
     {
         result.m_it.object_iterator = m_value.object->find(key);
     }

    return result;
}

[nlohmann]

Yes, the documentation of find is also clear about this:

Finds an element in a JSON object with key equivalent to key. If the element is not found** or the JSON value is not an objec**t, end() is returned.

To find elements in an array, you can use std::find.

[pboettch]

What confused me was the silent returning of find() giving me end(). Naively I'd expect an error somehow. I'm still discovering your style but would an assert not be the better solution for newbies like me. Please enlighten, it'll help me understand better your design decisions.

[nlohmann]

The design decisions are difficult, because we basically try to "merge" the behavior of a std::vector and a std::map. As find as member function is only available to std::map and I do not want to accidentally degrade to linear complexity, there is no member find for arrays.

I understand that silently returning end() is confusing. Maybe changing this to an exception could be a change for the 3.0.0 release.

[nlohmann]

Should the semantics be changed to throw in this case?

[gregmarr]

"The basic_json object represents a json object" is a precondition of calling this function.
Is there a general philosophy in this library on what to do in the case of a precondition violation?

[nlohmann]

@gregmarr In similar issues where we mix the behavior of std::map and std::vector we have the range of

• defining the missing behavior
• throw an exception if called on the "wrong" value type

In this case, there is just a comment that when called on anything but an object, find will always behave as if nothing was found.

As searching in an array is rather expensive, I tend to throw an exception in this case.

[pboettch]

I just ran into the same problem with

size_type size() const noexcept

for a string-element. I accidently took it for the length of the string.

[nlohmann]

@pboettch This is also clearly documented.

[pboettch]

After some thinking I agree with you. find() and size() work as they should and my expectations were wrong. I even can explain what mislead me: The slogan: "What if json was part of modern C++?". I understood that json is trying to integrate as much as possible with std::-containers and algorithms, but no. It actually means what is says: how would we design json to be a container class in std::? Lite this.

As all cases are documented and well described, one just has to learn it. As for all the other containers.

You can close this "issue" from my point of view. I would have rather discussed this on a mailing list than in an issue. Thanks again for your work.