Polars Series slice() – Usage & Examples

In polars, the Series.slice() function is used to extract a portion (or slice) of a Series, similar to slicing a list in Python. It returns a new Series that begins at a specified offset and contains a defined number of elements.

In this article, I will explain the slice() method of the Polars Series, covering its syntax, parameters, and how it works. This method returns a new Series that represents a subset of the original.

Key Points –

• The slice() method allows you to extract a subset of values from a Series based on position.
• It takes two parameters: offset (start index) and length (number of elements to return).
• If length is omitted or set to None, the slice will extend from offset to the end of the Series.
• The offset must be an integer and represents the starting index (0-based).
• A negative offset counts positions backward from the end of the Series.
• If length=None, the slice includes all elements from the offset to the end of the Series.
• If the offset exceeds the Series length, the result is an empty Series.
• Slicing does not modify the original Series; it returns a new sliced Series.

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Polars Series slice() Introduction

Let’s know the syntax of the series slice() method.

# Syntax of Series slice()
Series.slice(offset: int, length: int | None = None) → Series

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Parameters of the Polars series slice()

Following are the parameters of series slice() method.

• offset (int) - Starting index of the slice. Can be negative to count from the end.
• length (int or None, optional) - Number of elements to include in the slice. If None, slice continues to the end.

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Return Value

This function returns a new Series containing the sliced elements.

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Usage of Polars Series slice() Method

The Series.slice() function in Polars returns a subset of values from the Series, starting at a given index and taking a specific number of elements.

First, let’s create a Polars Series.

import polars as pl

# Sample Series with nulls
ser = pl.Series("values", [10, 20, 30, 40, 50])
print("Original Series:\n", ser)

Yields below output.

To perform a basic slice on a Polars Series starting from index 2 and taking 3 elements, you can use the slice() function.

# Slice from index 2, length 3
ser2 = ser.slice(offset=2, length=3)
print("Sliced Series:\n", ser2)

# Slice from index 2, take 3 elements
ser2 = ser.slice(2, 3)
print("Sliced Series:\n", ser2)

Here,

• offset=2 starts at the third element (indexing starts at 0).
• length=3 takes 3 consecutive elements from that point onward.
• The resulting Series contains: 30, 40, 50.

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Slice From the Beginning

To slice a Polars Series from the start, set offset=0 in the slice() method and specify the number of elements you want to retrieve.

# Slice from beginning (index 0), take 3 elements
ser2 = ser.slice(offset=0, length=3)
print("Sliced Series:\n", ser2)

# Output:
# Sliced Series:
# shape: (3,)
# Series: 'values' [i64]
# [
#	10
#	20
#	30
# ]

Here,

• slice(0, 3) gives you the first 3 values of the Series.
• Great for previewing or limiting data during processing.

Alternatively, to slice from the beginning of a Polars Series and include all elements, simply set the offset to 0 and omit the length.

# Slice from beginning (index 0)
ser2 = ser.slice(0)
print("Sliced Series:\n", ser2)

# Output:
# Sliced Series:
# shape: (5,)
# Series: 'values' [i64]
# [
#	10
#	20
#	30
#   40
#	50
# ]

Here,

• offset = 0: Start at the first element.
• length = None (default): Take all elements from that point to the end.

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Slice to the End (Length=None)

To slice to the end of a Polars Series, set the offset to your starting point and let length=None (which is the default behavior). This tells Polars to return all elements from that offset onward.

# Slice from index 2 to the end
ser2 = ser.slice(2)
print("Sliced Series:\n", ser2)

# Slice from index 2 to the end
ser2 = ser.slice(offset=2, length=None)
print("Sliced Series:\n", ser2)

# Output:
# Sliced Series:
# shape: (3,)
# Series: 'values' [i64]
# [
#	30
#	40
#	50
# ]

Here,

• offset = 2: Start from the 3rd element (30).
• length = None: Take all remaining elements.

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Use Negative Offset

Using a negative offset with slice() instructs Polars to begin slicing from the end of the Series, counting positions backward.

# Slice from 3rd last element to the end
ser2 = ser.slice(offset=-3, length=None)
print("Sliced Series:\n", ser2)

# Slice last 2 elements
ser2 = ser.slice(-2)
print("Sliced Series:\n", ser2)

# Output:
# Sliced Series:
# shape: (3,)
# Series: 'values' [i64]
# [
#	30
#	40
#	50
# ]

Here,

• offset=-3: starts from the third element from the end (i.e., value 30).
• length=None: returns elements to the end.

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Conclusion

In conclusion, the Series.slice() method in Polars offers a straightforward and efficient way to retrieve a portion of data based on position.

Happy Learning!!

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Related Articles

• Polars quantile() – Usage & Examples
• Polars extend() Usage & Examples
• Polars abs() – Explained by Examples
• Polars Shape Usage & Examples
• Polars Series mean() – Usage & Examples
• Polars Series min() – Explained by Examples
• Polars clear() – Explained by Examples
• Polars interpolate() Function with Examples
• Polars rolling_mean() – Usage & Examples
• Polars Series limit() – Explained by Examples
• Polars struct.schema() – Explained by Examples
• Polars Series fill_null() Function with Examples

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References

• https://docs.pola.rs/api/python/stable/reference/series/api/polars.Series.shuffle.html