Storage elements from matrix in other one, with indexing.

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

[KSSV]

What output you expect?

[Isai Fernandez]

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

Answer 1

[KSSV]

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

[Isai Fernandez]

Thank you so much, but; could it be with loops, as I tried?. Because I have to use it with a lot of data.

[KSSV]

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]

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

Answer 2

[Stephen23]

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

[Isai Fernandez]

Wow, I have to figure out to implement this short code.

[KSSV]

I missed the common sense....