Bed of Nails from vectors - inverse of find()?

Hello,

I would like to do the following:

X = zeros(N,M); % Say N = 50, M = 100

Then have N sets of indices, one for each row of X:

a(1,:) = [ 1 5 10 12 19 32 56 ...]; % These may vary substantially
a(2,:) = [ 1 7 14 19 29 45 76 ...];
...

These are then indices that will indicate where there should be a "1" in each row of X.

Is there a function that will take the indices and then put a "1" in an array? This is kind of like the inverse of find().

Thanks!

Doug Anderson

[Jos (10584)]

As your variable a is a N-by-K matrix, this assumes that all rows of X will hold K ones. Is that so?

[Douglas Anderson]

Yes, that is true.

Answer 1

[Jos (10584)]

% input
N = 5 ; M = 4 ; 
a = [1 3 ; 2 4 ; 1 4 ; 1 2 ; 3 4]

% engine in one line
XX = full(sparse(repmat(1:N, size(a,2), 1)', a, 1,N,M))

[Douglas Anderson]

Thanks to everyone. Of course, this is only the deepest level of a bunch of calculations to determine the "a" values, so this one-line engine is nice.

Can you tell me why this works? :) Thank you!

[Jos (10584)]

This might not be the most efficient way though.

See the documentation of sparse for more details.

Answer 2

[Sean de Wolski]

for ii = 1:size(x, 1)
  x(ii, a(ii,:)) = 1;
end

Answer 3

[Jos (10584)]

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

X = zeros(N,M)
X(sub2ind([N M], repmat(1:N, size(a,2), 1)', a)) = 1