Given two numpy arrays of equal shape, I want to track how elements from the first array have moved in the second array. Specifically, for each element in the second array, I want to find its original position in the first array. Example 1: All elements present a1 = np.array([1, 2, 3, 4]) # original array a2 = np.array([2, 1, 3, 4]) # new array # Result: [1, 0, 2, 3] # Explanation: # - 2 was originally at index 1 # - 1 was originally at index 0 # - 3 was originally at index 2 # - 4 was originally at index 3 Example 2: With new elements a1 = np.array([1, 2, 3, 4]) a2 = np.array([2, 1, 33, 4]) # Result: [1, 0, -1, 3] # The value 33 wasn't in the original array, so it gets -1 My solution is: [a1.tolist().index(v) if v in a1 else -1 for v in a2] or np.where(a2[:, None] == a1)[1] but this will not work in example 2 Is there a better way to do this? In real life my arrays have million of rows. Columns not that many, less than 10. Edit: Arrays are not sorted Continue reading...
