Back track the index of a matrix

Hi everybody, I'm writing a program to output results in a matrix(n x n).Let's call it matrix "A". I need to sort all the elements in matrix A in ascending order and select the first n elements. Let's put this elements in to a vector "B".

How should I write the code to back track the original index (row and column)of the elements of B in A ?

example:

A=[ 1 3 5; 9 7 6; 2 8 4];

B= [7 4 3];

elements of B in A: row= 2 3 1

column = 2 3 2

Thank you in advanced for the attention.

Dulan

Answer 1

[Bruno Pop-Stefanov]

The sort function can also output the linear indices of the sorted elements. Use these indices to access the corresponding elements in A. You can transform linear indices into subscript indices using the ind2sub function.

% Input matrix
A = [ 1 3 5; 9 7 6; 2 8 4];

% Sort. Transform A into a vector with (:)
[B,IX] = sort(A(:), 'ascend');

% Convert linear indices to subscript
[I,J] = ind2sub(size(A),IX);

% Display n first elements
for i=1:3
  fprintf('Element %d: %d at row %d and column %d\n', i, B(i), I(i), J(i));
end

[Adam Silva]

Hi Bruno. Thank you for the quick response. This code almost work. But the elements of my matrix A are complex numbers.

sample element set

 -0.0154 -45.4596i  -0.0164 -45.4599i  -0.0159 -45.4598i  -0.0158 -45.4597i
  -0.0150 -45.4594i  -0.0160 -45.4598i  -0.0155 -45.4596i  -0.0154 -45.4596i
  -0.0136 -45.4589i  -0.0147 -45.4592i  -0.0142 -45.4590i  -0.0141 -45.4590i
  -0.0160 -45.4599i  -0.0171 -45.4602i  -0.0165 -45.4600i  -0.0164 -45.4600i
  -0.0172 -45.4604i  -0.0183 -45.4607i  -0.0177 -45.4605i  -0.0176 -45.4605i
  -0.0099 -45.4573i  -0.0110 -45.4576i  -0.0105 -45.4575i  -0.0104 -45.4575i
  -0.0058 -45.4556i  -0.0069 -45.4559i  -0.0063 -45.4558i  -0.0062 -45.4558i
  -0.0074 -45.4561i  -0.0085 -45.4564i  -0.0080 -45.4562i  -0.0079 -45.4562i
   0.0260 -45.4466i   0.0249 -45.4470i   0.0254 -45.4468i   0.0255 -45.4468i
   0.0249 -45.4470i   0.0477 -45.4405i   0.0243 -45.4471i   0.0244 -45.4471i
   0.0254 -45.4468i   0.0243 -45.4471i   0.0374 -45.4435i   0.0250 -45.4469i
   0.0255 -45.4468i   0.0244 -45.4471i   0.0250 -45.4469i   0.0353 -45.4440i

when I used your code the answers were in the very end n elements. How can we change the code to select those elements.

[Azzi Abdelmalek]

You have to explain how do you want to sort your complex number, by real part? by modulus? ...

[Adam Silva]

Sorry for the incomplete question.

Complex number should be sort "by modules"

[Bruno Pop-Stefanov]

By default, sort orders complex numbers by modulus first, and, if two numbers have same modulus, it orders them by angle.

If you want to sort your list by angle only, then use sort on angle(A(:)) instead:

% Input matrix
A = [-0.0154-45.4596i, -0.0164-45.4599i, -0.0159-45.4598i; ...
     -0.0150-45.4594i, -0.0160-45.4598i, -0.0155-45.4596i; ...
     -0.0136-45.4589i, -0.0147-45.4592i, -0.0142-45.4590i];

% Sort. Transform A into a vector with (:)
[B,IX] = sort(angle(A(:)), 'ascend');

% Convert linear indices to subscript
[I,J] = ind2sub(size(A),IX);

% Display n first elements
for i=1:3
  fprintf('Element %d: %d at row %d and column %d\n', i, B(i), I(i), J(i));
end

Answer 2

[Azzi Abdelmalek]

A=[ 1 3 5; 9 7 6; 2 8 4];
B= [7 4 3];
idx=[];
idy=[];
for k=1:numel(B)
  ij=ismember(A,B(k));
  [ii,jj]=find(ij);
  idx=[idx ii];
  idy=[idy jj];
end
idx
idy

[Adam Silva]

Thank you Azzi

Answer 3

[Andrei Bobrov]

A = [ 1 3 5; 9 7 6; 2 3 4];
B = [7 4 3];

[a,b] = ismember(A,B);
[r,c] = find(a);
out = accumarray(b(a),1:numel(b(a)),[],@(ii){[r(ii),c(ii)]});

Answer 4

[Adam Silva]

Thank you all for quick responses and help. I got the answer for my assignment and learn some new codes in Matlab.