CODEWITHSUNDEEP
pythonloopsdate
Mel
Mel

Reputation: 113

Create a Python loop to find start and stop values

I want to create a list with dates when the following happens to the data in Python.

I gave a sample dataset in the table below. However, this happens multiple times and that is where I am struggling. I tried a for-loop to create the data but it only gives back the first time that this occurs, while I am interested in all events.

Sample data set:

Date and Time Process value
2020-06-07 00:00 8.2
2020-06-07 01:00 6.5
2020-06-07 02:00 4.3
2020-06-07 03:00 3.6
2020-06-07 04:00 0
2020-06-07 05:00 0
2020-06-07 06:00 0
2020-06-07 07:00 3.2
2020-06-07 08:00 4.8
2020-06-07 09:00 0

Expected result:

Date and Time Process value
2020-06-07 03:00 3.6
2020-06-07 04:00 0
2020-06-07 08:00 4.8
2020-06-07 09:00 0

Any tips?

Upvotes: 1

Views: 130

Answers (2)

cosmic_inquiry
cosmic_inquiry

Reputation: 2684

Fun way to do this one:

Setup:

import pandas as pd

data = {
  "Date and Time": ["2020-06-07 00:00", "2020-06-07 01:00", "2020-06-07 02:00", "2020-06-07 03:00",
                    "2020-06-07 04:00", "2020-06-07 05:00", "2020-06-07 06:00", "2020-06-07 07:00",
                    "2020-06-07 08:00", "2020-06-07 09:00"],
  "Process value": [8.2, 6.5, 4.3, 3.6, 0, 0, 0, 3.2, 4.8, 0]
}
df = pd.DataFrame(data)

Shift to find previous values:

below_4 = (df["Process value"] < 4) & (df["Process value"].shift(1) > 4)
above_4 = (df["Process value"] > 4) & (df["Process value"].shift(1) < 4)
reach_0 = (df["Process value"] == 0) & (df["Process value"].shift(1) != 0)
df.loc[below_4 | above_4 | reach_0]

Yields:

      Date and Time  Process value
3  2020-06-07 03:00            3.6
4  2020-06-07 04:00            0.0
8  2020-06-07 08:00            4.8
9  2020-06-07 09:00            0.0

Upvotes: 0

C14L
C14L

Reputation: 12558

You need to remember state.

result = []
last_event = "init"

for item in items:
    if last_event == "init" and item["value"] < 4.0:
        result.append(item)
        last_event = "below_4"
    elif last_event == "below_4" and item["value"] == 0:
        result.append(item)
        last_event = "is_0"
    elif last_event == "is_0" and item["value"] > 4.0:
        result.append(item)
        last_event = "above_4"

Normally, you'd define the possible states separately in a dict or dataclass, so that new states are not randomly introduced across a larger program.

Upvotes: 1

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