Adding row to matrix in a loop

I have a folder with many files (file_1, file_2 ... file_n), each of them contains variables (A, B, C ...). I need to work with several files (number of files varies) that I specifically choose from the folder (eg. file_1, file_7, file_22, ... file_m) - load the file, calculate some stuff and save the result as a new row to a matrix and then do this for another file etc.

I ´ve been thinking of using for loop to load and calculate, but I can´t figure out how to add a new row to a matrix after every loop. I tried "cat", but it didn´t work the way I need. Or is there a better solution?

And furthemore I need to plot the results in one graph the way I can specify the plot details for a group of files (eg. plot the first 10 results in red, the next ones in green, etc.). I can´t do it manually, because there are dozens of files.

Thanks in advance!

Answer 1

[Adam Danz]

If and only if the matrices have the same number of columns, you can concatenate two matrices vertically using,

M = [m1; m2];  % where m1 and m2 are matrices with the same number of columns.

To add a new row,

M(end+1,:) = NaN;  % or 0, or any other scalar value.

Another solution would be to save the matrices within a cell array and concatenate them later, after the loop; again, equal column numbers are required.

C = cell(n,1); 
for i = 1:n
    C{i} = mat;  % mat is a matrix
end
M = vertcat(C{:});

Once the matrix is complete, the plot() function will plot one line object per column

plot(M, '-') 

[Peter Till]

Thank you for your answer, Adam.

Yes, both matrices have the same number of columns. I use the second solution you mentioned, it works, but I had to use

C{i} = mat 

instead of

C{1} = mat 

to get a new row after every loop. And also I could omit

C = cell(n,1)

So my code is now

for i = 1:n;
    % calculating stuff
    mat = [X Y Z];
    C{i} = mat; 
end
M = vertcat(C{:});

[Adam Danz]

Thanks for pointing out that typo, Peter. I fixed it in the answer so it doesn't cause future confusion.

Your updated version looks good except for two small stylistic corrections you can make:

1. Indent the lines within the for-loop (ctrl + a to select all; ctrl + i for smart indentation),
2. Suppress the output from the first line of the for-loop by adding a semicolon to the end of the line (I realize it's probably just a demo-line).

[Peter Till]

Corrections done. Thanks for the great tip, I didn´t know about the ctr+i shortcut.