MATLAB: Matrices

indexingmatrices

I want to perform a Principal Component Analysis (PCA) and it is need that matrix has the same number of rows and columns. However, I got a matrix (1142×2) being that first column is composed by number of molecules and second column, species from plants. So, sometimes I got more than one molecule for each plant . In that way, I need to put all species in the rows (173) and the molecules in the columns (712) and put a value (1) only in the correct position, e.g. The 5 speciehave the substances 171, 300, 530, 700, So I need to put the value 1 in (5, 171), (5,300), (5, 530), so on… In the end, I want to construct a matrix of (173×712) which it possesses only values where there is a substance Someone help me Thanks

Best Answer

Please try the following:

   X = [ 5 171 ; 5 300 ; 5 530 ; 5 700 ];
   M = 173;
   N = 712;
   Y = zeros(M,N);
   idx = M*(X(:,2)-1) + X(:,1) ;
   Y(idx) = 1;
   sum(Y(:))

HTH.

Rick

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MATLAB: I am having matrix of i rows by 3 columns ,now i want to multiple only i rows of 1 columns with 5 upto 100th row after the 100th row ,values has to be as it is.

Let's assume that you want to multiply the first 'x' rows of the first column with a scalar 'n' and update the values in 'A'.

You can use the following command to do this:

>> A(1:x,1) = n*A(1:x,1);

If you want to do this for multiple columns (say first y columns) at once, you can do similar operation by indexing into columns also:

>> A(1:x,1:y) = n*A(1:x,1:y)

MATLAB: Convert matrix into the tensor

This is not a tensor, which has a specific meaning. If you had the same number of experiments per substance, you could turn it into a 3D array. For a variable number of experiments you will need a cell array. Here is one approach:

First, determine the distinct species and substances. species = unique(M(:,1)); substances = unique(M(:,2));

Create a cell array of the appropriate size.

nspec = numel(species);
nsub = numel(substances);
Marray = cell(nspec,nsub);

Populate each combination of species and substance with a matrix of experimental results.

for ii=1:nspec
  for jj=1:nsub
      [r,c] = find(M(:,1)==species(ii) & M(:,2) == substances(jj));
      Marray{ii,jj} = M(r,3:end);
  end
end

Here is how you retrieve the results for species 3 and substance 22:

ii = find(species==3); jj = find(substances==22);
Marray{ii,jj}
ans =
     1     1     1     0

Best Answer

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MATLAB: I am having matrix of i rows by 3 columns ,now i want to multiple only i rows of 1 columns with 5 upto 100th row after the 100th row ,values has to be as it is.

MATLAB: Convert matrix into the tensor

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