I have a large (4GB+) data file I'd like to access. It contains samples i of a number of different signals {a, b, c} as follows:
a_1 b_1 c_1 a_2 b_2 c_2 .... a_n b_n c_n
I would like to use memmapfile to retrieve, say, the a stream. Since I know the number of signals and the number of samples, I tried:
m = memmapfile('data.dat','Format',{'int16',[nSignals 1],'sid'},'repeat',nSamples);
but this returns the useless m field
Data: nSamples x 1 struct array with fields:
sid
Of course the following works fine, but is very slow:
m = memmapfile('data.dat','Format','int16');
a = m.Data(1:nSignals:end);
How can I recover a without having to access the full data matrix?
How about using FREAD and specifying the appropriate skip
value. The following will read the signal a
in one go:
% 3 interleaved signals each of type int16
nSignals = 3;
% amount of bytes to skip after reading each sample
szINT16 = 2; % sizeof(int16)=2
skipBytes = (nSignals-1)*szINT16;
% number of samples in each signal (Inf to read all samples)
nSamples = Inf;
fid = fopen('data.dat','rb');
a = fread(fid, nSamples, '*int16', skipBytes);
fclose(fid);
You could do the same for the other two signals, you just have to seek the right start location:
fseek(fid, szINT16*1, 'bof');
b = fread(fid, nSamples, '*int16', skipBytes);
fseek(fid, szINT16*2, 'bof');
c = fread(fid, nSamples, '*int16', skipBytes);