python - 如何找到数据帧列中的最大值？

Question

我有一个如下所示的数据框:

Date                           Location          NO2
2017-11-24 23:00:00             toronto          0.038
2017-11-24 22:00:00             toronto          0.031
2017-11-24 21:00:00             toronto          0.025
2017-11-24 20:00:00             toronto          0.033
2017-11-24 19:00:00             toronto          0.026
2017-11-24 18:00:00             toronto          0.021
2017-11-24 17:00:00             toronto          0.017

这是一周 24 小时内每天的记录。如何找到这段时间内 NO2 的最高值？

Answer 1

您可以使用np.where():

导入数据:

import sys
if sys.version_info[0] < 3: 
    from StringIO import StringIO
else:
    from io import StringIO

data = StringIO('''Date,Location,NO2
2017-11-24 23:00:00,toronto,0.038
2017-11-24 22:00:00,toronto,0.031
2017-11-24 21:00:00,toronto,0.025
2017-11-24 20:00:00,toronto,0.033
2017-11-24 19:00:00,toronto,0.026
2017-11-24 18:00:00,toronto,0.021
2017-11-24 17:00:00,toronto,0.017''')

df = pd.read_csv(data, sep=',')

使用np.where()查找与最大NO2值匹配的行的索引:

max_time = df.loc[np.where(df.NO2.values == df.NO2.max())[0], 'Date'].values[0]
max_time = df.loc[np.where(df.NO2.values == df.NO2.max())[0], 'Date'].values[0]
print('Max time:',max_time)
print('Max NO2:',df.NO2.max())

Max time: 2017-11-24 23:00:00
Max NO2: 0.038