Loop to extract rows from matrix

2017 8 29
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更新时间：2017 8 30
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Hi, I need to extract each rows of a matrix creating different variables. How a can do it? Thanks in advance!

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Jan
Jan 2017-8-29

2 个投票

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.

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GiuBa
GiuBa 2017-8-30
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
Adam 2017-8-30
Why can't you just create the text files from the one initial array, taking a row at a time?
Jan
Jan 2017-8-30
编辑：Jan 2017-8-30
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

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