python save numpy array只包含非零元素

Question

我有一個數組，大多數元素都是零。

A =  [ 1,0,2
       2,3,0
       0,0,4 ]

我想把它保存為

rowid[0] colid[0] 1
rowid[0] colid[2] 2
rowid[1] colid[0] 2
rowid[1] colid[1] 3
rowid[2] colid[2] 4

這里的rowid和colid是將數組索引映射到原始文件中的實際條目的數組。

如何在不使用for循環的情況下執行此操作？

Answer 1

A = np.array(A).reshape(3, 3) # make A a 3x3 numpy array 
i, j = np.where(A != 0) # find indices where it is nonzero 
v = A[i, j] # extract nonzero values of the array 
np.savetxt('file.csv', np.vstack((i, j, v)).T, delimiter = ',') # stack and save 

# @Daniel F suggestion is to make header with array shape and add delimiter kwarg
np.savetxt('file.csv', np.vstack((i, j, v)).T, delimiter = ',', header = str(A.shape))