在 pandas 中有效地使用替换 [英] Using replace efficiently in pandas

问题描述

我希望在 python3 中以有效的方式使用 replace 函数.我拥有的代码正在完成任务，但速度太慢，因为我正在处理一个大型数据集.因此，在权衡时，我的首要任务是效率而不是优雅.这是我想做的一个玩具:

I am looking to use the replace function in an efficient way in python3. The code I have is achieving the task, but is much too slow, as I am working with a large dataset. Thus, my priority is efficiency over elegancy whenever there is a tradeoff. Here is a toy of what I would like to do:

import pandas as pd
df = pd.DataFrame([[1,2],[3,4],[5,6]], columns = ['1st', '2nd'])

       1st  2nd
   0    1    2
   1    3    4
   2    5    6


idxDict= dict()
idxDict[1] = 'a'
idxDict[3] = 'b'
idxDict[5] = 'c'

for k,v in idxDict.items():
    df ['1st'] = df ['1st'].replace(k, v)

哪个给了

     1st  2nd
   0   a    2
   1   b    4
   2   c    6

如我所愿，但需要的时间太长.最快的方法是什么?

as I desire, but it takes way too long. What would be the fastest way?

这是一个比这个更集中和干净的问题，对于解决方案类似.

this is a more focused and clean question than this one, for which the solution is similar.

推荐答案

use map 执行查找:

use map to perform a lookup:

In [46]:
df['1st'] = df['1st'].map(idxDict)
df
Out[46]:
  1st  2nd
0   a    2
1   b    4
2   c    6

为了避免没有有效密钥的情况，您可以传递 na_action='ignore'

to avoid the situation where there is no valid key you can pass na_action='ignore'

您也可以使用 df['1st'].replace(idxDict) 但要回答您关于效率的问题:

You can also use df['1st'].replace(idxDict) but to answer you question about efficiency:

时间

In [69]:
%timeit df['1st'].replace(idxDict)
%timeit df['1st'].map(idxDict)

1000 loops, best of 3: 1.57 ms per loop
1000 loops, best of 3: 1.08 ms per loop

In [70]:    
%%timeit
for k,v in idxDict.items():
    df ['1st'] = df ['1st'].replace(k, v)

100 loops, best of 3: 3.25 ms per loop

所以在这里使用 map 比这里快 3 倍以上

So using map is over 3x faster here

在更大的数据集上:

In [3]:
df = pd.concat([df]*10000, ignore_index=True)
df.shape

Out[3]:
(30000, 2)

In [4]:    
%timeit df['1st'].replace(idxDict)
%timeit df['1st'].map(idxDict)

100 loops, best of 3: 18 ms per loop
100 loops, best of 3: 4.31 ms per loop

In [5]:    
%%timeit
for k,v in idxDict.items():
    df ['1st'] = df ['1st'].replace(k, v)

100 loops, best of 3: 18.2 ms per loop

对于 30K 行的 df，map 快约 4 倍，因此它比 replace 或循环更好地扩展

For 30K row df, map is ~4x faster so it scales better than replace or looping

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