How do Negative Indices Work in Python Lists?

Negative List Indexing in Python

Negative list indices are an intriguing concept in Python, often leaving beginners puzzled. To unravel this topic, let's delve into a specific code snippet that showcases their usage:

<code class="python"># Node list
n = []
for i in xrange(1, numnodes + 1):
    tmp = session.newobject();
    n.append(tmp)
link(n[0], n[-1])</code>

Query: Unveiling the Negative Index

The focal point of confusion pertains to the index -1. If index 0 denotes the first element of the list, what significance does -1 hold?

Answer: Counting from the Right

The enigma lies in the fact that negative indices signify counting from the right end of the list instead of the left. Consequently, list[-1] points to the last element, list[-2] signifies the second-to-last element, and so forth.

To grasp this concept fully, consider the list below:

[10, 20, 30, 40, 50]

Using positive indices, you can access elements like this:

n[0] # 10
n[1] # 20

Now, let's explore negative indices:

n[-1] # 50
n[-2] # 40

As you can observe, -1 successfully retrieves the last element, while -2 captures the second-to-last element. This revised understanding enables you to effectively manipulate lists, irrespective of their size.

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