How To Count The Number Of Following Rows In Pandas

I have dataframe below A B 1 a0 2 b0 3 b1 4 a1 5 b2 6 a2 First,I would like to cut df.with startswith('a') df1 A B 1 a0 2 b0 3 b1 df2 A B 4 a1 5 b2 df3 A B 6 a2 And I would lik

Solution 1:

You can convert cells not starting with a to missing values and forward fill the series and then do value_counts:

df.B.where(df.B.str.startswith("a"), None).ffill().value_counts()
​
#a0    3
#a1    2
#a2    1
#Name: B, dtype: int64

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If you have duplicated as appear, to differentiate them, you can create an additional group variable with cumsum:

start_a = df.B.str.startswith("a")
df.groupby(by = [df.B.where(start_a, None).ffill(), start_a.cumsum().rename('g')]).size()

#B   g        # here is an extra group variable to differentiate possible duplicated a rows
#a0  1    3
#a1  2    2
#a2  3    1
#dtype: int64