Storage elements from matrix in other one, with indexing.

Question

Isai Fernandez 2021-5-10

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/826380-storage-elements-from-matrix-in-other-one-with-indexing

评论： KSSV 2021-5-10

I want to storage information from one matrix to another. In this example, from A matrix I have to extract to storage in B matrix. The solution should be:

B = [6 4
    6 8]

I tried so hard, I don't get it.

Please help me.

A = [
    8 3 0 2
    9 6 1 4
    10 6 2 1
    1 6 2 8 
];
a=2;
b=4;
B = zeros(2)
m=1
while m<=2
    k= 1
    while k<=2
        i=1
        while i<=4
            if i==a
                n1 = i
                if n1<=2
                    j = 1
                    while j<=4
                        if j==b
                            n2=j
                            if n2<=2
                            B(m,k) = B(m,k)+A(n1,n2)
                            %B(m,k) = B(m,k)+A(n1,n2)
                            end
                        end
                        j = j+1
                    end
                end
            end
            i = i+1
        end
    k = k+1
    end
    m=m+1
end

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KSSV 2021-5-10

What output you expect?

Isai Fernandez 2021-5-10

B matrix with elements from A with index a and b.

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Answer 1

KSSV 2021-5-10

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/826380-storage-elements-from-matrix-in-other-one-with-indexing#answer_696245

在 MATLAB Online 中打开

A = [8 3 0 2
    9 6 1 4
    10 6 2 1
    1 6 2 8 
];
a=2;
b=4;
[X,Y] = meshgrid([a b],[a b]) ;
id = sub2ind(size(A),Y,X) ;
B = A(id) ;
B
B = 2×2
     6     4
     6     8

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KSSV 2021-5-10

在 MATLAB Online 中打开

A = [8 3 0 2
    9 6 1 4
    10 6 2 1
    1 6 2 8 
];
a=2;
b=4;
[X,Y] = meshgrid([a b],[a b]) ;
[m,n] = size(X) ; 
B = zeros(m,n) ; 
for i = 1:m
    for j = 1:n
        B(i,j) = A(Y(i,j),X(i,j)) ;
    end
end

Loop will be slower....the solution shown first will be fast.

KSSV 2021-5-10

The answer is good.... You use that answer.

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Answer 2

Stephen23 2021-5-10

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/826380-storage-elements-from-matrix-in-other-one-with-indexing#answer_696285

在 MATLAB Online 中打开

The MATLAB approach:

A = [8,3,0,2;9,6,1,4;10,6,2,1;1,6,2,8]
A = 4×4
     8     3     0     2
     9     6     1     4
    10     6     2     1
     1     6     2     8
v = [2,4];
out = A(v,v)
out = 2×2
     6     4
     6     8