Loop to extract rows from matrix

Hi, I need to extract each rows of a matrix creating different variables. How a can do it? Thanks in advance!

Answer 1

[Jan]

Don't do this. The topic is discussed daily in this forum, see e.g. https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval.

The dynamic creation of a bunch of vectors causes troubles. It is even worse, if you want to use these vectors later on. To access a column of a matrix, simply use an index: A(:, i) is the i.th column. This is fast, clean and simple.

Please explain, why you assume that creating a set of variables is useful in your case. I'm sure, that the forum can give you better options to solve the actual problem.

[GiuBa]

Many thanks for the link. I have a matrix with 228 rows and a need to create a 228 matrix with one row in order to create a txt file from each of them.

As example: I have this matrix

A=[1,2.30,43.66,21.42,6.63;1,2.30,24.22,34.37,1.20;1,2.30,33.72,28.54,12.38;];

and I need to create those matrices:

1=[1,2.30,43.66,21.42,6.63];
2=[6.63;1,2.30,24.22,34.37,1.20];
3=[1,2.30,33.72,28.54,12.38];

and finally create from each of those matrices a .txt files using the following code but implemented for a loop:

% the data
     fnam='test.txt'; % 
     hdr={'a ','b','c'}; % First header%
     hdr2={'(m)','(%)','(%)'}; % Second Header%
     m = data %rows from matrix

% the engine
     fmt = repmat('%s\t ', 1, length(hdr));
   fmt(end:end+1) = '\n';
   fid = fopen(fnam, 'w');
   fprintf(fid, fmt, hdr{:});
   fprintf(fid, fmt, hdr2{:});
   fclose(fid);
   dlmwrite(fnam,m,'-append','delimiter','\t');
% the result
     type(fnam)

The final result that I want obtait could be a three different .txt files: 1.txt 2.txt 3.txt Thanks in advance!

[Adam]

Why can't you just create the text files from the one initial array, taking a row at a time?

[Jan]

Again: No, you do not need to create the vectors explicitly. Trying to do so is a shot in your knee. A(iRow, :) is perfect already.

Do you want to create 228 files? Then:

hdr  = {'a ','b','c'}; % First header%
hdr2 = {'(m)','(%)','(%)'}; % Second Header%
fmt  = repmat('%s\t ', 1, length(hdr));
fmt(end:end+1) = '\n';

for iRow = 1:size(A, 1);
  fnam = sprintf('test%03d.txt', iRow);
  fid = fopen(fnam, 'w');
  fprintf(fid, fmt, hdr{:});
  fprintf(fid, fmt, hdr2{:});
  fclose(fid);
  dlmwrite(fnam, A(iRow, :), '-append', 'delimiter', '\t');
end

This can be done directly also without dlmwrite:

fmtData = [repmat('%g\t', 1, size(A, 2) - 1), '%g\n'];
...
  fprintf(fid, fmt, hdr2{:});
  fprintf(fid, fmtData, A(iRow, :));
  fclose(fid);
end