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function [I, J, X] = find (G, k, search)
%FIND extract entries from a matrix.
% [I, J, X] = find (G) extracts the nonzeros from a matrix G.
% X has the same type as G ('double', 'single', 'int8', ...).
%
% Linear 1D indexing (I = find (S) for the built-in matrix S) is not yet
% supported.
%
% A GraphBLAS matrix G may contain explicit zero entries, and by default
% these are excluded from the result. Use GrB.extracttuples (G) to return
% these explicit zero entries.
%
% For a column vector, I = find (G) returns I as a list of the row indices
% of nonzeros in G. For a row vector, I = find (G) returns I as a list of
% the column indices of nonzeros in G.
%
% [...] = find (G, k, 'first') returns the first k nonozeros of G.
% [...] = find (G, k, 'last') returns the last k nonozeros of G.
% For this usage, the first and last k are in terms of nonzeros in the
% column-major order.
%
% See also sparse, GrB.build, GrB.extracttuples.
% SuiteSparse:GraphBLAS, Timothy A. Davis, (c) 2017-2022, All Rights Reserved.
% SPDX-License-Identifier: Apache-2.0
% FUTURE: find (G,k,'first') and find (G,k,'last') are slow.
% They are currently implemented, all entries are extracted and then the
% first or last k are selected from the extracted tuples. It would be
% faster to use a mexFunction that directly accesses the opaque content
% of G, instead of using GrB_Matrix_extractTuples_*, which always extracts
% the entire matrix.
if (isobject (G))
G = G.opaque ;
end
if (nargin > 1)
k = ceil (double (gb_get_scalar (k))) ;
if (k < 1)
error ('GrB:error', 'k must be positive') ;
end
if (~isequal (gbformat (G), 'by col'))
% find (G, k) assumes the matrix is stored by column, so reformat G
% if it is stored by row.
G = gbnew (G, 'by col') ;
end
end
% prune explicit zeros
G = gbselect (G, 'nonzero') ;
[m, n] = gbsize (G) ;
if (nargout == 3)
[I, J, X] = gbextracttuples (G) ;
if (m == 1)
I = I' ;
J = J' ;
X = X' ;
end
elseif (nargout == 2)
[I, J] = gbextracttuples (G) ;
if (m == 1)
I = I' ;
J = J' ;
end
else
if (m == 1)
% extract indices from a row vector
[~, I] = gbextracttuples (G) ;
I = I' ;
elseif (n == 1)
% extract indices from a column vector
I = gbextracttuples (G) ;
else
% extract linear indices from a matrix
[I, J] = gbextracttuples (G) ;
% use the built-in sub2ind to convert the 2D indices to linear indices
I = sub2ind ([m n], I, J) ;
end
end
if (nargin > 1)
% find (G, k, ...): get the first or last k entries
if (nargin < 3)
search = 'first' ;
end
n = length (I) ;
if (k >= n)
% output already has all k first or last entries;
% nothing more to do
elseif (isequal (search, 'first'))
% find (G, k, 'first'): get the first k entries
I = I (1:k) ;
if (nargout > 1)
J = J (1:k) ;
end
if (nargout > 2)
X = X (1:k) ;
end
elseif (isequal (search, 'last'))
% find (G, k, 'last'): get the last k entries
I = I (n-k+1:n) ;
if (nargout > 1)
J = J (n-k+1:n) ;
end
if (nargout > 2)
X = X (n-k+1:n) ;
end
else
error ('GrB:error', ...
'invalid search option; must be ''first'' or ''last''') ;
end
end
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