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

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Douglas Anderson on 27 Feb 2018
Commented: Jos (10584) on 28 Feb 2018
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
##### 2 CommentsShowHide 1 older comment
Douglas Anderson on 27 Feb 2018
Yes, that is true.

Jos (10584) on 27 Feb 2018
% 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))
##### 2 CommentsShowHide 1 older comment
Jos (10584) on 28 Feb 2018
This might not be the most efficient way though.
See the documentation of sparse for more details.

Sean de Wolski on 27 Feb 2018
for ii = 1:size(x, 1)
x(ii, a(ii,:)) = 1;
end

Jos (10584) on 27 Feb 2018
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

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