MATLAB: Matrix indexing without loop

matrix indexing

Hello,

Suppose I have two matrices, A and B:

 A = [2 3 1 4;
      1 4 1 3];
 B = [1 2 3 4;
      4 3 2 1];

Now I get the max indices for the matrix A:

 [~,ind] = max(A,[],2);

I want to obtain the corresponding values of matrix B with ind:

 for ii = 1:2
  v(ii) = B(ii,ind(ii));
 end

How to get the values of B without a for loop?

Best Answer

v = B(sub2ind(size(B), 1:size(B,1), ind))

Related Solutions

MATLAB: I have a marix of zeros(100100) and i have 5 values of rows and cols in rc.(getting these values from another function). Now i want to make a new matrix B of same size as A(100100) and want ones on that rows and cols provided by rc and zeros else

rc = [2  5
      3  5
      7  9
     15 69
     95 6];
B = full(sparse(rc(:, 1), rc(:, 2), 1, size(A, 1), size(A, 2)));

is one way of doing it. Another way:

B = zeros(size(A));
B(sub2ind(size(B), rc(:, 1), rc(:, 2))) = 1;

MATLAB: Question about vectorization using indexes

Note that ind and b must be transposed for this to work:

>> a   = [4;2;1;3;1;4;4;0]; % must be column!
>> ind = [1,0;2,3;4,0;3,3;5,3].'; % transposed!

>> b   = [5,0;2,2;1,0;2,2;2,2].'; % transposed!
>> idx  = b~=0;
>> XC = ind(idx);
>> bC =   b(idx);
>> [~,idc] = find(idx);
>> out = accumarray(idc,a(XC).^bC,[],@prod)
out =
  1024
  4
  3
  1
  1

Best Answer

Related Solutions

MATLAB: I have a marix of zeros(100*100) and i have 5 values of rows and cols in rc.(getting these values from another function). Now i want to make a new matrix B of same size as A(100*100) and want ones on that rows and cols provided by rc and zeros else

MATLAB: Question about vectorization using indexes

Related Question

MATLAB: I have a marix of zeros(100100) and i have 5 values of rows and cols in rc.(getting these values from another function). Now i want to make a new matrix B of same size as A(100100) and want ones on that rows and cols provided by rc and zeros else