What's fid in "Training a Model from Scratch"?

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For this DL example,
What's the 'fid' in the first code block?
rawImgDataTrain = uint8 (fread(fid, numImg * numRows * numCols, 'uint8'));
% Reshape the data part into a 4D array
rawImgDataTrain = reshape(rawImgDataTrain, [numRows, numCols, numImgs]);
imgDataTrain(:,:,1,ii) = uint8(rawImgDataTrain(:,:,ii));

Accepted Answer

Walter Roberson
Walter Roberson on 31 Jul 2022
rawImgDataTrain = uint8 (fread(fid, numImg * numRows * numCols, 'uint8'));
In this context, fid would be
fid = fopen('Some_MNIST_file_name_goes_here');
That is, it is a "file identifier" returned by fopen() when you ask to open one of the MNIST files.
numImg and numRows and numCols would all have to be known ahead of time, some-how. Perhaps there is a header file that contains those values.
The fread() call would read binary data, one unsigned 8-bit integer per entry. The number of entries to read is numImg * numRows * numCols . The uint8() call would not be necessary if the line had been properly coded as
rawImgDataTrain = fread(fid, numImg * numRows * numCols, '*uint8');
  1 Comment
Runcong Kuang
Runcong Kuang on 1 Aug 2022
Cool. But I really want to know what exactly is the mnist file here.
I download the .gz files and don't know what to do with them.

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More Answers (1)

Atsushi Ueno
Atsushi Ueno on 31 Jul 2022
> What's the 'fid' in the first code block?
It is fileID — File identifier of an open file
When you open the file by fopen function, you can get the ID number.
You need the ID number to access the opened file by fread function or something else.

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