5 Best Ways to Concatenate Two Pandas Series Without Repeating Indices

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πŸ’‘ Problem Formulation: When working with data in Python’s Pandas library, analysts often need to concatenate two Series objects into a single Series. A common challenge arises when both Series share index labels, which typically results in repeated indices in the concatenated result. This article addresses how to concatenate two Series such that the resulting Series has unique index labels, thereby preventing index duplication. For example, combining Series A with index [1, 2, 3] and Series B with index [2, 3, 4] should yield a single Series with a non-repeating index sequence.

Method 1: Reset Index and Concatenate

Resetting the indices of the Series before concatenation is a straightforward method to ensure unique index labels in the combined Series. This involves assigning a new default integer index to each Series, which avoids the possibility of overlapping indices when concatenating.

Here’s an example:

import pandas as pd

series_a = pd.Series([1, 2, 3])
series_b = pd.Series([4, 5, 6])

reset_a = series_a.reset_index(drop=True)
reset_b = series_b.reset_index(drop=True)
combined_series = pd.concat([reset_a, reset_b])

print(combined_series)

Output:

0    1
1    2
2    3
3    4
4    5
5    6
dtype: int64

In this example, we first call reset_index(drop=True) on both Series to replace their indices with a default range index. Then we use pd.concat() to combine them into a single Series. drop=True parameter is used to discard the old index labels.

Method 2: Concatenation with Ignore Index

Another method is to use the ignore_index=True parameter of the pd.concat() function directly. This allows for the concatenation of two Series into one, with the new Series having a continuous index regardless of any overlap in the original Series’ indices.

Here’s an example:

import pandas as pd

series_a = pd.Series([7, 8, 9], index=[1, 2, 3])
series_b = pd.Series([10, 11, 12], index=[2, 3, 4])

combined_series = pd.concat([series_a, series_b], ignore_index=True)

print(combined_series)

Output:

0     7
1     8
2     9
3    10
4    11
5    12
dtype: int64

Here, the ignore_index=True option in pd.concat() discards the original Series indices and assigns a new range index to the result, ensuring there are no duplicate indices.

Method 3: Use of append with Ignore Index

The append() method in Pandas, along with the ignore_index=True parameter, provides another intuitive way to concatenate Series while automatically generating a new index. This may be more readable to some users versus the concat() function.

Here’s an example:

import pandas as pd

series_a = pd.Series([13, 14, 15])
series_b = pd.Series([16, 17, 18])

combined_series = series_a.append(series_b, ignore_index=True)

print(combined_series)

Output:

0    13
1    14
2    15
3    16
4    17
5    18
dtype: int64

This code snippet illustrates appending one Series to another using append(), where ignore_index=True indicates that the Series should not retain their original indices post concatenation.

Method 4: Manual Index Assignment

Manually assigning a new index while combining Series allows you to have complete control over the resulting Series’ index. It is particularly useful if you need a specific index pattern or labels that are not simply a range of numbers.

Here’s an example:

import pandas as pd

series_a = pd.Series([19, 20, 21])
series_b = pd.Series([22, 23, 24])

combined_series = pd.Series(series_a.tolist() + series_b.tolist(), index=range(6))

print(combined_series)

Output:

0    19
1    20
2    21
3    22
4    23
5    24
dtype: int64

In this case, tolist() is used to convert each Series to a list which are then concatenated using the + operator. A new Series is created from this combined list with a manually assigned index.

Bonus One-Liner Method 5: Chain and Rename

A more advanced, one-liner method to achieve unique indexing involves chaining the Series together and immediately renaming the index. It’s a quick solution for data scientists who prefer concise code.

Here’s an example:

import pandas as pd

series_a = pd.Series([25, 26, 27])
series_b = pd.Series([28, 29, 30])

combined_series = series_a.append(series_b).rename(lambda x: x + 1)

print(combined_series)

Output:

1    25
2    26
3    27
4    28
5    29
6    30
dtype: int64

This compact code snippet takes advantage of the chainability of methods in pandas, using append() for concatenation followed by rename() with a lambda function to shift the index.

Summary/Discussion

  • Method 1: Resetting Indices. Ensures unique numeric indices. Additional step of resetting necessitates more verbose code.
  • Method 2: Concat with Ignore Index. Concise and utilizes pandas’ core functionality. However, does not allow custom indices.
  • Method 3: Append with Ignore Index. Readable and straightforward, though less versatile than concat() for multiple Series or DataFrames.
  • Method 4: Manual Index Assignment. Offers full control over the result’s index. Requires extra handling, potentially error-prone if mishandled.
  • Method 5: Chain and Rename. A quick one-liner, best for experienced users who prefer brevity in their code. May sacrifice readability for less seasoned users.