Slice a List with Negative Indices in Python

In Python, you can slice a list using negative indices to access elements from the end of the list. The syntax list[start:stop:step] works with negative indices, where -1 refers to the last element, -2 to the second last, and so on.

Examples

1. Slicing the Last N Elements Using Negative Indices

Using negative indices, we can extract a portion of the list starting from the end.

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# Original list
numbers = [10, 20, 30, 40, 50, 60, 70]

# Extracting the last 3 elements
last_three = numbers[-3:]

# Printing the result
print("Last three elements:", last_three)

Explanation

Here, we define a list numbers containing seven elements. The slicing operation numbers[-3:] selects elements starting from the third last element (-3) up to the end of the list. Since no stop index is provided, it defaults to the last element.

Output:

Last three elements: [50, 60, 70]

2. Extracting a Middle Section Using Negative Indices

Negative indices can also be used to extract a section of a list from within.

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# Original list
letters = ["a", "b", "c", "d", "e", "f", "g"]

# Extracting elements from the third last to the second last
middle_section = letters[-4:-1]

# Printing the result
print("Middle section:", middle_section)

Explanation

We use letters[-4:-1] to extract a subsection of the list. Here:

  • -4 corresponds to the fourth last element (“d”).
  • -1 corresponds to the last element, but since slicing excludes the stop index, it stops at “f”.

Output:

Middle section: ['d', 'e', 'f']

3. Reversing a List Using Negative Step

A negative step value allows slicing the list in reverse order.

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# Original list
words = ["Python", "Java", "C++", "JavaScript", "Ruby"]

# Reversing the list using slicing
reversed_list = words[::-1]

# Printing the result
print("Reversed list:", reversed_list)

Explanation

The slice words[::-1] reverses the list by using:

  • An empty start and stop index, selecting the whole list.
  • A step value of -1, which moves from right to left.

Output:

Reversed list: ['Ruby', 'JavaScript', 'C++', 'Java', 'Python']

4. Extracting Every Second Element in Reverse Order

Using a negative step, we can extract elements at regular intervals in reverse.

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# Original list
numbers = [10, 20, 30, 40, 50, 60, 70, 80]

# Extracting every second element in reverse
reverse_step = numbers[-1::-2]

# Printing the result
print("Every second element in reverse:", reverse_step)

Explanation

Here, numbers[-1::-2] does the following:

  • -1 starts from the last element (80).
  • ::-2 selects every second element moving backward.

Output:

Every second element in reverse: [80, 60, 40, 20]

Conclusion

Slicing with negative indices provides an efficient way to work with lists:

  1. Extracting last elements: list[-n:] retrieves the last n elements.
  2. Extracting a middle section: list[-x:-y] selects a subset.
  3. Reversing a list: list[::-1] inverts the order.
  4. Skipping elements in reverse: list[-1::-step] selects elements at intervals from the end.