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POAppend to a JSON array in a JSON file on disk, every second using C++
 

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  1. POAppend to a JSON array in a JSON file on disk, every second using C++
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    <p>This is my first post here, so please bear with me.</p> <p>I have searched high and low on the internet for an answer, but I've not been able to resolve my issue, so I have decided to write a post here.</p> <p>I am trying to write(append) to a JSON array on file using C++ and JZON, at intervals of 1 write each second. The JSON file is initially written by a “Prepare” function. Another function is then called each second to a add an array to the JSON file and append an new object to the array every second.</p> <p>I have tried many things, most of which resulted in all sorts of issues. My latest attempt gave me the best results and this is the code that I have included below. However, the approach I took is very inefficient as I am writing an entire array every second. This is having a massive hit on CPU utilisation as the array grows, but not so much on memory as I had first anticipated. </p> <p>What I really would like to be able to do is to append to an existing array contained in a JSON file on disk, line by line, rather than having to clear the entire array from the JSON object and rewriting the entire file, each and every second.</p> <p>I am hoping that some of the geniuses on this website will be able to point me in the right direction.</p> <p>Thank you very much in advance.</p> <p>Here is my code:</p> <pre><code>//Create some object somewhere at the top of the cpp file Jzon::Object jsonFlight; Jzon::Array jsonFlightPath; Jzon::Object jsonCoordinates; int PrepareFlight(const char* jsonfilename) { //...SOME PREPARE FUNCTION STUFF GOES HERE... //Add the Flight Information to the jsonFlight root JSON Object jsonFlight.Add("Flight Number", flightnum); jsonFlight.Add("Origin", originicao); jsonFlight.Add("Destination", desticao); jsonFlight.Add("Pilot in Command", pic); //Write the jsonFlight object to a .json file on disk. Filename is passed in as a param of the function. Jzon::FileWriter::WriteFile(jsonfilename, jsonFlight, Jzon::NoFormat); return 0; } int UpdateJSON_FlightPath(ACFT_PARAM* pS, const char* jsonfilename) { //Add the current returned coordinates to the jsonCoordinates jzon object jsonCoordinates.Add("altitude", pS-&gt; altitude); jsonCoordinates.Add("latitude", pS-&gt; latitude); jsonCoordinates.Add("longitude", pS-&gt; longitude); //Add the Coordinates to the FlightPath then clear the coordinates. jsonFlightPath.Add(jsonCoordinates); jsonCoordinates.Clear(); //Now add the entire flightpath array to the jsonFlight object. jsonFlight.Add("Flightpath", jsonFlightPath); //write the jsonFlight object to a JSON file on disk. Jzon::FileWriter::WriteFile(jsonfilename, jsonFlight, Jzon::NoFormat); //Remove the entire jsonFlighPath array from the jsonFlight object to avoid duplicaiton next time the function executes. jsonFlight.Remove("Flightpath"); return 0; } </code></pre>
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    1. COYou can't "append" to a JSON string. Once you've closed the string (matching "[" with "]", eg) then you must start a new JSON string.
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    2. CO(You can, on the other hand, leave the file "open" -- write an initial "[", then append your first string. To append the 2nd string first write a ",". After the last string write "]". Or use file update mode to change a closing "]" to ",", write the appended data, then another "]".)
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    3. COThanks. This is pretty much what I was thinking, hence me actually writing the entire array to file every time I have to append to the array. I was just hoping that I was suffering from amateur programming symptoms and that I was missing something obvious. Thanks for the response.
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