How to get/set elements in a numpy matrix by multiple indices without for-loop?












2















For example, suppose we have



a = zeros((5,5))


Then I want to set elements at positions (1,2), (3,4), (0,3) as 1,2,3 respectively at the same time, but the following would not work,



# I expect this to be the same as a[(1,2)] = 1, a[(3,4)] = 2, a[(0,3)] = 3

a[[(1,2),(3,4),(0,3)]] = [1,2,3]


It will complain "too many indices for array". I do not want to involve a for-loop for efficiency concern (the real problem is a large matrix and I need to set or get elements in that matrix with a long list of randomly generated indexes).






python numpy







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  • 1





    You want, in way or other, to index with a[[1,3,0], [2,4,3)]], in other words, a list (or array) of the i indices, and another of the j indices. This is, effectively, a transpose of your list of size 2 tuples.

    – hpaulj
    Jan 1 at 0:08


















2















For example, suppose we have



a = zeros((5,5))


Then I want to set elements at positions (1,2), (3,4), (0,3) as 1,2,3 respectively at the same time, but the following would not work,



# I expect this to be the same as a[(1,2)] = 1, a[(3,4)] = 2, a[(0,3)] = 3

a[[(1,2),(3,4),(0,3)]] = [1,2,3]


It will complain "too many indices for array". I do not want to involve a for-loop for efficiency concern (the real problem is a large matrix and I need to set or get elements in that matrix with a long list of randomly generated indexes).






python numpy







share|improve this question




















  • 1





    You want, in way or other, to index with a[[1,3,0], [2,4,3)]], in other words, a list (or array) of the i indices, and another of the j indices. This is, effectively, a transpose of your list of size 2 tuples.

    – hpaulj
    Jan 1 at 0:08
















2












2








2








For example, suppose we have



a = zeros((5,5))


Then I want to set elements at positions (1,2), (3,4), (0,3) as 1,2,3 respectively at the same time, but the following would not work,



# I expect this to be the same as a[(1,2)] = 1, a[(3,4)] = 2, a[(0,3)] = 3

a[[(1,2),(3,4),(0,3)]] = [1,2,3]


It will complain "too many indices for array". I do not want to involve a for-loop for efficiency concern (the real problem is a large matrix and I need to set or get elements in that matrix with a long list of randomly generated indexes).






python numpy







share|improve this question
















For example, suppose we have



a = zeros((5,5))


Then I want to set elements at positions (1,2), (3,4), (0,3) as 1,2,3 respectively at the same time, but the following would not work,



# I expect this to be the same as a[(1,2)] = 1, a[(3,4)] = 2, a[(0,3)] = 3

a[[(1,2),(3,4),(0,3)]] = [1,2,3]


It will complain "too many indices for array". I do not want to involve a for-loop for efficiency concern (the real problem is a large matrix and I need to set or get elements in that matrix with a long list of randomly generated indexes).






python numpy




python numpy






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Dec 31 '18 at 23:33







Tony

















asked Dec 31 '18 at 23:27









TonyTony

292311




292311








  • 1





    You want, in way or other, to index with a[[1,3,0], [2,4,3)]], in other words, a list (or array) of the i indices, and another of the j indices. This is, effectively, a transpose of your list of size 2 tuples.

    – hpaulj
    Jan 1 at 0:08
















  • 1





    You want, in way or other, to index with a[[1,3,0], [2,4,3)]], in other words, a list (or array) of the i indices, and another of the j indices. This is, effectively, a transpose of your list of size 2 tuples.

    – hpaulj
    Jan 1 at 0:08










1




1





You want, in way or other, to index with a[[1,3,0], [2,4,3)]], in other words, a list (or array) of the i indices, and another of the j indices. This is, effectively, a transpose of your list of size 2 tuples.

– hpaulj
Jan 1 at 0:08







You want, in way or other, to index with a[[1,3,0], [2,4,3)]], in other words, a list (or array) of the i indices, and another of the j indices. This is, effectively, a transpose of your list of size 2 tuples.

– hpaulj
Jan 1 at 0:08














1 Answer
1






active

oldest

votes


















6














You can do the following:



import numpy as np



rows, cols = zip(*[(1, 2), (3, 4), (0, 3)])



a = np.zeros((5,5))

a[rows, cols] = [1, 2, 3]



print(a)


Output



[[0. 0. 0. 3. 0.]

 [0. 0. 1. 0. 0.]

 [0. 0. 0. 0. 0.]

 [0. 0. 0. 0. 2.]

 [0. 0. 0. 0. 0.]]





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    1 Answer
    1






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    6














    You can do the following:



    import numpy as np
    

    
rows, cols = zip(*[(1, 2), (3, 4), (0, 3)])
    

    
a = np.zeros((5,5))
    
a[rows, cols] = [1, 2, 3]
    

    
print(a)


    Output



    [[0. 0. 0. 3. 0.]
    
 [0. 0. 1. 0. 0.]
    
 [0. 0. 0. 0. 0.]
    
 [0. 0. 0. 0. 2.]
    
 [0. 0. 0. 0. 0.]]





    share|improve this answer




























      6














      You can do the following:



      import numpy as np
      

      
rows, cols = zip(*[(1, 2), (3, 4), (0, 3)])
      

      
a = np.zeros((5,5))
      
a[rows, cols] = [1, 2, 3]
      

      
print(a)


      Output



      [[0. 0. 0. 3. 0.]
      
 [0. 0. 1. 0. 0.]
      
 [0. 0. 0. 0. 0.]
      
 [0. 0. 0. 0. 2.]
      
 [0. 0. 0. 0. 0.]]





      share|improve this answer


























        6












        6








        6







        You can do the following:



        import numpy as np
        

        
rows, cols = zip(*[(1, 2), (3, 4), (0, 3)])
        

        
a = np.zeros((5,5))
        
a[rows, cols] = [1, 2, 3]
        

        
print(a)


        Output



        [[0. 0. 0. 3. 0.]
        
 [0. 0. 1. 0. 0.]
        
 [0. 0. 0. 0. 0.]
        
 [0. 0. 0. 0. 2.]
        
 [0. 0. 0. 0. 0.]]





        share|improve this answer













        You can do the following:



        import numpy as np
        

        
rows, cols = zip(*[(1, 2), (3, 4), (0, 3)])
        

        
a = np.zeros((5,5))
        
a[rows, cols] = [1, 2, 3]
        

        
print(a)


        Output



        [[0. 0. 0. 3. 0.]
        
 [0. 0. 1. 0. 0.]
        
 [0. 0. 0. 0. 0.]
        
 [0. 0. 0. 0. 2.]
        
 [0. 0. 0. 0. 0.]]






        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Dec 31 '18 at 23:36









        Daniel MesejoDaniel Mesejo

        18.3k21431




        18.3k21431
































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