MATLAB: Extract coloumns from .mat file

r2017a

I have a .mat file which consist of a 22253519×5 double. I need to extract first three coloums only and save them in another .mat file having 2253519×3 double data. All the rows are to be considered.

I have no clue about this. New to MATLAB. Any help would be appreciated.

Best Answer

The below would be easier if you only had one variable stored in the .mat file and you knew the name of the variable. The below code does not assume that there is only one variable and does not assume that the variables have any particular names; the code takes the first three columns of all of the variables that are present, and creates the new .mat file using all of the same variable names.

filename = 'YourFile.mat';
newfile = 'YourNewFile.mat';
datastruct = load(filename);
newstruct = structfun(@(M) M(:,1:3), datastruct, 'uniform', 0);
save(newfile, 'newstruct', '-struct');

Related Solutions

MATLAB: How to add Filename to all variables automatically

Assuming you want to process a whole directory like this:

projectdir = 'File';   %location to save into
if ~exist(projectdir, 'dir'); mkdir(projectdir); end
dinfo = dir('*.mat');
for fidx = 1 : length(dinfo)
  filename = dinfo(fidx).name;
  [~, basename] = fileparts(filename);
  datastruct = load(filename);
  fn = fieldnames(datastruct);
  for nidx = 1 : length(fn)
    varname = fn{nidx};
    outvarname = [basename '_' varname];
    outfilename = fullfile(projectdir, [outvarname '.mat']);
    outstruct = struct( outvarname, datastruct.(varname) );
    save(outfilename, '-struct', outstruct);
  end
end

This preserves variable names inside the .mat files.

MATLAB: Save new field to a structure?!

There are a couple good ways to do this. If your MAT-file has a lot of data stored in it, save using the -append flag. Another approach, particularly if you need to modify more than one of the struct arrays stored in the MAT-file, is to load into a struct and save that struct using the -struct flag.

data = load('myfile.mat');
data.struct1.newfield = 42;
save('myfile.mat', '-struct', 'data')

With this approach you don't need to worry about overwriting a variable in your workspace that happens to have the same name as one of the variables in the MAT-file.

Best Answer

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MATLAB: How to add Filename to all variables automatically

MATLAB: Save new field to a structure?!

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