delete zeros from a SparseArray












1














Why doesn't SparseArray automatically remove zero entries? The code



a = SparseArray[{2->22,3->33,5->55,7->77,11->1111,13->1313},13];

b = SparseArray[{1->11,3->33,5->55,7->77,9->99,11->1111},13];

r = DeleteCases[a-b,0]

r //ArrayRules


returns {{1}->-11,{3}->0,{5}->0,{7}->0,{9}->-99,{11}->0,{2}->22,{13}->1313,{_}->0} instead of {{1}->-11,{9}->-99,{2}->22,{13}->1313,{_}->0}.



Is there a nice way to get rid of zero entries? For instance, if on $A!in!mathbb{Z}^{mtimes n}_2$ I perform Gaussian elimination, I use A=Mod[A,2];, suddenly many entries are zero and I don't want to waste memory on them.
Of course I could use A=SparseArray[DeleteCases[ArrayRules[A],#[[2]]==0&],{m,n}], but it seems clumsy to use this often...



Also, if I have a very long sparse row that gets updated frequently and fills up somewhat (Gaussian elimination), is it better to use SparseArray with + or Association with Merge?






matrix associations sparse-arrays rule







share|improve this question


















  • 1




    Possible duplicate of How to prevent a SparseArray entry from being specified when it coincides with the default value?
    – MeMyselfI
    3 hours ago










  • It does not do it automatically because rebuilding a sparse array is expensive. It is left to you to decide when you want to rebuild it and eliminate unnecessary entries. Just apply SparseArray again.
    – Szabolcs
    1 hour ago
















1














Why doesn't SparseArray automatically remove zero entries? The code



a = SparseArray[{2->22,3->33,5->55,7->77,11->1111,13->1313},13];

b = SparseArray[{1->11,3->33,5->55,7->77,9->99,11->1111},13];

r = DeleteCases[a-b,0]

r //ArrayRules


returns {{1}->-11,{3}->0,{5}->0,{7}->0,{9}->-99,{11}->0,{2}->22,{13}->1313,{_}->0} instead of {{1}->-11,{9}->-99,{2}->22,{13}->1313,{_}->0}.



Is there a nice way to get rid of zero entries? For instance, if on $A!in!mathbb{Z}^{mtimes n}_2$ I perform Gaussian elimination, I use A=Mod[A,2];, suddenly many entries are zero and I don't want to waste memory on them.
Of course I could use A=SparseArray[DeleteCases[ArrayRules[A],#[[2]]==0&],{m,n}], but it seems clumsy to use this often...



Also, if I have a very long sparse row that gets updated frequently and fills up somewhat (Gaussian elimination), is it better to use SparseArray with + or Association with Merge?






matrix associations sparse-arrays rule







share|improve this question


















  • 1




    Possible duplicate of How to prevent a SparseArray entry from being specified when it coincides with the default value?
    – MeMyselfI
    3 hours ago










  • It does not do it automatically because rebuilding a sparse array is expensive. It is left to you to decide when you want to rebuild it and eliminate unnecessary entries. Just apply SparseArray again.
    – Szabolcs
    1 hour ago














1












1








1







Why doesn't SparseArray automatically remove zero entries? The code



a = SparseArray[{2->22,3->33,5->55,7->77,11->1111,13->1313},13];

b = SparseArray[{1->11,3->33,5->55,7->77,9->99,11->1111},13];

r = DeleteCases[a-b,0]

r //ArrayRules


returns {{1}->-11,{3}->0,{5}->0,{7}->0,{9}->-99,{11}->0,{2}->22,{13}->1313,{_}->0} instead of {{1}->-11,{9}->-99,{2}->22,{13}->1313,{_}->0}.



Is there a nice way to get rid of zero entries? For instance, if on $A!in!mathbb{Z}^{mtimes n}_2$ I perform Gaussian elimination, I use A=Mod[A,2];, suddenly many entries are zero and I don't want to waste memory on them.
Of course I could use A=SparseArray[DeleteCases[ArrayRules[A],#[[2]]==0&],{m,n}], but it seems clumsy to use this often...



Also, if I have a very long sparse row that gets updated frequently and fills up somewhat (Gaussian elimination), is it better to use SparseArray with + or Association with Merge?






matrix associations sparse-arrays rule







share|improve this question













Why doesn't SparseArray automatically remove zero entries? The code



a = SparseArray[{2->22,3->33,5->55,7->77,11->1111,13->1313},13];

b = SparseArray[{1->11,3->33,5->55,7->77,9->99,11->1111},13];

r = DeleteCases[a-b,0]

r //ArrayRules


returns {{1}->-11,{3}->0,{5}->0,{7}->0,{9}->-99,{11}->0,{2}->22,{13}->1313,{_}->0} instead of {{1}->-11,{9}->-99,{2}->22,{13}->1313,{_}->0}.



Is there a nice way to get rid of zero entries? For instance, if on $A!in!mathbb{Z}^{mtimes n}_2$ I perform Gaussian elimination, I use A=Mod[A,2];, suddenly many entries are zero and I don't want to waste memory on them.
Of course I could use A=SparseArray[DeleteCases[ArrayRules[A],#[[2]]==0&],{m,n}], but it seems clumsy to use this often...



Also, if I have a very long sparse row that gets updated frequently and fills up somewhat (Gaussian elimination), is it better to use SparseArray with + or Association with Merge?






matrix associations sparse-arrays rule




matrix associations sparse-arrays rule






share|improve this question













share|improve this question











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asked 7 hours ago









Leon

347111




347111








  • 1




    Possible duplicate of How to prevent a SparseArray entry from being specified when it coincides with the default value?
    – MeMyselfI
    3 hours ago










  • It does not do it automatically because rebuilding a sparse array is expensive. It is left to you to decide when you want to rebuild it and eliminate unnecessary entries. Just apply SparseArray again.
    – Szabolcs
    1 hour ago














  • 1




    Possible duplicate of How to prevent a SparseArray entry from being specified when it coincides with the default value?
    – MeMyselfI
    3 hours ago










  • It does not do it automatically because rebuilding a sparse array is expensive. It is left to you to decide when you want to rebuild it and eliminate unnecessary entries. Just apply SparseArray again.
    – Szabolcs
    1 hour ago








1




1




Possible duplicate of How to prevent a SparseArray entry from being specified when it coincides with the default value?
– MeMyselfI
3 hours ago




Possible duplicate of How to prevent a SparseArray entry from being specified when it coincides with the default value?
– MeMyselfI
3 hours ago












It does not do it automatically because rebuilding a sparse array is expensive. It is left to you to decide when you want to rebuild it and eliminate unnecessary entries. Just apply SparseArray again.
– Szabolcs
1 hour ago




It does not do it automatically because rebuilding a sparse array is expensive. It is left to you to decide when you want to rebuild it and eliminate unnecessary entries. Just apply SparseArray again.
– Szabolcs
1 hour ago










1 Answer
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You can just apply SparseArray to a SparseArray object to normalize it. For your example:



a = SparseArray[{2->22,3->33,5->55,7->77,11->1111,13->1313},13];

b = SparseArray[{1->11,3->33,5->55,7->77,9->99,11->1111},13];

r = DeleteCases[a-b,0];



SparseArray[r] //ArrayRules



{{1} -> -11, {2} -> 22, {9} -> -99, {13} -> 1313, {_} -> 0}







share|improve this answer





















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    4














    You can just apply SparseArray to a SparseArray object to normalize it. For your example:



    a = SparseArray[{2->22,3->33,5->55,7->77,11->1111,13->1313},13];
    
b = SparseArray[{1->11,3->33,5->55,7->77,9->99,11->1111},13];
    
r = DeleteCases[a-b,0];
    

    
SparseArray[r] //ArrayRules



    {{1} -> -11, {2} -> 22, {9} -> -99, {13} -> 1313, {_} -> 0}







    share|improve this answer


























      4














      You can just apply SparseArray to a SparseArray object to normalize it. For your example:



      a = SparseArray[{2->22,3->33,5->55,7->77,11->1111,13->1313},13];
      
b = SparseArray[{1->11,3->33,5->55,7->77,9->99,11->1111},13];
      
r = DeleteCases[a-b,0];
      

      
SparseArray[r] //ArrayRules



      {{1} -> -11, {2} -> 22, {9} -> -99, {13} -> 1313, {_} -> 0}







      share|improve this answer
























        4












        4








        4






        You can just apply SparseArray to a SparseArray object to normalize it. For your example:



        a = SparseArray[{2->22,3->33,5->55,7->77,11->1111,13->1313},13];
        
b = SparseArray[{1->11,3->33,5->55,7->77,9->99,11->1111},13];
        
r = DeleteCases[a-b,0];
        

        
SparseArray[r] //ArrayRules



        {{1} -> -11, {2} -> 22, {9} -> -99, {13} -> 1313, {_} -> 0}







        share|improve this answer












        You can just apply SparseArray to a SparseArray object to normalize it. For your example:



        a = SparseArray[{2->22,3->33,5->55,7->77,11->1111,13->1313},13];
        
b = SparseArray[{1->11,3->33,5->55,7->77,9->99,11->1111},13];
        
r = DeleteCases[a-b,0];
        

        
SparseArray[r] //ArrayRules



        {{1} -> -11, {2} -> 22, {9} -> -99, {13} -> 1313, {_} -> 0}








        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 7 hours ago









        Carl Woll

        67k387175




        67k387175






























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