How do I operate on a DataFrame with a Series for every column

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Objective and Motivation

I've seen this kind of question several times over and have seen many other questions that involve some element of this. Most recently, I had to spend a bit of time explaining this concept in comments while looking for an appropriate canonical Q&A. I did not find one and so I thought I'd write one.

This question usually arises with respect to a specific operation but equally applies to most arithmetic operations.

  • How do I subtract a Series from every column in a DataFrame?
  • How do I add a Series from every column in a DataFrame?
  • How do I multiply a Series from every column in a DataFrame?
  • How do I divide a Series from every column in a DataFrame?

The Question

Given a Series s and DataFrame df. How do I operate on each column of df with s?

df = pd.DataFrame(     [[1, 2, 3], [4, 5, 6]],     index=[0, 1],     columns=['a', 'b', 'c'] )  s = pd.Series([3, 14], index=[0, 1]) 

When I attempt to add them, I get all np.nan

df + s      a   b   c   0   1 0 NaN NaN NaN NaN NaN 1 NaN NaN NaN NaN NaN 

What I thought I should get is

    a   b   c 0   4   5   6 1  18  19  20 

 


Please bear the preamble. It's important to address some higher level concepts first. Since my motivation is to share knowledge and teach, I wanted to make this as clear as possible.


It is helpful to create a mental model of what Series and DataFrame objects are.

Anatomy of a Series

A Series should be thought of as an enhanced dictionary. This isn't always a perfect analogy, but we'll start here. Also, there are other analogies that you can make but I am targetting a dictionary in order to demonstrate the purpose of this post.

index

These are the keys that we can reference to get at the corresponding values. When the elements of the index are unique, the comparison to a dictionary becomes very close.

values

These are the corresponding values that are keyed by the index.

Anatomy of a DataFrame

A DataFrame should be thought of as a dictionary of Series or a Series of Series. In this case the keys are the column names and the values are the columns themselves as Series objects. Each Series agrees to share the same index which is the index of the DataFrame.

columns

These are the keys that we can reference to get at the corresponding Series.

index

This the the index that all of the Series values agree to share.

Note: RE: columns and index objects

They are the same kind of things. A DataFrames index can be used as another DataFrames columns. In fact, this happens when you do df.T to get a transpose.

values

This is a 2 dimensional array that contains the data in a DataFrame. The reality is that values is NOT what is stored inside the DataFrame object. (Well sometimes it is, but I'm not about to try to describe the block mananager). The point is, it is better to think of this as access to a 2 dimensional array of the data.


Define Sample Data

These are sample pandas.Index objects that can be used as the index of a Series or DataFrame or can be used as the columns of a DataFrame

idx_lower = pd.Index([*'abcde'], name='lower') idx_range = pd.RangeIndex(5, name='range') 

These are sample pandas.Series objects that use the pandas.Index objects above

s0 = pd.Series(range(10, 15), idx_lower) s1 = pd.Series(range(30, 40, 2), idx_lower) s2 = pd.Series(range(50, 10, -8), idx_range) 

These are sample pandas.DataFrame objects that use the pandas.Index objects above

df0 = pd.DataFrame(100, index=idx_range, columns=idx_lower) df1 = pd.DataFrame(     np.arange(np.product(df0.shape)).reshape(df0.shape),     index=idx_range, columns=idx_lower ) 

Series on Series

When operating on two Series, the alignment is obvious. You align the index of one Series with the index of the other.

s1 + s0  lower a    40 b    43 c    46 d    49 e    52 dtype: int64 

Which is the same as when I randomly shuffle one before I operate. The indices will sitll align.

s1 + s0.sample(frac=1)  lower a    40 b    43 c    46 d    49 e    52 dtype: int64 

And is NOT the case when instead I operate with the values of the shuffled Series. In this case, Pandas doesn't have the index to align with and therefore operates from a positions.

s1 + s0.sample(frac=1).values  lower a    42 b    42 c    47 d    50 e    49 dtype: int64 

Add a scalar

s1 + 1  lower a    31 b    33 c    35 d    37 e    39 dtype: int64 

DataFrame on DataFrame

Similar is true when operating between two DataFrames
The alignment is obvious and does what we think it should do

df0 + df1  lower    a    b    c    d    e range                          0      100  101  102  103  104 1      105  106  107  108  109 2      110  111  112  113  114 3      115  116  117  118  119 4      120  121  122  123  124 

Shuffle second DataFrame on both axes. The index and columns will still align and give us the same thing.

df0 + df1.sample(frac=1).sample(frac=1, axis=1)  lower    a    b    c    d    e range                          0      100  101  102  103  104 1      105  106  107  108  109 2      110  111  112  113  114 3      115  116  117  118  119 4      120  121  122  123  124 

Same shuffling but add the array and not the DataFrame. No longer aligned and will get different results.

df0 + df1.sample(frac=1).sample(frac=1, axis=1).values  lower    a    b    c    d    e range                          0      123  124  121  122  120 1      118  119  116  117  115 2      108  109  106  107  105 3      103  104  101  102  100 4      113  114  111  112  110 

Add 1 dimensional array. Will align with columns and broadcast across rows.

df0 + [*range(2, df0.shape[1] + 2)]  lower    a    b    c    d    e range                          0      102  103  104  105  106 1      102  103  104  105  106 2      102  103  104  105  106 3      102  103  104  105  106 4      102  103  104  105  106 

Add a scalar. Nothing to align with so broadcasts to everything

df0 + 1  lower    a    b    c    d    e range                          0      101  101  101  101  101 1      101  101  101  101  101 2      101  101  101  101  101 3      101  101  101  101  101 4      101  101  101  101  101 

DataFrame on Series

If DataFrames are to be though of as dictionaries of Series and Series are to be thought of as dictionaries of values, then it is natural that when operating between a DataFrame and Series that they should be aligned by their "keys".

s0: lower    a    b    c    d    e         10   11   12   13   14  df0: lower    a    b    c    d    e range                          0      100  100  100  100  100 1      100  100  100  100  100 2      100  100  100  100  100 3      100  100  100  100  100 4      100  100  100  100  100 

And when we operate, the 10 in s0['a'] gets added to the entire column of df0['a']

df0 + s0  lower    a    b    c    d    e range                          0      110  111  112  113  114 1      110  111  112  113  114 2      110  111  112  113  114 3      110  111  112  113  114 4      110  111  112  113  114 

Heart of the issue and point of the post

What about if I want s2 and df0?

s2:               df0:               |    lower    a    b    c    d    e range        |    range                          0      50    |    0      100  100  100  100  100 1      42    |    1      100  100  100  100  100 2      34    |    2      100  100  100  100  100 3      26    |    3      100  100  100  100  100 4      18    |    4      100  100  100  100  100 

When I operate, I get the all np.nan as cited in the question

df0 + s2          a   b   c   d   e   0   1   2   3   4 range                                         0     NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN 1     NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN 2     NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN 3     NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN 4     NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN 

This does not produce what we wanted. Because Pandas is aligning the index of s2 with the columns of df0. The columns of the result includes a union of the index of s2 and the columns of df0.

We could fake it out with tricky transposition

(df0.T + s2).T  lower    a    b    c    d    e range                          0      150  150  150  150  150 1      142  142  142  142  142 2      134  134  134  134  134 3      126  126  126  126  126 4      118  118  118  118  118 

But it turns out Pandas has a better solution. There are operation methods that allow us to pass an axis argument to specify the axis to align with.

- sub
+ add
* mul
/ div
** pow

And so the answer is simply

df0.add(s2, axis='index')  lower    a    b    c    d    e range                          0      150  150  150  150  150 1      142  142  142  142  142 2      134  134  134  134  134 3      126  126  126  126  126 4      118  118  118  118  118 

Turns out axis='index' is synomynous with axis=0.
As is axis='columns' synomynous with axis=1

df0.add(s2, axis=0)  lower    a    b    c    d    e range                          0      150  150  150  150  150 1      142  142  142  142  142 2      134  134  134  134  134 3      126  126  126  126  126 4      118  118  118  118  118 

Rest of the operations

df0.sub(s2, axis=0)  lower   a   b   c   d   e range                     0      50  50  50  50  50 1      58  58  58  58  58 2      66  66  66  66  66 3      74  74  74  74  74 4      82  82  82  82  82 

df0.mul(s2, axis=0)  lower     a     b     c     d     e range                               0      5000  5000  5000  5000  5000 1      4200  4200  4200  4200  4200 2      3400  3400  3400  3400  3400 3      2600  2600  2600  2600  2600 4      1800  1800  1800  1800  1800 

df0.div(s2, axis=0)  lower         a         b         c         d         e range                                                   0      2.000000  2.000000  2.000000  2.000000  2.000000 1      2.380952  2.380952  2.380952  2.380952  2.380952 2      2.941176  2.941176  2.941176  2.941176  2.941176 3      3.846154  3.846154  3.846154  3.846154  3.846154 4      5.555556  5.555556  5.555556  5.555556  5.555556 

df0.pow(1 / s2, axis=0)  lower         a         b         c         d         e range                                                   0      1.096478  1.096478  1.096478  1.096478  1.096478 1      1.115884  1.115884  1.115884  1.115884  1.115884 2      1.145048  1.145048  1.145048  1.145048  1.145048 3      1.193777  1.193777  1.193777  1.193777  1.193777 4      1.291550  1.291550  1.291550  1.291550  1.291550 

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