How can I read a .json file?

Hello I found some data on the web that requires me to open a .gz file and a .json. I was able to unzip the gz file in MATLAB but have not been able to read the .json.

My Code:

>> fname = 'meta_Electronics.json';
>> fid = fopen(fname);
>> raw = fread(fid,inf);
>> str = char(raw');
>> fclose(fid);
>> data = JSON.parse(str)

and this is the error that is displayed:

Undefined variable "JSON" or class "JSON.parse".

Can someone point me in the right direction, as far as what i need to change?

Cheers.

[Adam]

I looked on the File Exchange when I wanted to read a json file, then downloaded something from there and made various edits to it to speed it up a bit. There are a few submissions on there that will read json files.

If you haven't defined anything called JSON though then

data = JSON.parse(str);

is highly optimistic coding. It's a very good way to code from the top level down using idealised code, but only if you write the code that you are calling too rather than just assume it will appear somehow!

Answer 1

[Alka Nair]

 fname = 'meta_Electronics.json'; 
fid = fopen(fname); 
raw = fread(fid,inf); 
str = char(raw'); 
fclose(fid); 
val = jsondecode(str);

worked for me.

[Sean Mahnken]

Slightly simpler:

fname = 'meta_Electronics.json';
val = jsondecode(fileread(fname));

[Victor Quezada]

Thanks!

[Tolga Karabiyikoglu]

Cool- seems to work for me too

[Juan Miguel Serrano Rodríguez]

Thanks!

Answer 2

[Elbi Mutluoglu]

How can I read this json file as array.

{"flowSegmentData":{"frc":"FRC0","currentSpeed":30,"freeFlowSpeed":85,"currentTravelTime":28,"freeFlowTravelTime":10,"confidence":0.9900000095367432,"roadClosure":false,"coordinates":{"coordinate":[{"latitude":40.99520713231141,"longitude":29.11316083692566},{"latitude":40.9951128720138,"longitude":29.11348934584032},{"latitude":40.99499782625885,"longitude":29.113922597143414},{"latitude":40.99492447823734,"longitude":29.11425589372996},{"latitude":40.99481265425299,"longitude":29.114754874308034},{"latitude":40.9947156413892,"longitude":29.115239700057685},{"latitude":40.99469244741794,"longitude":29.115362205564793},{"latitude":40.99466308297875,"longitude":29.115527466260318},{"latitude":40.99465731538117,"longitude":29.11556668313179},{"latitude":40.99465324816397,"longitude":29.115588820406316},{"latitude":40.9946365659001,"longitude":29.115684510886638},{"latitude":40.994617484549394,"longitude":29.11581043877149},{"latitude":40.99460761712758,"longitude":29.115881261564937}]},"@version":"traffic-service 3.2.001"}}

[Nitesh Panchal]

please use----- {jsondecode('[ ----------------please paste your json code here------------- ]')}

{jsondecode('[{"flowSegmentData":{"frc":"FRC0","currentSpeed":30,"freeFlowSpeed":85,"currentTravelTime":28,"freeFlowTravelTime":10,"confidence":0.9900000095367432,"roadClosure":false,"coordinates":{"coordinate":[{"latitude":40.99520713231141,"longitude":29.11316083692566},{"latitude":40.9951128720138,"longitude":29.11348934584032},{"latitude":40.99499782625885,"longitude":29.113922597143414},{"latitude":40.99492447823734,"longitude":29.11425589372996},{"latitude":40.99481265425299,"longitude":29.114754874308034},{"latitude":40.9947156413892,"longitude":29.115239700057685},{"latitude":40.99469244741794,"longitude":29.115362205564793},{"latitude":40.99466308297875,"longitude":29.115527466260318},{"latitude":40.99465731538117,"longitude":29.11556668313179},{"latitude":40.99465324816397,"longitude":29.115588820406316},{"latitude":40.9946365659001,"longitude":29.115684510886638},{"latitude":40.994617484549394,"longitude":29.11581043877149},{"latitude":40.99460761712758,"longitude":29.115881261564937}]},"@version":"traffic-service 3.2.001"}}]')}

Answer 3

[cui,xingxing]

My personal opinion is that there is still no direct mapping from a generic readable json file to MATLAB's built-in types (struct or dictionary). The readstruct function is expected to support more file formats.

But on the other hand, you don't really need to call low-level functions such as fread, you can just use the fileread function to read the content and then decode it. For example, the following operation:

!unzip test.zip # see attachment file,use @Elbi Mutluoglu data for example

Archive:  test.zip
  inflating: test.json               

View json file contents, preview it.

!cat test.json

{
    "flowSegmentData": {
        "frc": "FRC0",
        "currentSpeed": 30,
        "freeFlowSpeed": 85,
        "currentTravelTime": 28,
        "freeFlowTravelTime": 10,
        "confidence": 0.9900000095367432,
        "roadClosure": false,
        "coordinates": {
            "coordinate": [
                {
                    "latitude": 40.99520713231141,
                    "longitude": 29.11316083692566
                },
                {
                    "latitude": 40.9951128720138,
                    "longitude": 29.11348934584032
                },
                {
                    "latitude": 40.99499782625885,
                    "longitude": 29.113922597143414
                },
                {
                    "latitude": 40.99492447823734,
                    "longitude": 29.11425589372996
                },
                {
                    "latitude": 40.99481265425299,
                    "longitude": 29.114754874308034
                },
                {
                    "latitude": 40.9947156413892,
                    "longitude": 29.115239700057685
                },
                {
                    "latitude": 40.99469244741794,
                    "longitude": 29.115362205564793
                },
                {
                    "latitude": 40.99466308297875,
                    "longitude": 29.115527466260318
                },
                {
                    "latitude": 40.99465731538117,
                    "longitude": 29.11556668313179
                },
                {
                    "latitude": 40.99465324816397,
                    "longitude": 29.115588820406316
                },
                {
                    "latitude": 40.9946365659001,
                    "longitude": 29.115684510886638
                },
                {
                    "latitude": 40.994617484549394,
                    "longitude": 29.11581043877149
                },
                {
                    "latitude": 40.99460761712758,
                    "longitude": 29.115881261564937
                }
            ]
        },
        "@version": "traffic-service 3.2.001"
    }
}

myJsonFile = "test.json";
text = fileread(myJsonFile);
data = jsondecode(text)

data = struct with fields:
    flowSegmentData: [1×1 struct]

% Access to the next level
fieldnames(data.flowSegmentData)

ans = 9×1 cell array
    {'frc'               }
    {'currentSpeed'      }
    {'freeFlowSpeed'     }
    {'currentTravelTime' }
    {'freeFlowTravelTime'}
    {'confidence'        }
    {'roadClosure'       }
    {'coordinates'       }
    {'x_version'         }

[cui,xingxing]

Luckily, my expectations above (readstruct support for json reads, writestruct support for json saves) have been implemented by the new version, which has been supported since R2023b!

Answer 4

[Hammad Ur Rahman]

% Read JSON data from a file

jsonFileName = 'data.json';

jsonStr = fileread(jsonFileName);

% Convert JSON string to MATLAB variables

jsonData = jsondecode(jsonStr);

% Access JSON data using MATLAB variables

value = jsonData.key; % Replace 'key' with the actual key name