How we count number of matrices with respect to some condition?

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Ammy
Ammy 2018-2-7
评论： Steven Lord 2018-2-7
I have 1000 rows
for i=1:1000
B=A(i,:);
C=reshape(B,4,4);
D=bsxfun(@minus,C,max(C(:)))
end
Now I have 1000 D matrices , How can I check that
1- How many of D matrices are Null matrix (having all the entries are zero);
2- How many of D matrices have all the 4 distinct column;
3-How many of D matrices have 3 distinct column;
4- How many of D matrices have 2 distinct column;
5- How many of D matrices have 0 on its diagonal ;
6-How many of D matrices have non-zero entry on its diagonal.
Thanking for anticipation
  2 个评论
Guillaume
Guillaume 2018-2-7
Now I have 1000 D matrices
Well, actually, you have only one D matrix that you overwrite 999 times, leaving only the one created at step i = 1000.
D=bsxfun(@minus,C,max(C(:)))
That's a very convoluted way of doing
D = C - max(C(:));
Now if you were to get rid of the loop and operate on the whole A at once which would be a lot faster, and indeed create your 1000 matrices, then yes bsxfun could be useful:
D = permute(reshape(A.', 4, 4, []), [2 1 3]); %creates a 4x4x1000 matrix
D = bsxfun(@minus, D, max(max(D, [], 1), [], 2)));
Ammy
Ammy 2018-2-7
I have a lots of matrices in which some matrices are null matrices, How can I get total number of null matrices.

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回答（1 个）

KSSV
KSSV 2018-2-7
1- How many of D matrices are Null matrix (having all the entries are zero);
Try using nnz()...nnz gives you number of non-zero sin the matrix.
2- How many of D matrices have all the 4 distinct column;
Try using unique. Read about it. You can use unique along rows and columns.
3-How many of D matrices have 3 distinct column;
Read abiut unique
4- How many of D matrices have 2 distinct column;
Read about unique
5- How many of D matrices have 0 on its diagonal ;
Read about diag. This gives you diagonal elements. And then you can use nnz.
6-How many of D matrices have non-zero entry on its diagonal.
Read about diag and nnz.
  2 个评论
Ammy
Ammy 2018-2-7
nnz counts the number of entries in a matrix, but I want to count number of matrices having all the entries 0.
Steven Lord
Steven Lord 2018-2-7
The nnz function counts the Number of NonZero elements in the array. It ignores zero elements.
When all elements are non-zero, nnz is the same as numel which returns the NUMber of ELements.
A = magic(4)
nnzA = nnz(A)
nA = numel(A)
When some elements are non-zero and some are zero, nnz and numel return different values.
B = eye(4)
nnzB = nnz(B)
nB = numel(B)
When all elements are zero, nnz should return ...
C = zeros(4)
nncZ = nnz(C)
nC = numel(C)

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