how to append a mat file

Hi - I am using the save command to save a variable in my workplace which is two rows and one column worth of data (2 by 1). However, I would like the 'append' command to move one column the right evertime it is run (as not to replace my first column worth of data), however reading the 'save' help file, it only 'appends' if it is ASCI but does not 'append' when it is a .mat file. How do I prevent my data from being overwritten and instead having new data keep being added to the right (this way, I can just load it, and plot all of first row and use the second row as 'labels' (for the legend))...

save(data.mat,datacell,'-append')

Answer 1

[Walter Roberson]

You cannot do that with save()

Have a look at https://www.mathworks.com/help/matlab/ref/matfile.html

Answer 2

[nas illmatic]

There should be a way to read back the mat file, then update it to keep the existing column, and write a new column, with the data to plot and it's label, then save it back to the matfile.

[Walter Roberson]

Sure. just load the appropriate variable from the mat file , make the change and save with append (which does work for mat files by replacing the specified variables and leaving the rest in place . ) The built in functionality to handle that is matfile() but you can write your own function .

Answer 3

[nas illmatic]

I would like to use built-in functions whenever possible. Will this work?

% mydata.mat only has a 'variable' - 
% it is a 2 rows, 1 column cell by cell named mydatacell 
% which is row 1 data, and row 2 the label name

%Define Matfile
m = matfile(mydata.mat,'Writable',isWritable);
%Find Size
[nrows,ncols] = size(m,'mydata');
%Append the data
m.mydatacell[m.mydatacell ; ncols+1] = [1.2 ; 'mylabel']  % I think it's semicolon here to go to column
save(mydata.mat,mydatacell,'-append');

[Walter Roberson]

You can click on "Comment on this Answer" to reply to someone, instead of creating a new Answer each time.

[Walter Roberson]

newdata = {1.2; 'mylabel'};
if ismember('mydata', fieldnames(m))         %is the variable already present?
  [nrows, ncols] = size(m, 'mydatacell');
  m.mydata(:,ncols+1) = newdata;             %yes, extend it
else
    m.mydata = newdata;                      %no, assign it
end

save is not needed.

At the moment I do not see any nice way of checking whether a variable exists in a matfile, so I use the work-around of looking for the variable name in the fieldnames . fieldnames will always exist even if the file has not been written to yet, and will always contain the field 'Properties' at least.

[nas illmatic]

I created a simple example and utilized the 'matfile' that you pointed to me, but for some reason, my 'mydatacell' which I expected it to grow from a 2x1 cell to a 2x2 cell (where the second column contains 1.2 and 'mynewlabel') but for some reason it's not growing....

mydatacell= {1.1; 'myoldlabel'};
save('mymat.mat','mydatacell')
m = matfile('mymat.mat','Writable',true);

newdata = {1.2; 'mynewlabel'};
if ismember('mymat', fieldnames(m))         %is the variable already present?
  [nrows, ncols] = size(m, 'mydatacell');
  m.mydatacell(:,ncols+1) = newdata;             %yes, extend it
else
    m.mydatacell = newdata;                      %no, assign it
end

save('mymat.mat','mydatacell')

[Maxwell Rosen]

I think you have to delete that last save() function. Matlab automatically appends your array that you opened m. See the function https://www.mathworks.com/help/matlab/ref/matlab.io.matfile.html in the example Load and Save Parts of Variables