获取第一个和最后一个元素pd.石斑鱼()

Question

我有一个时间序列，我正在重新采样到5s窗口，例如：

INDEX                   size           price
2018-05-07 21:53:13.731 0.365127    9391.800000
2018-05-07 21:53:16.201 0.666127    9391.800000
2018-05-07 21:53:18.038 0.143104    9391.800000
2018-05-07 21:53:18.243 0.025643    9391.800000
2018-05-07 21:53:18.265 0.640484    9391.800000
2018-05-07 21:53:18.906 -0.100000   9391.793421
2018-05-07 21:53:19.829 0.559516    9391.800000
2018-05-07 21:53:19.846 0.100000    9391.800000
2018-05-07 21:53:19.870 0.006560    9391.800000
2018-05-07 21:53:20.734 0.666076    9391.800000
2018-05-07 21:53:20.775 0.666076    9391.800000
2018-05-07 21:53:28.607 0.100000    9391.800000
2018-05-07 21:53:28.610 0.041991    9391.800000
2018-05-07 21:53:29.283 -0.053518   9391.793421
2018-05-07 21:53:47.322 -0.046302   9391.793421
2018-05-07 21:53:49.182 0.100000    9391.800000

def tick_features(x):
    volume = np.abs(x['size']).sum()
    num_trades = x['size'].count()
    return pd.Series([volume,num_trades], index=['volume','num_trades'])


tick = tick.groupby(pd.Grouper(freq='5S')).apply(tick_features)

如何通过pd.Grouper()和.apply()获取每个5S的第一个和最后一个元素？你知道吗

我可以用.resample().agg()和{'price':'first'}做类似的事情，但是出于其他原因，如果可能的话，我想通过pd.Grouper()来做。你知道吗

Answer 1

我建议将^{}与元组和函数列表first和last一起使用：

tick_features = [('volume', lambda x: x.abs().sum()),
                 ('num_trades', 'count'),
                 ('first_trade', 'first'),
                 ('last_trade', 'last')]
tick = tick.groupby(pd.Grouper(freq='5S'))['size'].agg(tick_features)
print (tick)
                       volume  num_trades   first_trade   last_trade
INDEX                                                               
2018-05-07 21:53:10  0.365127           1      0.365127     0.365127
2018-05-07 21:53:15  2.241434           8      0.666127     0.006560
2018-05-07 21:53:20  1.332152           2      0.666076     0.666076
2018-05-07 21:53:25  0.195509           3      0.100000    -0.053518
2018-05-07 21:53:30  0.000000           0           NaN          NaN
2018-05-07 21:53:35  0.000000           0           NaN          NaN
2018-05-07 21:53:40  0.000000           0           NaN          NaN
2018-05-07 21:53:45  0.146302           2     -0.046302     0.100000

apply解决方案是可能的，但需要if-else声明：

def tick_features(x):
    volume = np.abs(x['size']).sum()
    num_trades = x['size'].count()
    if not x.empty:
        f = x['size'].iloc[0]
        l = x['size'].iloc[-1]
    else:
        f = np.nan
        l = np.nan
    return pd.Series([volume,num_trades, f, l], 
                      index=['volume','num_trades', 'first_trade', 'last_trade'])


tick = tick.groupby(pd.Grouper(freq='5S')).apply(tick_features)
print (tick)
                       volume  num_trades  first_trade  last_trade
INDEX                                                             
2018-05-07 21:53:10  0.365127         1.0     0.365127    0.365127
2018-05-07 21:53:15  2.241434         8.0     0.666127    0.006560
2018-05-07 21:53:20  1.332152         2.0     0.666076    0.666076
2018-05-07 21:53:25  0.195509         3.0     0.100000   -0.053518
2018-05-07 21:53:30  0.000000         0.0          NaN         NaN
2018-05-07 21:53:35  0.000000         0.0          NaN         NaN
2018-05-07 21:53:40  0.000000         0.0          NaN         NaN
2018-05-07 21:53:45  0.146302         2.0    -0.046302    0.100000