fread and fwrite application in reading file from disk

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Siamack 2013-12-6

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https://ww2.mathworks.cn/matlabcentral/answers/108855-fread-and-fwrite-application-in-reading-file-from-disk

评论： Siamack 2013-12-8

hello everyone, can you read data from disk (say a pdf file) with a precision other than uint8 (say integer*4) and then re-write the numerical array back into a meaningful file on the disk?

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Walter Roberson 2013-12-6

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https://ww2.mathworks.cn/matlabcentral/answers/108855-fread-and-fwrite-application-in-reading-file-from-disk#answer_117486

在 MATLAB Online 中打开

input_data = fread(input_fid, '*uint32');
fwrite(output_fid, input_data);

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Walter Roberson 2013-12-7

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No information should be lost when you convert to 64 bit chunks. However, if you try to do arithmetic on 64 bit integers, and accidentally convert them to double precision in the process, then the result is only going to have 53 bits of precision instead of 64.

For example if you have

X = zeros(1024,1);
X(1:20) = fread(file, 20, '*uint64');

then the data would be read as uint64 but then converted to double to be stored in X because X is double. To fix you would use

X = zeros(1024,1,'uint64')

Siamack 2013-12-8

Hi! You are right! the information doesnt get lost, I just used the "typecast" function and casted the data back to uint8 (two bytes short than the original and had to be added to the end of the array) and then used fwrite to write it onto the hard and the pdf file was readable. I simply don't know where this can go wrong hence wont assume this might work for every file undergone the transitions. nonetheless thanks for the answer Walter

Regards, Siamack

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