Generating a matrix with specific sums
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Hylke Dijkstra
on 2 Jan 2022
Commented: Image Analyst
on 3 Jan 2022
Dear all,
I am trying to generate a matrix based on specific sums. Let's say I generate the following matrix
X = randi([0 400], 6, 6);
Which gives me the matrix X. Now I want to sum x11 and x21, x12 and x22, x21 and x22, and so on. This should give me a 3x6 matrix. Does anyone have any clue how to do this effectively?
Best
4 Comments
Accepted Answer
Image Analyst
on 2 Jan 2022
The pattern is confusing, like Torsten says. Is this what you mean?
X = randi([0 400], 6, 6)
X2 = [...
sum(X(1:2, :), 1); ...
sum(X(3:4, :), 1); ...
sum(X(5:6, :), 1); ]
4 Comments
Image Analyst
on 3 Jan 2022
If you have some other even number of rows, you can use a for loop:
X = randi([0 400], 8, 3)
[rows, columns] = size(X);
X2 = zeros(rows/2, columns);
newRow = 1;
for row = 1 : 2 : rows
X2(newRow, :) = sum(X(row : row+1, :), 1);
newRow = newRow + 1; % Increment pointer
end
X2 % Show in command window.
More Answers (1)
Paul
on 2 Jan 2022
Another solution:
X = randi([0 400], 6, 6)
kron(eye(3),[1 1])*X
7 Comments
Paul
on 3 Jan 2022
If you want to know more about the kron() function, start with its doc page.
Mathematically, all that's happening is matrix multiplication. Is that the part of the solution you're asking about?
FWIW, this solution probably isn't the most efficient what with all the multiplications by zero and summing up terms that are zero. Probably better to just pre-allocate the answer and use a simple loop over the sum() function (basically a minor extension of Image Analyst's solution) to fill in the rows of the anwer.
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