Pandas中map，applymap和apply方法之间的区别 [英] Difference between map, applymap and apply methods in Pandas

问题描述

您能否通过基本示例告诉我何时使用这些矢量化方法?

Can you tell me when to use these vectorization methods with basic examples?

我看到map是Series方法，而其余的是DataFrame方法.我对apply和applymap方法感到困惑.为什么我们有两种将函数应用于DataFrame的方法?同样，简单的例子可以很好地说明用法！

I see that map is a Series method whereas the rest are DataFrame methods. I got confused about apply and applymap methods though. Why do we have two methods for applying a function to a DataFrame? Again, simple examples which illustrate the usage would be great!

推荐答案

Wes McKinney的 Python for数据分析书，第pg. 132(我强烈推荐这本书):

Straight from Wes McKinney's Python for Data Analysis book, pg. 132 (I highly recommended this book):

另一种常见的操作是将一维数组上的函数应用于每一列或每一行. DataFrame的apply方法可以做到这一点:

Another frequent operation is applying a function on 1D arrays to each column or row. DataFrame’s apply method does exactly this:

In [116]: frame = DataFrame(np.random.randn(4, 3), columns=list('bde'), index=['Utah', 'Ohio', 'Texas', 'Oregon'])

In [117]: frame
Out[117]: 
               b         d         e
Utah   -0.029638  1.081563  1.280300
Ohio    0.647747  0.831136 -1.549481
Texas   0.513416 -0.884417  0.195343
Oregon -0.485454 -0.477388 -0.309548

In [118]: f = lambda x: x.max() - x.min()

In [119]: frame.apply(f)
Out[119]: 
b    1.133201
d    1.965980
e    2.829781
dtype: float64

许多最常见的数组统计信息(例如sum和mean)是DataFrame方法， 因此不必使用apply.

Many of the most common array statistics (like sum and mean) are DataFrame methods, so using apply is not necessary.

也可以使用基于元素的Python函数.假设您要根据帧中的每个浮点值来计算格式化的字符串.您可以使用applymap做到这一点:

Element-wise Python functions can be used, too. Suppose you wanted to compute a formatted string from each floating point value in frame. You can do this with applymap:

In [120]: format = lambda x: '%.2f' % x

In [121]: frame.applymap(format)
Out[121]: 
            b      d      e
Utah    -0.03   1.08   1.28
Ohio     0.65   0.83  -1.55
Texas    0.51  -0.88   0.20
Oregon  -0.49  -0.48  -0.31

之所以使用applymap之所以命名，是因为Series具有用于应用逐元素函数的map方法:

The reason for the name applymap is that Series has a map method for applying an element-wise function:

In [122]: frame['e'].map(format)
Out[122]: 
Utah       1.28
Ohio      -1.55
Texas      0.20
Oregon    -0.31
Name: e, dtype: object

总结起来，apply在DataFrame的行/列基础上工作，applymap在DataFrame的元素基础上工作，而map在Series的元素基础上工作.

Summing up, apply works on a row / column basis of a DataFrame, applymap works element-wise on a DataFrame, and map works element-wise on a Series.

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