Reading a large binary file

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Dominik Rhiem
Dominik Rhiem on 18 Oct 2023
Commented: Dominik Rhiem on 19 Oct 2023
I want to load a 8 GB binary data file into Matlab. My computer has 24 GB of RAM. It runs out of memory, despite me closing literally everything besides Matlab, and I don't understand why. The file should not take more than its size, right?
Code for loading is simply:
fd = fopen(['data.bin'], 'r');
data = fread(fd, 'int16');

Accepted Answer

James Tursa
James Tursa on 18 Oct 2023
Edited: James Tursa on 18 Oct 2023
data = fread(fd, '*int16');
When you use 'int16' for the type to read, MATLAB reads as int16 and then converts to double (four times the memory). When you use '*int16', MATLAB keeps the type as int16. E.g.,
% Create a small sample file
fid = fopen('junk.bin','wb');
% read it with 'int16'
fid = fopen('junk.bin','rb');
x = fread(fid,'int16')
x = 4
class(x) % double, not what you wanted
ans = 'double'
% read it with '*int16'
x = fread(fid,'*int16')
x = int16 4
class(x) % int16, this is what you wanted
ans = 'int16'
  1 Comment
Dominik Rhiem
Dominik Rhiem on 19 Oct 2023
Positive side effect: This is extremely fast.

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