如何使用dataFrame查找一組數據的平均值？

Question

import numpy as np 
import panda as pd 

dates = pd.date_range('20161104', periods = 10) 
df = pd.DataFrame(np.random.randn(10, 4), index = dates, columns = list('ABCD')) 

我想找到C的平均值，當C大於0時，這意味着如果相應的C小於0，我將不會將該A計入我的計算。

有誰知道如何使它沒有創建一個新的數據集或使用groupby？謝謝！

Answer 1

我不知道到底需要：

np.random.seed(1) 
dates = pd.date_range('20161104', periods = 10) 
df = pd.DataFrame(np.random.randn(10, 4), index = dates, columns = list('ABCD')) 
print (df) 
        A   B   C   D 
2016-11-04 1.624345 -0.611756 -0.528172 -1.072969 
2016-11-05 0.865408 -2.301539 1.744812 -0.761207 
2016-11-06 0.319039 -0.249370 1.462108 -2.060141 
2016-11-07 -0.322417 -0.384054 1.133769 -1.099891 
2016-11-08 -0.172428 -0.877858 0.042214 0.582815 
2016-11-09 -1.100619 1.144724 0.901591 0.502494 
2016-11-10 0.900856 -0.683728 -0.122890 -0.935769 
2016-11-11 -0.267888 0.530355 -0.691661 -0.396754 
2016-11-12 -0.687173 -0.845206 -0.671246 -0.012665 
2016-11-13 -1.117310 0.234416 1.659802 0.742044 

#mean of A where C is larger than 0 
print (df.ix[df.C > 0, 'A'].mean()) 
-0.2547213686717275 

#mean of A where C is less than 0 
print (df.ix[df.C < 0, 'A'].mean()) 
0.3925351332955095 

#mean of A where C is larger than 0 and C is less than 0, co condition never return True 
print (df.ix[(df.C > 0) & (df.C < 0), 'A'].mean()) 
nan 

#mean of A where A is larger than 0 and C is less than 0 
print (df.ix[(df.A > 0) & (df.C < 0), 'A'].mean()) 
1.2626006564638268 

Answer 2

你可以試試這個：

df[df.C> 0]['A'].mean()