I am puzzled why this is not working:
a = np.array([[1,2,3], [1,20, 30], [4, 5, 6]]) a[a[:, 0] == 1][:, 2] = [9999, 9999] I expect:
a = [[1, 2, 9999], [1, 20, 9999], [4, 5, 6]] But nothing changes. What is wrong?
1 Answer
As explained in the comments, you're currently updating an in-memory copy, not your original array.
You can use direct indexing:
a[a[:,0]==1, 2] = 9999 or with numpy.where (not necessary here):
a[np.where(a[:,0]==1)[0], 2] = 9999 updated a:
array([[ 1, 2, 9999], [ 1, 20, 9999], [ 4, 5, 6]]) 
u = a[a[:, 0] == 1]copies the data becausea[:, 0] == 1is an advanced index. Next,u[:, 2]indexes into that copy, so assignment affects the copy, not the original array.