Count consecutive repeated values in pandas

user_11024951 0
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I'm trying to highlight areas in Matplotlib where the data in a pandas data frame is same over a consecutive number of rows, so given the data frame below and a threshold of 3:

In

    days = pd.date_range(dt.datetime.now(), dt.datetime.now() + dt.timedelta(13), freq='D')
    data = [2,3,3,3,2,2,3.4,3.1,2.7,np.nan,4,4,4,4.5]
    df = pd.DataFrame({'cat': data})
    df = df.set_index(days)

out:

                            col
2021-03-12 15:13:24.727074  2.0
2021-03-13 15:13:24.727074  3.0
2021-03-14 15:13:24.727074  3.0
2021-03-15 15:13:24.727074  3.0
2021-03-16 15:13:24.727074  2.0
2021-03-17 15:13:24.727074  2.0
2021-03-18 15:13:24.727074  3.4
2021-03-19 15:13:24.727074  3.1
2021-03-20 15:13:24.727074  2.7
2021-03-21 15:13:24.727074  NaN
2021-03-22 15:13:24.727074  4.0
2021-03-23 15:13:24.727074  4.0
2021-03-24 15:13:24.727074  4.0
2021-03-25 15:13:24.727074  4.5

The ultimate objective would be to return the following dataframe, where 'result' was a test to see if the data in 'col' was not changing. The 2 consecutive values of 2.0 don't flag because they are only 2 consecutive instances vs our threshold of >= 3.

                            col  result
2021-03-12 15:13:24.727074  2.0  False
2021-03-13 15:13:24.727074  3.0  True
2021-03-14 15:13:24.727074  3.0  True
2021-03-15 15:13:24.727074  3.0  True
2021-03-16 15:13:24.727074  2.0  False
2021-03-17 15:13:24.727074  2.0  False
2021-03-18 15:13:24.727074  3.4  False
2021-03-19 15:13:24.727074  3.1  False
2021-03-20 15:13:24.727074  2.7  False
2021-03-21 15:13:24.727074  NaN  False
2021-03-22 15:13:24.727074  4.0  True
2021-03-23 15:13:24.727074  4.0  True
2021-03-24 15:13:24.727074  4.0  True
2021-03-25 15:13:24.727074  4.5  False

I tried using cumsum() below and incrmented by 1 when there is a difference. Using the following code:

df['increment'] = (df['col'].diff(1) != 0).astype('int').cumsum()

This works to get the size of the consecutive blocks using

df.groupby('increment').size() >= threshold

This gets me close but the problem is it breaks my link with my original dataframe datetime index, which means i can't plot the boolean data together with the original df['col'].

pandas matplotlib python
2 Answers

Use cumsum() on the comparison with shift to identify the blocks:

# groupby exact match of values
blocks = df['col'].ne(df['col'].shift()).cumsum()

df['result'] = blocks.groupby(blocks).transform('size') >= 3

Output:

                            col  result
2021-03-12 15:13:24.727074  2.0   False
2021-03-13 15:13:24.727074  3.0    True
2021-03-14 15:13:24.727074  3.0    True
2021-03-15 15:13:24.727074  3.0    True
2021-03-16 15:13:24.727074  2.0   False
2021-03-17 15:13:24.727074  2.0   False
2021-03-18 15:13:24.727074  3.4   False
2021-03-19 15:13:24.727074  3.1   False
2021-03-20 15:13:24.727074  2.7   False
2021-03-21 15:13:24.727074  NaN   False
2021-03-22 15:13:24.727074  4.0    True
2021-03-23 15:13:24.727074  4.0    True
2021-03-24 15:13:24.727074  4.0    True
2021-03-25 15:13:24.727074  4.5   False

Note It's not ideal to use == to compare floats. Instead, we can use threshold, something like:

# groupby consecutive rows if the differences are not significant
blocks = df['col'].diff().abs().gt(1e-6).cumsum()
user_42384080

Boolean select by testing consecutive similarity using shift. Apply cumsum to convert to groups. Use the resulting group to groupby. Apply transform to find size.

  df=df.assign(result=df.groupby((~df.cat.eq(df.cat.shift())).cumsum())['cat'].transform('size').ge(3))

             

                            cat  result
2021-03-13 05:32:30.309303  2.0   False
2021-03-14 05:32:30.309303  3.0    True
2021-03-15 05:32:30.309303  3.0    True
2021-03-16 05:32:30.309303  3.0    True
2021-03-17 05:32:30.309303  2.0   False
2021-03-18 05:32:30.309303  2.0   False
2021-03-19 05:32:30.309303  3.4   False
2021-03-20 05:32:30.309303  3.1   False
2021-03-21 05:32:30.309303  2.7   False
2021-03-22 05:32:30.309303  NaN   False
2021-03-23 05:32:30.309303  4.0    True
2021-03-24 05:32:30.309303  4.0    True
2021-03-25 05:32:30.309303  4.0    True
2021-03-26 05:32:30.309303  4.5   False
user_89869750
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