IND2SUB4UP Subscripts from linear index for upper triangular matrix (only elements above diagonal)

Subscripts from a given single index for a 2D upper matrix (no elements over the diagonal)

Description

[I, J] = IND2SUB4UP(IND) returns vectors I and J containing equivalent row and column subscripts corresponding to the index vector IND.

Let ind be a vector of indexes for entries of some upper triangular matrix. The entries are selected vertically so that:

       ind = 1 is associated to entry (1, 2)
       ind = 2 is associated to entry (1, 3)
       ind = 3 is associated to entry (2, 3)
       ind = 4 is associated to entry (1, 4)
       ...
       ind = N * (N - 1) / 2 is associated to entry (N - 1, N)

***********************************************************************

EXAMPLE

If
             A = rand(10);
and
             b = A(find(triu(A, 1)));
then, given indices
             IND = [1:45];
for vector b, these are equivalent to subscripts
             [I, J] = ind2sub4up(IND);
for matrix A. In fact:
             all(A(sub2ind(size(A), I, J)) == b(IND))

ans =
        1

This is obtained without even knowing about size(A)

***********************************************************************

See also SUB2IND, IND2SUB, FIND.

• MATLAB release: MATLAB 7 (R14)

[Francesco Pozzi]

% Note that if
A = rand(10);
% and
b = A(find(triu(A, 1)));
% then, given indices
IND = [1:45];
% for vector b, these are equivalent to subscripts
[I, J] = ind2sub4up(IND);
% for matrix A. In fact:
all(A(sub2ind([10, 10], I, J)) == b(IND))
%
% ans =
%
% 1
%
% This is obtained without even knowing about size(A)

[Bruno Luong]

Same comment as the other twin package