问如何计算有序排序元素之间的平均值

EN

df = df.sort_values('time_ref')
s = df.groupby('f_idx').cumcount()

out =( df.set_index(['f_idx',s])['time_ref'].unstack().diff(axis=1).mean(1)
      .reset_index(name='output'))

print(out)

  f_idx  output
0    f1     4.0
1    f2     2.0
2    f3     NaN

df1 = df.sort_values(['f_idx','time_ref'], ascending=[True,False])

df2 = (df1.groupby('f_idx')['time_ref'].agg(lambda x: x.diff(-1).mean())
          .reset_index(name='res_mean'))

print (df2)
  f_idx  res_mean
0    f1       4.0
1    f2       2.0
2    f3       NaN

df = (df
      .sort_values(['f_idx','time_ref'], ascending=[True,False])
      .groupby('f_idx')
      ['time_ref']
      .rolling(2)                                  # Take every 2 items
      .apply(lambda x: x.iloc[0] - x.iloc[1])      # Get the difference
      .groupby('f_idx')
      .mean()                                      # Get the mean
      .rename('res_mean')
      .to_frame())

f_idx   res_mean
f1  4.0
f2  2.0
f3  NaN