MATLAB: I have a 60316 matrix an to extract 100 random data compounds to build a 3100 matrix.What should I do

matrix

randi(100,3,A)

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Here is what you directed in your comment:

A = rand(6031, 6); % Some random data
% "extract the second and third columns of A to build a new matrix B:
B = A(:, 2:3);
% "extract 3 random rows from B to build a new matrix C"
rowsToExtract = randperm(size(B, 1), 3)
C = B(rowsToExtract, :)

Note that this does not give 100 columns since you extracted only columns 2 and 3, not 100 columns.

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MATLAB: How to reshape matrix

Your goal isn't clear (see comment under your question). But I think I understand what you're looking for.

Here I make the following assumptions

You have a variable that identifies the subject for each row of your data.
The rows of your 2340x5 matrix is organized by subject. So, the first 90 rows are s1, then 90 rows of s2, etc.
The number of variables (in your case, 90) will always be even. If this is untrue, you'll have to use a cell array rather than a matrix and the two columns will be uneven.
You want to extract data from 1 column at a time. ( This one was really unclear but since you want two columns in a matrix, this has to be the case).

If this is correct, you are not resizing the matrix (ie, using resize()), you are extracting a subset of data to create a new, smaller matrix.

Here I create fake data that fit your description and in 3 lines, I build the 2-column matrix extracting data from column 3 of your data.

% create fake data
nSubj = 26;                          %number of subjects
nVars = 90;                          %number of variables
data = rand(nSubj * nVars, 5);       %your data (fake)
subjID = meshgrid(1:nSubj, 1:nVars); 
subjID = subjID(:);                  %identifies the subject # for each row of data
% create index that chooses rows from first 1/2 of each subject
firstHalfIdx = repmat([true,false], floor(nVars/2), 1); %floor() in case nVars isn't even.
firstHalfIdx = repmat(firstHalfIdx(:), nSubj, 1); 
% Put data into new matrix 
col = 3;     %choose column to extract
dataHalves = [data(firstHalfIdx, col), data(~firstHalfIdx, col)];

dataHalves is a [1170-by-2] matrix where column 1 is the first half of data from all subjects and column 2 is the 2nd half - extracting data from column 3 of your data.

MATLAB: Take three matrices into a single matrix

A=[a1;a2;a3]

a1 through a3 are single line vectors

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MATLAB: How to reshape matrix

MATLAB: Take three matrices into a single matrix

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