read in .json file yields strange sizes

[CDitzel]

I am trying to read in an .json file into an json object via the overloaded >> operator

I can also write it to another .json file with << which reproduces the same file, as expected.

however, when I try to find out about the size of the json-container, it yields 3, although my file is quite big

json input; i >> input; std::cout << input.size() << input << std::endl;

it gives the correct content but the size is obviously wrong

[CDitzel]

Ok I narrowed down the problem. My input looks as follows, and apparently only the imgHeight, imgWidt and objectsare counted as entries. How can I then access the inner elements in the json container?

{ "imgHeight": 1024, "imgWidth": 2048, "objects": [ { "label": "road", "polygon": [ [ 0, 769 ], [ 290, 574 ], [ 93, 528 ], [ 0, 524 ], [ 0, 448 ], [ 0, 448 ], [ 210, 453 ], [ 511, 451 ], [ 782, 459 ], [ 892, 495 ], [ 943, 498 ], [ 1044, 471 ], [ 1490, 486 ], [ 1880, 519 ], [ 1922, 512 ], [ 1975, 522 ], [ 2047, 524 ], [ 2047, 570 ], [ 1668, 553 ], [ 2047, 597 ], [ 2047, 1023 ], [ 892, 1023 ], [ 0, 1023 ] ] } ] }

[nlohmann]

You can try input["objects"].size().

[CDitzel]

does not work. I get 0% returned

[nlohmann]

What gives input.dump()?

[CDitzel]

thank you for your quick reply!

it yields the content of the entire file

[nlohmann]

Of what do you want to get the size then?

[CDitzel]

cuz I want to access the range of numeric tuples in the inner most element and write them into a new json file

[nlohmann]

I do not understand your problem. If the input file is the one of your second comment and you want to have the number of elements in the objects array, then input["objects"].size() would give you this.

[CDitzel]

well for me it does not, it gives 1%.

I somehow need a way to access the tuples in the inner array

[nlohmann]

How can this function return a percentage? What array do you mean? I tried to be concrete, but you only told me that it does not work.

[CDitzel]

I will try to elaborate, wait a second

[CDitzel]

well ok in the shell it showed 1%, in a file it is displayed as 1, which is correct I guess.

But the main problem remains: How can I access elements of a json file that are cascaded deep in several layers of arrays, e.g. how can I access in my given file just the number pairs by means of the json-container?

[nlohmann]

input["objects"][0]["polygon"].size()

[gregmarr]

well ok in the shell it showed 1%, in a file it is displayed as 1, which is correct I guess.

My Psychic Debugging skills (see The Old New Thing) says that you did

printf("%d", input["objects"].size());

at the end of your program, and that because you didn't include "\n", you got the 1 immediately followed by your shell prompt %.

[CDitzel]

Danke Niels und viele Grüße nach Berlin!

@gregmarr: No my shell is not the culprit

[nlohmann]

Grüße zurück! Does it work now?

[CDitzel]

I think so, I will have to further investigate. I am given a messed up json file that I have to clean up and bring into a shape comparable to the one I posted above, so I will see if I can manage

[CDitzel]

one last question: I am trying to achieve a similar output format as in my example above, where there are two braces following each other, namely " [ {". But I cannot figure out how to put this in the code in order to get the desired output

[nlohmann]

You can use [] for both arrays and objects. For arrays, you pass the index (starting with 0) as parameter, for objects the key (i.e., a string). In the example above:

input["objects"][0]["polygon"]
  ^       ^      ^      ^
  |     a key    |      |
  |          an index   |
an object             a key

• input is an object
• input["objects"] is an array
• input["objects"][0] is an object
• input["objects"][0]["polygon"] is an array

[CDitzel]

awesome. Thank you so much niels

[nlohmann]

Can I close the issue?

[CDitzel]

if I am allowed to open up another one in case of further questions, yes ; )

[nlohmann]

Of course :)