Anomalous indices loop

[Anastasis-Iliopoulos]

Hello, I have a question about this snippet of code (the loop helps to find the anomalies):

# data i is an anomaly if samples [(i - timesteps + 1) to (i)] are anomalies

anomalous_data = cnn_residuals > (3/2 * UCL)
anomalous_data_indices = []
    for data_idx in range(N_STEPS - 1, len(X) - N_STEPS + 1):
        if np.all(anomalous_data[data_idx - N_STEPS + 1 : data_idx]):
            anomalous_data_indices.append(data_idx)

comment says that given a point i, if all samples between i-timesteps+1 and i are anomalies then the point i is anomaly.
This loop starts from N_STEPS - 1 which is (lets say N_STEPS=10 ) from 9 and ends to len(X) - N_STEPS + 1 which is (lets say len(X)=100 ) the number 91.

Then the if statement going to check (for each iteration) the samples from data_idx - N_STEPS + 1 to data_idx
So
iteration 1: all samples from 9-10+1 which is 0 to 9 (not inclusive)
iteration 2: all samples from 10-10+1 which is 1 to 10 (not inclusive)
....[going at the end].....
iteration 82: all samples from 10-10+1 which is 81 to 90 (not inclusive)

In other words:
We check i=9 with samples 0 to 8 (inclusive)
We check i=10 with samples 1 to 9 (inclusive)
....[going at the end].....
We check i=90 with samples 81 to 89 (inclusive)

Question
I think we are missing i=92, 93, 94,....,99. Am I wrong? And why?
If am not wrong shouldn't the for data_idx in range(N_STEPS - 1, len(X) - N_STEPS + 1) be replaced with for data_idx in range(N_STEPS - 1, len(X)): in order to iterate until the end?

Thank you in advance

[Anastasis-Iliopoulos]

Ok. I figured it out.
There is no error.

Basically what we have is:
Time_window 0: [0 to 9]
Time_window 1: [1 to 10]
Time_window 2: [2 to 11]
....
Time_window 90: [90 to 99]

Each window has a score
Given an i if scores of all 10 windows before [i to i+10-1] are above the threshold then i is anomaly.

10 Windows before [i to i+10-1] are all windows that include i. So this makes sense. Which means that every window that includes i exceeds the threshold means that i is a "problem".

Further more:

1. we need 10 windows to decide. Because the number 2 belongs to window [0 to 9], window [1 to 10] and window [2 to 11] we do not have enough windows to decide if 2 is anomaly. The same applies to all number from 0 to window_size.
2. on the other end.... number 98 belongs to windows [89 to 98] and [90 to 99]. So we do not have enough windows to decide if 98 is anomaly. The same applies to all number from len(X)-window_size+1 to len(X)-1.

Thats why we care just for the middle numbers/windows.

[YKatser]

@Anastasis-Iliopoulos Thank you for your interest! We will try to respond sooner if any questions arise in the future.