CODEWITHSUNDEEP
matlabloopsmatrix
Tony YEe
Tony YEe

Reputation: 235

Extract the sequence matrix elements?

I have a matrix as shown in below:

A=[2 4;1 3;8 6;5 1;4 9]

now i need to extract the matrix A into 2 parts:

newpoint=[2 4];
rest=[1 3;8 6;5 1;4 9];

then apply loop again to extract the second column as new point :

newpoint=[1 3];
rest=[2 4;8 6;5 1;4 9];

Applying loop again to take third column number as new point :

newpoint=[8 6];
rest=[2 4;1 3;5 1;4 9];

Take the number in row sequence until the last row . Can someone be kind enough to help.Thanks~

Upvotes: 0

Views: 192

Answers (2)

George Aprilis
George Aprilis

Reputation: 1200

Apart from HebeleHododo's answer, if you have big matrices maybe you can try this:

A = [2 4; 1 3; 8 6; 5 1; 4 9];

B = zeros(size(A,1)-1,size(A,2));

for idx = 1:size(A, 1)
    newpoint = A(idx, :);
    B(1:idx-1,:) = A(1:idx-1,:);
    B(idx:end,:) = A(idx+1:end,:);
    % do stuff
end

It doesn't get rid of the for loop, but the temporary B matrix is pre-allocated and the copy between A and B is clear, which makes it quicker.

For A = rand(100000,2); HebeleHododo's method takes ~123 seconds in my computer and the one above takes ~85 seconds.

Edit: Just for reference, the timing is done using Intel Core i5-3450 CPU @ 3.10GHz and Matlab R2011b

Upvotes: 2

HebeleHododo
HebeleHododo

Reputation: 3640

You said you want to extract columns, but gave examples with rows. I am going ahead and assuming you meant rows.

You can do it with a for loop.

A = [2 4; 1 3; 8 6; 5 1; 4 9];

for idx = 1:size(A, 1)
    newpoint = A(idx, :);
    rest = A; % Copy A to rest
    rest(idx, :) = []; % Remove newpoint line
    % do stuff
end

Results of first two iterations:

newpoint =

     2     4

rest =

     1     3
     8     6
     5     1
     4     9

newpoint =

     1     3

rest =

     2     4
     8     6
     5     1
     4     9

This is not a good method if your A matrix is big.

Edit: In fact, do not use this method. George Aprilis timed it and found 123 seconds for a 100000x2 matrix. I guess my computer is much slower. It took 216 seconds. I repeat, do not use this.

Upvotes: 2

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