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There is an numpy calculation process as below, which has been approved correctly, but I don't understand the process clearly and would like to ask for help. The steps as following:

  1. There is user_item_train matrix as below which created through unstack: enter image description here
  2. There is a test data index list as test_idx as below: enter image description here
  3. Below code: look for the user_id which are in test_idx from train dataset, and return boolen values enter image description here
  4. There is u_train dataset as below which generated through np.linalg.svd(user_item_train,full_matrices=False): enter image description here
  5. I don't understand this step: enter image description here

May I ask for your help to explain it a bit?
Thanks.

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u_train is a 2-dimensional array. In NumPy, you can use lists as slices. It's easiest to imagine with an example: Assume I have a 4x4 array named arr. Then,

  • arr[3, 1] gives me the value in row 3, column 1 (remember that we count from 0).
  • arr[[1, 3], [0, 2]] gives me a 2x1 array with the following original indices: that is, row 1 and 3, column 0 and 2.
+--------+--------+
| (1, 0) | (1, 2) |
+--------+--------+
| (3, 0) | (3, 2) |
+--------+--------+
  • arr[[0, 3], :] gives me the row 0 and 3, all columns, since : implies everything in that axis:
+--------+--------+--------+--------+
| (0, 0) | (0, 1) | (0, 2) | (0, 3) |
+--------+--------+--------+--------+
| (3, 0) | (3, 1) | (3, 2) | (3, 3) |
+--------+--------+--------+--------+
  • arr[3] is identical to arr[3, :], this is just syntactic sugar.
  • For using a boolean list as indices, a True in the list means include the row of the same index as it, and vice versa. For example, arr[:, [True, True, False, False]] is equivalent to arr[:, [0, 1]]. Note that, perhaps obviously, the length of the boolean list must be the same as the dimension, so arr[:, [True, False, True]] will raise an IndexError.
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5 Comments

Thank you, and how to understand the index is a boolean array?
@Annie it's an int array. Indices are integers, just like with lists and tuples.
Hello kwkt, if I am right, what you mean is that it automatically recognizer False is 0 and True is 1 at step5? As y can see from step 3, test_idxs is a boolean array.
@Annie Ah, sorry I missed that. With a boolean list, it's acting as a gate: if an index i of the list is True, the row of that index is included in the result, and vice versa. Note that the boolean list must have the same length as the corresponding dimension (understandably). I'm updating my answer.
That is much clearly. Thank you very much.

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