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Closed 11 years ago.
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How do I divide the rows of a matrix by different values in MATLAB (array division)
I have a matrix A (size MxN) in Matlab and a Vector b with M rows and now I want to divide all elemtens in the i-th row of A by the i-th entry in b like a(i,:)/b(i) but I really don't want to use this sort since I than use a for-loop and I definitly need a FAST solution!
Could anybody help out? Thanks!
Edit: Somehow I just came up with it after posting... My solution is bsxfun(#rdivide, [1 1; 2 2; 3 3], [2 2 6]'), do you think that's a good and fast one?
You want to use bsxfunc :
bsxfun(#rdivide,A,B)
http://www.mathworks.com/help/techdoc/ref/bsxfun.html
Related
This question already has answers here:
Multiply a 3D matrix with a 2D matrix
(10 answers)
Closed 5 years ago.
I try to use svd in my work which will return the vector as below.
A=rand(10,5);
b=rand(10,5,100);
[U, S , V]=svd(A);
What I'm trying to do is to multiply each slice of b with U. With one specific slice, this operation is valid:
c=U'*b(:,:,1);
However when I try to use a vectorized method as below, it return array dimensions mismatch error.
Utran=U.';
c = cellfun(#(x) x.*b,num2cell(Utran,[1 2]),'UniformOutput',false);
I could probably use loop for the first method, but it's not efficient if I have a large matrix. Any idea what's my mistake here?
The following is a vectorized solution.
Not sure if it's faster than using loops; and even if it is, note that it uses more memorry for intermediate computations:
result = permute(sum(permute(conj(U), [1 3 4 2]).*b), [4 2 3 1]);
This question already has answers here:
Sum over blocks in a 2D matrix - MATLAB
(4 answers)
Closed 6 years ago.
I'm sure this is basic, but I can't come up to an easy way to reduce a matrix of population count by grid cell by adding up all the elements of the "small" cells which would fit in the new ones.
My matrix is 720x360 and would like to change it to 360x180. I thought about using imresize with 0.5 scale, and then assume the value per new grid is 4 times the old one.
I fell uneasy doing so though, as I can't find examples of similar cases. It'll be nice to have "sum" or "average" as an "interpolation" option or something like that.
Thanks in advance for any help
Cheers
Maria
If you have the Image Processing Toolbox:
x = randi(10,6,8); % example matrix
bs = [2 2]; % block size
y = col2im(sum(im2col(x, bs, 'distinct'), 1), [1 1], size(x)./bs, 'distinct');
How this works:
im2col(... 'distinct') arranges each distinct block of the given size into a column. sum(...,1) then sums each column. Finally, im2col(..., 'distinct') arranges the results back into a matrix of reduced size.
This question already has an answer here:
MATLAB one liner for batch assignment in 2D matrix?
(1 answer)
Closed 7 years ago.
So I'm a beginner in MATLAB so this question might be trivial.
Suppose x=[1 2 3 4 5] and y=[3 4 2 5 1] and img = zeros(5,5). I want to set img(1,3),(2,4),(3,2),(4,5),(5,1) to 1. How do I do this? When I simply try img(x,y), it takes all the combinations of indices like (1,3),(1,4),(1,2) etc. which is not what I want.
As you have experienced, MATLABs indexing doesn't work that way. To get a feeling for the ways indexing works in MATLAB, please have a look at this nice article from Mathworks.
Now how to tackle your problem: The solution is to use linear indexing. You can always index a 2-dimensional matrix either by (i,j) or with a linear index k which increases column-wise. You can convert between matrix-indexes and linear indexes using the sub2ind function. To get the correct indexes for your questions, use
img = zeros(5,5)
x = [1 2 3 4 5];
y = [3 4 2 5 1];
ind = sub2ind(size(img),x,y);
img(ind) = 1;
This question already has answers here:
How to divide an image into blocks in MATLAB?
(5 answers)
Closed 8 years ago.
I was wondering: I have a 100x100 matrix. I would like to split it in several 10x10 sub-matrices the first including columns and rows 1-10, then second including columns 11-20 and rows 1-10 and son on until eventually I have a set of 10x10 matrices.
Is there any way of doing this without needing to build an extremely complex array of for loops?
Thanks :)
If you need to extract the sub-matrices explicitly than mat2cell would be a reasonable choice:
sm = mat2cell( M, 10*ones(1,size(M,1)/10), 10*ones(1,size(M,2)/10) );
However, if you only need these submatrices for a local processing you can use blockproc
blockproc( M, [10 10], #myFun );
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applying norm function to rows of matrix - Matlab [duplicate]
(3 answers)
Closed 9 years ago.
I have a Nx3 matrix (A) the columns are X,Y,Z respectively. I want to calculate the norm that is sqrt(X^2+Y^2+Z^2) for each row. I did a for loop for that:
for i = 1:length(A)
Result(i) = norm(A(i,:))
end
is there any other way to do it avoiding for loop?
Thanks
You can do it like this:
sqrt(sum(A.^2, 2))
Your method returns a 1x3 where this returns a 3x1. So if you want you can transpose it but I doubt you really need to.