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matrixoctaveconstruction
HappyCoding
HappyCoding

Reputation: 13

construct a matrix in Octave

I have a vector idx = [3; 5; 3; 4; 3; 2; 5; 1]. The number is from 1:k with k = 5. I want to make a "k by m" matrix A (m is the number of elements in the vector idx). Each row of A contains either '0' or '1' with '1' indicated by the index of the vector idx. For example, the third row of A (k = 3) is "1" at columns 1, 3, 5 because those are the indexes of "3" in idx. So that A =

[0 0 0 0 0 0 0 1; 0 0 0 0 1 0 0 0; 1 0 1 0 1 0 0 0; 0 0 0 1 0 0 0 0; 0 1 0 0 0 0 1 0]

How can I do this in Octave? Thank you!

Upvotes: 0

Views: 165

Answers (2)

Andy
Andy

Reputation: 8091

Or another way:

idx = [3; 5; 3; 4; 3; 2; 5; 1];
A = sparse (idx, [1:numel(idx)], 1)
A =   Compressed Column Sparse (rows = 5, cols = 8, nnz = 8 [20%])
(3, 1) ->  1
(5, 2) ->  1
(3, 3) ->  1
(4, 4) ->  1
(3, 5) ->  1
(2, 6) ->  1
(5, 7) ->  1
(1, 8) ->  1

Which gives you a compressed column sparse (very efficient), you can convert this to a "normal, full matrix":

B = full (A)
B =

0   0   0   0   0   0   0   1
0   0   0   0   0   1   0   0
1   0   1   0   1   0   0   0
0   0   0   1   0   0   0   0
0   1   0   0   0   0   1   0

Upvotes: 1

beaker
beaker

Reputation: 16810

Try this:

idx = [3; 5; 3; 4; 3; 2; 5; 1];
n = numel(idx);
k = 5;
A=zeros(k,n);

A(sub2ind(size(A), idx, [1:n]')) = 1

Output is:

A =

   0   0   0   0   0   0   0   1
   0   0   0   0   0   1   0   0
   1   0   1   0   1   0   0   0
   0   0   0   1   0   0   0   0
   0   1   0   0   0   0   1   0

Upvotes: 0

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