Storing sparse matrices in cell or struct without converting them to full

I noticed recently, to my surprise, that sparse matrices are silently converted to full when put into a cell or a stuct. This is especially problematic when using the hand functionality of varargin and varargout that tend to collect variables as cell-arrays. Is there a way to disable or bypass this behavior?

[Jan Neggers]

Thank you @Chunru,

I think I got lost somewhere, or my analysis was very sloppy and thus my conclusion wrong. Sparse matrices are NOT converted to full when storing in cells or structs. Something else in my code is doing so. Since this is a large project, I need to dive back in and do some bug hunting, this time I'll make sure to have a coffee before I start.

If the problem is related, I'll come back here, but in case of silence assume that it was not a related issue.

I think what tripped me up was the output of display():

>> s.a = speye(3)

s = 

  struct with fields:

    a: [3×3 double]

>> a{1} = speye(3)

a =

  1×1 cell array

    {3×3 double}

Which does not include the "sparse" attribute. But, I should have looked inside at the matrix itself.

[Chunru]

When the structure and cell are displayed as a whole, it shows the size and type of the data.

When the elements are shown, it shows the sparse storage.

s.a = speye(3)

s = struct with fields:
    a: [3×3 double]

s.a

ans = 
   (1,1)        1
   (2,2)        1
   (3,3)        1

a{1} = speye(3)

a = 1×1 cell array
    {3×3 double}

c = a{1}

c = 
   (1,1)        1
   (2,2)        1
   (3,3)        1

Answer 1

[Chunru]

The sparse array in cell is still sparse array as shown below, Can you show the problem you have encountered?

a = sparse(eye(1000));          % create a sparse array
b{1} = a; b{2}=a;               % put it into cell array
c = b{1}                        % retrieve from the cell array

c = 
   (1,1)        1
   (2,2)        1
   (3,3)        1
   (4,4)        1
   (5,5)        1
   (6,6)        1
   (7,7)        1
   (8,8)        1
   (9,9)        1
  (10,10)       1
  (11,11)       1
  (12,12)       1
  (13,13)       1
  (14,14)       1
  (15,15)       1
  (16,16)       1
  (17,17)       1
  (18,18)       1
  (19,19)       1
  (20,20)       1
  (21,21)       1
  (22,22)       1
  (23,23)       1
  (24,24)       1
  (25,25)       1
  (26,26)       1
  (27,27)       1
  (28,28)       1
  (29,29)       1
  (30,30)       1
  (31,31)       1
  (32,32)       1
  (33,33)       1
  (34,34)       1
  (35,35)       1
  (36,36)       1
  (37,37)       1
  (38,38)       1
  (39,39)       1
  (40,40)       1
  (41,41)       1
  (42,42)       1
  (43,43)       1
  (44,44)       1
  (45,45)       1
  (46,46)       1
  (47,47)       1
  (48,48)       1
  (49,49)       1
  (50,50)       1
  (51,51)       1
  (52,52)       1
  (53,53)       1
  (54,54)       1
  (55,55)       1
  (56,56)       1
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  (66,66)       1
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  (70,70)       1
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 (823,823)      1
 (824,824)      1
 (825,825)      1
 (826,826)      1
 (827,827)      1
 (828,828)      1
 (829,829)      1
 (830,830)      1
 (831,831)      1
 (832,832)      1
 (833,833)      1
 (834,834)      1
 (835,835)      1
 (836,836)      1
 (837,837)      1
 (838,838)      1
 (839,839)      1
 (840,840)      1
 (841,841)      1
 (842,842)      1
 (843,843)      1
 (844,844)      1
 (845,845)      1
 (846,846)      1
 (847,847)      1
 (848,848)      1
 (849,849)      1
 (850,850)      1
 (851,851)      1
 (852,852)      1
 (853,853)      1
 (854,854)      1
 (855,855)      1
 (856,856)      1
 (857,857)      1
 (858,858)      1
 (859,859)      1
 (860,860)      1
 (861,861)      1
 (862,862)      1
 (863,863)      1
 (864,864)      1
 (865,865)      1
 (866,866)      1
 (867,867)      1
 (868,868)      1
 (869,869)      1
 (870,870)      1
 (871,871)      1
 (872,872)      1
 (873,873)      1
 (874,874)      1
 (875,875)      1
 (876,876)      1
 (877,877)      1
 (878,878)      1
 (879,879)      1
 (880,880)      1
 (881,881)      1
 (882,882)      1
 (883,883)      1
 (884,884)      1
 (885,885)      1
 (886,886)      1
 (887,887)      1
 (888,888)      1
 (889,889)      1
 (890,890)      1
 (891,891)      1
 (892,892)      1
 (893,893)      1
 (894,894)      1
 (895,895)      1
 (896,896)      1
 (897,897)      1
 (898,898)      1
 (899,899)      1
 (900,900)      1
 (901,901)      1
 (902,902)      1
 (903,903)      1
 (904,904)      1
 (905,905)      1
 (906,906)      1
 (907,907)      1
 (908,908)      1
 (909,909)      1
 (910,910)      1
 (911,911)      1
 (912,912)      1
 (913,913)      1
 (914,914)      1
 (915,915)      1
 (916,916)      1
 (917,917)      1
 (918,918)      1
 (919,919)      1
 (920,920)      1
 (921,921)      1
 (922,922)      1
 (923,923)      1
 (924,924)      1
 (925,925)      1
 (926,926)      1
 (927,927)      1
 (928,928)      1
 (929,929)      1
 (930,930)      1
 (931,931)      1
 (932,932)      1
 (933,933)      1
 (934,934)      1
 (935,935)      1
 (936,936)      1
 (937,937)      1
 (938,938)      1
 (939,939)      1
 (940,940)      1
 (941,941)      1
 (942,942)      1
 (943,943)      1
 (944,944)      1
 (945,945)      1
 (946,946)      1
 (947,947)      1
 (948,948)      1
 (949,949)      1
 (950,950)      1
 (951,951)      1
 (952,952)      1
 (953,953)      1
 (954,954)      1
 (955,955)      1
 (956,956)      1
 (957,957)      1
 (958,958)      1
 (959,959)      1
 (960,960)      1
 (961,961)      1
 (962,962)      1
 (963,963)      1
 (964,964)      1
 (965,965)      1
 (966,966)      1
 (967,967)      1
 (968,968)      1
 (969,969)      1
 (970,970)      1
 (971,971)      1
 (972,972)      1
 (973,973)      1
 (974,974)      1
 (975,975)      1
 (976,976)      1
 (977,977)      1
 (978,978)      1
 (979,979)      1
 (980,980)      1
 (981,981)      1
 (982,982)      1
 (983,983)      1
 (984,984)      1
 (985,985)      1
 (986,986)      1
 (987,987)      1
 (988,988)      1
 (989,989)      1
 (990,990)      1
 (991,991)      1
 (992,992)      1
 (993,993)      1
 (994,994)      1
 (995,995)      1
 (996,996)      1
 (997,997)      1
 (998,998)      1
 (999,999)      1
(1000,1000)     1

whos

  Name           Size              Bytes  Class     Attributes

  a           1000x1000            24008  double    sparse    
  b              1x2               48224  cell                
  c           1000x1000            24008  double    sparse    
  cmdout         1x33                 66  char