how to read interleaved data with memmapfile()?

Question

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?

Answer 1

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);

Answer 2

Maybe try something as:

m = memmapfile('data.dat', 
               'Format', 
               { 'int16'  [1 1] 'a'; 'int16' [1 1] 'b'; 'int16' [1 1] 'c'}, 
               'Repeat', nSamples);

a = m.Data(:).a; % extract all instances of a