string formatting - Matlab file name with zero-padded numbers

Question

I have 11x11 matrices and I saved them as .mat files from F01_01 to F11_11. I have to run a function Func on each file. Since it takes a long time, I want to write a script to run the function automatically:

for i=01:11  
    for j=01:11  
        filename=['F',num2str(i), '_', num2str(j),'.mat'];  
        load(filename);  
        Func(Fi_j);   % run the function for each file  Fi_j  
    end  
end  

But it doesn't work, Matlab cannot find the mat-files.
Could somebody please help ?

Answer 1

The problem

i=01; 
j=01; 
['F',num2str(i), '_', num2str(j),'.mat']

evaluates to

F1_1.mat

and not to

F01_01.mat

as expected.

The reason for this is that i=01 is a double type assignment and i equals to 1 - there are no leading zeros for these types of variables.

A solution

a possible solution for the problem would be

for ii = 1:11
    for jj= 1:11
        filename = sprintf('F_%02d_%02d.mat', ii, jj );
        load(filename);  
        Func(Fi_j);   % run the function for each file  Fi_j  
     end  
end

Several comments:

1. Note the use of sprintf to format the double ii and jj with leading zero using %02d.

2. You can use the second argument of num2str to format its output, e.g.: num2str(ii,'%02d').

3. It is a good practice to use string formatting tools when dealing with strings.

4. It is a better practice in matlab not to use i and j as loop counters, since their default value in matlab is sqrt(-1).