Implementation of csr_to_sparse.m

Contents.m

@@ -29,9 +29,10 @@
 %
 % Helper functions
 % sparse_to_csr              - Compressed sparse row arrays from a matrix
+% csr_to_sparse              - Convert back to Matlab sparse matrices
 % load_gaimc_graph           - Loads a sample graph from the library
 
-% David Gleich
+% David F. Gleich
 % Copyright, Stanford University, 2008-2009
 
 % History
@@ -47,4 +48,4 @@
 % Future todos
 % Implement weighted core nums
 % More testing
-% Implement all pairs shortest paths with Floyd Warshall
+% Implement all pairs shortest paths with Floyd Warshall

csr_to_sparse.m

@@ -0,0 +1,54 @@
+function [nzi,nzj,nzv] = csr_to_sparse(rp,ci,ai,ncols)
+% CSR_TO_SPARSE Convert from compressed row arrays to a sparse matrix
+%
+% A = csr_to_sparse(rp,ci,ai) returns the sparse matrix represented by the
+% compressed sparse row representation rp, ci, and ai.  The number of
+% columns of the output sparse matrix is max(max(ci),nrows).  See the call
+% below.
+%
+% A = csr_to_sparse(rp,ci,ai,ncol) While we can infer the number of rows 
+% in the matrix from this expression, you may want a
+% different number of 
+%
+% [nzi,nzj,nzv] = csr_to_sparse(...) returns the arrays that feed the
+% sparse call in matlab.  You can use this to avoid the sparse call and
+% customize the behavior.
+%
+% This command "inverts" the behavior of sparse_to_csr.
+% Repeated entries in the matrix are summed, just like sparse does.  
+%
+% See also SPARSE SPARSE_TO_CSR
+% 
+% Example:
+%   A=sparse(6,6); A(1,1)=5; A(1,5)=2; A(2,3)=-1; A(4,1)=1; A(5,6)=1; 
+%   [rp ci ai]=sparse_to_csr(A); 
+%   A2 = csr_to_sparse(rp,ci,ai)
+
+% David F. Gleich
+% Copyright, Stanford University, 2008-2009
+
+%  History
+%  2009-05-01: Initial version
+%  2009-05-16: Documentation and example
+
+nrows = length(rp)-1;
+nzi = zeros(length(ci),1);
+for i=1:nrows
+    for j=rp(i):rp(i+1)-1
+        nzi(j) = i;
+    end
+end
+
+if nargout<2,
+    if nargin>3,
+        nzi = sparse(nzi,ci,ai,nrows,ncols);
+    else
+        % we make the matrix square unless there are more columns
+        ncols = max(max(ci),nrows);
+        if isempty(ncols), ncols=0; end
+        nzi = sparse(nzi,ci,ai,nrows,ncols);
+    end
+else
+    nzj = ci;
+    nzv = ai;
+end 

test/test_csr_to_sparse.m

@@ -0,0 +1,33 @@
+function test_csr_to_sparse
+%% empty arguments
+A = csr_to_sparse(1,[],[]);
+if ~isequal(A,sparse([]))
+    error('gaimc:csr_to_sparse','failed to convert empty matrix');
+end
+
+A = csr_to_sparse(1,[],[],50);
+if size(A,2)~=50
+    error('gaimc:csr_to_sparse','incorrect empty size output');
+end
+
+%% exact test
+% clique to sparse
+rp = 1:5:26;
+ci = reshape(repmat(1:5,5,1)',25,1);
+ai = ones(25,1);
+A=csr_to_sparse(rp,ci,ai);
+if ~isequal(A,sparse(full(ones(5))))
+    error('gaimc:csr_to_sparse','failed to convert clique');
+end
+
+%% 100 random trials of round trips between sparse_to_csr and csr_to_sparse
+for t=1:100
+    A = sprand(100,80,0.01);
+    [rp ci ai]=sparse_to_csr(A);
+    A2 = csr_to_sparse(rp,ci,ai,80);
+    if ~isequal(A,A2)
+        error('gaimc:csr_to_sparse','random sparse test failed');
+    end
+end
+
+

test/test_examples.m

@@ -73,3 +73,11 @@
 load_gaimc_graph('cores_example'); % the graph A has three components
 corenums(A)
 
+%% sparse_to_csr
+A=sparse(6,6); A(1,1)=5; A(1,5)=2; A(2,3)=-1; A(4,1)=1; A(5,6)=1; 
+[rp ci ai]=sparse_to_csr(A);
+
+%% csr_to_sparse
+A=sparse(6,6); A(1,1)=5; A(1,5)=2; A(2,3)=-1; A(4,1)=1; A(5,6)=1; 
+[rp ci ai]=sparse_to_csr(A); 
+A2 = csr_to_sparse(rp,ci,ai);

test/test_main.m

@@ -1,8 +1,11 @@
 function test_main
 % TODO Check the directory
+test_examples
 test_sparse_to_csr
+test_csr_to_sparse % should always come after sparse_to_csr
 test_dfs
 test_load_gaimc_graph
 test_largest_component
+test_examples
 
-end
+end