deleting variables from a .mat file

Question

Does anyone here know how to delete a variable from a matlab file? I know that you can add variables to an existing matlab file using the save -append

Answer 1

Interestingly enough, you can use the -append option with SAVE to effectively erase data from a .mat file. Note this excerpt from the documentation (bold added by me):

For MAT-files, -append adds new variables to the file or replaces the saved values of existing variables with values in the workspace.

In other words, if a variable in your .mat file is called A, you can save over that variable with a new copy of A (that you've set to []) using the -append option. There will still be a variable called A in the .mat file, but it will be empty and thus reduce the total file size.

Here's an example:

>> A = rand(1000);            %# Create a 1000-by-1000 matrix of random values
>> save('savetest.mat','A');  %# Save A to a file
>> whos -file savetest.mat    %# Look at the .mat file contents
  Name         Size                Bytes  Class     Attributes

  A         1000x1000            8000000  double

The file size will be about 7.21 MB. Now do this:

>> A = [];                              %# Set the variable A to empty
>> save('savetest.mat','A','-append');  %# Overwrite A in the file
>> whos -file savetest.mat              %# Look at the .mat file contents
  Name      Size            Bytes  Class     Attributes

  A         0x0                 0  double

And now the file size will be around 169 bytes. The variable is still in there, but it is empty.