jsondecode not suitable to parse uint64 array values

Why jsondecode function is not able to correctly parse uint64 values? Is MathWorks going to solve this issue?

I was reading a json file with timestamps in nanoseconds:

json_data = fileread('my_json_file.json');
data = jsondecode(json_data);

When accessing to the value in matlab, it returned 1704972586391907328. My original value was 1704972586391907240.

So I asked myself, how is this possible. It turns out that what "jsondecode" function is doing is:

str2double(raw_values_str)

As Numerics of class double only have 15 decimal digits of precision (52 bits), it is not precisely representable using a double.

To be able to retrieve the uint64 value I did a manual parsing of json file:

function values = extract_uint64_array(json_data, field_name)
    % Construct the field search string
    field_search_str = sprintf('"%s": [', field_name);

    % Extract the specified field manually
    start_index = strfind(json_data, field_search_str) + length(field_search_str);
    end_index = strfind(json_data(start_index:end), ']') - 2 + start_index;
    raw_values_str = json_data(start_index:end_index);

    % Convert the field values to uint64 array using sscanf and a loop
    raw_values_cell = strsplit(raw_values_str, ',');
    values = uint64(zeros(1, numel(raw_values_cell)));

    for i = 1:numel(raw_values_cell)
        values(i) = sscanf(raw_values_cell{i}, '%lu');
    end
end

I hope it helps some of you. Thank you for reading!

Answer 1

[Sachin Lodhi]

Hello GonzaloMadrono,

I understand that you are trying to parse uint64 values using "jsondecode" function. I too was facing this issue and with the following workaround I was able to accurately parse uint64 values from JSON data: 

function values = extract_uint64_array(json_data, field_name)
    % Construct the field search string
    field_search_str = sprintf('"%s": [', field_name);
    % Extract the specified field manually
    start_index = strfind(json_data, field_search_str) + length(field_search_str);
    end_index = strfind(json_data(start_index:end), ']') - 2 + start_index;
    raw_values_str = json_data(start_index:end_index);
    
    % Convert the field values to uint64 array using sscanf and a loop
    raw_values_cell = strsplit(raw_values_str, ',');
    values = uint64(zeros(1, numel(raw_values_cell)));
    for i = 1:numel(raw_values_cell)
        values(i) = sscanf(raw_values_cell{i}, '%lu');
    end
end

This function manually extracts the array of uint64 values from the raw JSON data, ensuring accurate representation of the large integers.

I hope this helps!