CODEWITHSUNDEEP
matlabmatrixaddition
Tetra
Tetra

Reputation: 747

Adding multiple nth elements of a matrix repeatedly

I have a matrix 63 rows x 7 columns

I want to select every 7th,8th,9th ongoing value in each column and add them to create a new value.

i.e.

7 8 9th added to a new value

16 17 18th added to a new value...etc

25 26 27th

34 35 36th

43 44 45th

52 53 54th

61 62 63th

So I should end up with a 7x7 matrix.

Without doing this manually, is there a simple command so that if the dimensions of the matrix changes, the output will always be correct?

Upvotes: 0

Views: 944

Answers (3)

Jonas
Jonas

Reputation: 74930

You can do that easily with a bit of reshaping.

originalMatrix = (1:63)'*(1:7); %'
[nRows,nCols] = size(originalMatrix); %# =63 in this example
stepSize = 9;
nTriplets = floor(nRows/stepSize); %# =7 in this example

%# create index list
idx = bsxfun(@minus,stepSize:stepSize:nRows,[2 1 0]'); %'
idx = idx(:); %# reshape to a vector

%# create 3-by-7-by-7 array from original matrix
tmpMatrix = reshape(originalMatrix(idx,:),3,nTriplets,nCols);

%# sum along dim 1 (to sum e.g. the 7th, 8th, and 9th value)
result = squeeze(sum(tmpMatrix,1));

result =
      24          48          72          96         120         144         168
      51         102         153         204         255         306         357
      78         156         234         312         390         468         546
     105         210         315         420         525         630         735
     132         264         396         528         660         792         924
     159         318         477         636         795         954        1113
     186         372         558         744         930        1116        1302

Upvotes: 1

Marek Kurdej
Marek Kurdej

Reputation: 1499

If I had understood your question properly, this slip of code should do what you want. But I admit, maybe it's not the most efficient Matlab code ever-written...

k = 9; n = 7; m = k*n; % 63
A = randi(5,m,n);

startIdx = k*(1:n)+n-k;
endIdx = k*(1:n);

B = zeros(n,n);
for i = 1:n
    tmp = A(startIdx(i):endIdx(i),:);
    B(i,:) = sum(tmp,1);
end

Upvotes: 1

arne.b
arne.b

Reputation: 4330

matrix=(1:63)'*(1:7);
n=7;

startind = n:(n+2):size(matrix,1);
endind = (n+2):(n+2):size(matrix,1);
tmp=cumsum(matrix);
tmp(endind,:)-tmp(startind,:)

This will, of course, only work if startind and endind have the same length, which would not be the case for, say, a matrix of size 62x7.

Upvotes: 1

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