在dask数据框中创建if-else条件列 [英] Create an if-else condition column in dask dataframe

问题描述

我需要创建一个基于dask数据框上某些条件的列.在熊猫中，这非常简单:

I need to create a column which is based on some condition on dask dataframe. In pandas it is fairly straightforward:

ddf['TEST_VAR'] = ['THIS' if x == 200607 else  
              'NOT THIS' if x == 200608 else 
              'THAT' if x == 200609 else 'NONE'  
              for x in ddf['shop_week'] ]

在黄昏时，我必须做如下相同的事情:

While in dask I have to do same thing like below:

def f(x):
    if x == 200607:
         y= 'THIS'
    elif x == 200608 :
         y= 'THAT'
    else :
         y= 1 
    return y

ddf1 = ddf.assign(col1 = list(ddf.shop_week.apply(f).compute()))
ddf1.compute()

问题:

1. 是否有更好/更直接的方法来实现它?
2. 我无法修改第一个数据帧ddf，我需要创建ddf1来确保更改是dask数据帧不可变对象?

推荐答案

答案:

1. 您现在正在做什么几乎可以.在您准备好最终答案之前，无需调用 compute .

# ddf1 = ddf.assign(col1 = list(ddf.shop_week.apply(f).compute()))
ddf1 = ddf.assign(col1 = ddf.shop_week.apply(f))

在某些情况下， dd.Series.where 可能很合适

For some cases dd.Series.where might be a good fit

ddf1 = ddf.assign(col1 = ddf.shop_week.where(cond=ddf.balance > 0, other=0))

从0.10.2版本开始，您现在可以将列直接插入dask.dataframes

As of version 0.10.2 you can now insert columns directly into dask.dataframes

ddf['col'] = ddf.shop_week.apply(f)

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