How to append in .mat file row or columnwise cellmatrix

Question

I am running a simulation where i generate huge 2d sparse matrices and hence i use FIND function to only store nonzero values with their indices.

Now for each itera

Answer 1

You can't append data to an existing variable with save. You need different variables:

clear all
filename = 'data.mat';
save(filename) %// empty file, for now. We'll append variables within the loop
for n = 1:10  
    A = rand(5);  
    [i,j,vals] = find(A);  
    varname = ['data' num2str(n)]; %// varname is 'data1', 'data2' etc
    assignin('base',varname,[i,j,vals]); %// create that variable
    save(filename, varname, '-append') %// append it to file
end

Answer 2

You can use matfile with cells as long as you are not trying to index into the actual cells. Remember that cells are still arrays, so you can access each cell using array indexing. For example:

>> x = {'this', 'is', 'an', 'example'};
>> x{4}

ans =

example

>> x(4)

ans = 

    'example'

The following just initializes the data. Make sure to save it with the '-v7.3' tag, so that it supports efficient partial saving and loading.

data = {};
save('data.mat', 'data', '-v7.3');

Now you can access your data with the matfile

mf = matfile('data.mat', 'Writable', true);
for n=1:10  
   A=rand(5);  
   [i,j,vals]=find(A);  
   data={[i,j,vals]};
   mf.data(end+1, 1) = data;
end

Reference: matfile documentation