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How can I read a .json file?

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Carlyle Hickman
Carlyle Hickman on 24 Feb 2017
Edited: cui,xingxing on 27 Apr 2024 at 2:04
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?
  1 Comment
Adam on 24 Feb 2017
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!

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Accepted Answer

Alka Nair
Alka Nair on 22 Nov 2017
fname = 'meta_Electronics.json';
fid = fopen(fname);
raw = fread(fid,inf);
str = char(raw');
val = jsondecode(str);
worked for me.
Tolga Karabiyikoglu
Tolga Karabiyikoglu on 12 Nov 2020
Cool- seems to work for me too

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More Answers (3)

Elbi Mutluoglu
Elbi Mutluoglu on 13 Feb 2020
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"}}
  1 Comment
Nitesh Panchal
Nitesh Panchal on 26 Apr 2021
Edited: Nitesh Panchal on 26 Apr 2021
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"}}]')}

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cui,xingxing on 14 Aug 2023
Edited: cui,xingxing on 27 Apr 2024 at 2:04
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 # see attachment file,use @Elbi Mutluoglu data for example
Archive: 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
ans = 9×1 cell array
{'frc' } {'currentSpeed' } {'freeFlowSpeed' } {'currentTravelTime' } {'freeFlowTravelTime'} {'confidence' } {'roadClosure' } {'coordinates' } {'x_version' }
-------------------------Off-topic interlude, 2024-------------------------------
I am currently looking for a job in the field of CV algorithm development, based in Shenzhen, Guangdong, China,or a remote support position. I would be very grateful if anyone is willing to offer me a job or make a recommendation. My preliminary resume can be found at: . Thank you!
  1 Comment
cui,xingxing on 24 Aug 2023
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!

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Hammad Ur Rahman
Hammad Ur Rahman on 9 Feb 2024
% 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

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