grouping array based on similar row element

Hello,

I have a matrix A:

123 1970 1 1 12
123 1971 1 1 120
123 1972 1 12 300
240 1970 1 10 40
240 1971 2 1  45
240 1972 3 34 56
230 1970 1 2 120
230 1971 2 3 320
230 1972 1 14 360	

I wish to export 3 different matrices in a folder based on the common column element. This might be easy without a loop using an index operation. The resulting matrices

A =

123 1970 1 1 12
123 1971 1 1 120
123 1972 1 12 300

B =

240 1970 1 10 40
240 1971 2 1  45
240 1972 3 34 56

C =

230 1970 1 2 120
230 1971 2 3 320
230 1972 1 14 360

[Guillaume]

Is the last row of C correct, shouldn't that be in B?

[Poulomi Ganguli]

yes, the last row of C should be in B. sorry. Corrected now.

[Image Analyst]

It's pretty universal among experienced MATLAB users that that would be a bad idea.

See the FAQ.

Now, if you want ot do it in a loop

uniqueValues = unique(A(:, 1));
for k = 1 : length(uniqueValues)
    thisValue = uniqueValues(k);
    rowsWithThisValue = A(:, 1) == thisValue;
    subArray = A(rowsWithThisValue, :)
    % Now do something with the subArray
end

then that would be okay.

Answer 1

[Guillaume]

The easiest:

savelocation = 'C:\somewhere\somefolder';
fileformat = 'submatrices_%02d.txt';

group = findgroups(A(:, 1));
splitapply(@(subA, index) writematrix(subA, fullfile(savelocation, sprintf(fileformat, index(1)))), A, group, group); %passing group twice so we get it in the anonymous function to use as file number

[Poulomi Ganguli]

I wanted to specify 123.txt, 240.txt and 230.txt as file names, so I revised the last line a little:

splitapply(@(subA, index) writematrix(subA, fullfile(Save_Location, sprintf(fileformat, subA(index(1)))),'Delimiter',' '),A,group,group);

[Guillaume]

Or simpler:

splitapply(@(subA) writematrix(subA, fullfile(Save_Location, sprintf('%d.txt', subA(1))), 'Delimiter', ' '), A, group);

[lim xiang]

Hi, I am new to Matlab. May you help to explain the code ? Thank you