load several .mat files with n x n matrices and compute average

Hi there,

I'm new to working with matlab. I have experience with bash shell scripts. I have a bunch of .mat files that contain n x n matrices like this toy example:

>> load('matrix1.mat')

>> load('matrix2.mat')

>> m1

m1 =

       1     2     3
       2     1     5
       3     6     1

>> m2

m2 =

       1     3     5
       2     1     5
       4     9     1

I have two questions:

1. How can I automatically loop through -say- 100 of these to load them into matlab?

2. If they are loaded (in some way), how can I perform calculations on the cells in the matrices such as computing the average across them? In the example below I'm looking for

mAVG =

       1     2.5   4
       2     1     5
       3.5   7.5   1

Thanks so much!

Answer 1

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#How_can_I_process_a_sequence_of_files.3F

http://matlab.wikia.com/wiki/FAQ#How_can_I_create_variables_A1.2C_A2.2C....2CA10_in_a_loop.3F

If the variables are all the same 2D size, then the most efficient thing to do is load them one at a time and store the value into a slice of an array. Then once you have all of the data loaded, you can mean() across the third dimension.

dinfo = dir('matrix*.mat');
nfile = length(dinfo);
if nfile == 0
   avg = [];
   warning('There were no input files');
else
  filestruct = load(dinfo(1).name);
  fn = fieldnames(filestruct);
  thisvar = filestruct.(fn{1});
  orig_size = size(thisvar);
  input_data = [thisvar(:), zeros(numel(thisvar), nfile-1, class(thisvar))];
  for K = 2 : nfile
    filestruct = load(dinfo(K).name);
    fn = fieldnames(filestruct);
    thisvar = filestruct.(fn{1});
    if ~isequal(size(thisvar), orig_size)
       error('file "%s" has a different size of matrix than the first file "%s"', dinfo(K).name, dinfo(1).name);
    end
    input_data(:,K) = thisvar(:);
  end
  input_data = reshape(input_data, [orig_size, nfile]);
  avg = mean(input_data, ndim(input_data));
end

(with data of an unknown number of dimensions, it turned out to be easiest to store the data as columns and reshape afterwards.)

[Michael02139]

Thanks for your reply Walter.

I get one error message:

??? Undefined function or method 'ndim' for input arguments of type 'double'.

The input data variable looks good (and I understand that I've just created a 3x3x2 data structure).

Any idea what this means? (am I close?)

>> input_data

input_data(:,:,1) =

     1     2     3
     2     1     5
     3     6     1

input_data(:,:,2) =

     1     3     5
     2     1     5
     4     7     1

[Guillaume]

ndim should have been ndims

[Walter Roberson]

Yes, sorry, MATLAB is inconsistent between nelem (no 's') and 'ndims' (with 's'), and I keep getting them wrong because of that.

[Michael02139]

Perfect!! Thank you!