Convert an array of bounded integers to matrix of bit arrays

Question

Suppose I have an array a of bounded integers (in this case bounded by 5):

a = [3 4 4 2 1 5 5];

I want to convert this array of integers to a length(a) x 5 matrix A where each row is a bit array with a 1 in the column indexed by the integer from a:

A = [0 0 1 0 0;
     0 0 0 1 0;
     0 0 0 1 0;
     0 1 0 0 0;
     1 0 0 0 0;
     0 0 0 0 1;
     0 0 0 0 1];

This is easily accomplished with a for loop:

n = length(a)
A = zeros(n, max(a(:)));
for k = 1 : n
  A(k, a(k)) = 1;
end

I am looking for a vectorized implementation that does not use a for loop.

Shai · Accepted Answer · 2013-05-27 13:59:44Z

2

Two possible methods:

use sparse:

A = sparse( 1:n, a, 1, n, max(a(:)) );

if you want a non-sparse result

full(A);

Using sun2ind:

A = zeros( n, max(a(:)) );
A( sub2ind(size(A), 1:n, a ) ) = 1;

edited May 27, 2013 at 13:59

answered May 26, 2013 at 21:43

Shai

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1 Comment

user425495 Over a year ago

Thank you for the quick response and multiple solutions.

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