generate an array according to some sequence

Question

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Suppose I have an array [4 2 1 3] (length = 4) and I want to generate another array of length 6(= 0.5*4*(4-1)).

The vlaues of the new array represents the relationship between i and j, for i < j.

If i precedes j than (i,j) = 1; and 0, otherwise.

That is [(1,2) (1,3) (1,4) (2,3) (2,4) (3,4)] = [0 1 0 1 0 0].

Is there any way out to evade using loops?

Thanks in advance.

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Answer 1

Walter Roberson 2011-4-10

0 个投票

[v,idx] = sort(A);
T = triu(bsxfun(@lt,idx.',idx),1);
B = T(logical(triu(ones(length(A)),1))).';

One more note on Walter's solution.

B is built in a column-wise fashion.

B = [(1,2) (1,3) (2,3) (1,4) (2,4)....].

Walter Roberson 2011-4-10

The T(logical...) part returns a column, and then the .' changes that to a row.