Question

What will the following code display? numbers = [1, 2, 3, 4, 5] print(numbers[?2])

Short answer

Answer: 4

Step-by-step solution

Analyze the list and the index

We have a list named "numbers", which contains the elements [1, 2, 3, 4, 5]. We are asked to print the value at the index "?2". However, there is an error. The correct syntax should be "-2" instead of "?2". So, let's work on the correct index: -2. In Python, negative indices refer to counting positions from the end of the list.

Find the element at index -2

Now that we discern how negative indexing works in Python lists, let's find the element at index -2. Since starting from the end of the list is represented by -1, the second last element is represented by -2. 1. -1 (last element) -> 5 2. -2 (second last element) -> 4

Print the element at index -2

The element at index -2 is 4. Therefore, the code with the corrected index (numbers[-2]) will display the value 4 when executed.

Key concepts

Indexing

Indexing in Python allows you to access specific elements from a list using a numerical index. Each item in a list is assigned a unique index that starts from 0 and goes up to the length of the list minus one. For example, in the list `[1, 2, 3, 4, 5]`, the index of `1` is `0`, `2` is `1`, and so on.

Using indexing, you can retrieve, update, or remove elements at specified positions. For instance, `numbers[0]` would access the first element `1` and `numbers[3]` would access the fourth element `4`. Proper indexing is crucial for efficiently managing data stored within lists.

• Positive indexing starts from the beginning of the list.
• Provides a straightforward way to manipulate list entries directly.

Understanding list indexing is fundamental for anyone working with sequential data in Python.

Syntax Error

A syntax error in Python occurs when the code does not conform to the language's rules. Syntax errors are caught during the compilation step, before the program is run. It's like using incorrect grammar in spoken language, which makes the intended message hard to understand.

For instance, in the statement `print(numbers[?2])`, the use of `?` instead of a valid integer or negative sign leads to a syntax error. Python expects a clear and specific instruction on what to access from the list at the given index.

• Always ensure correct characters and syntax are used.
• Review the purpose and placement of operators, parentheses, and indexes.

Keeping an eye out for syntax errors helps avoid frustrating run-time issues and enhances code clarity.

Negative Indexing

Negative indexing is a powerful feature in Python that allows you to access list elements from the end instead of the start. It opens up a flexible means of traversal, especially when dealing with lists of unknown lengths.

By using negative indices, such as `-1`, `-2`, etc., you can easily access elements in reverse order. In a list like `[1, 2, 3, 4, 5]`, `numbers[-1]` returns `5` (the last element) and `numbers[-2]` returns `4` (the second last element).

• Negative indices begin with `-1` for the last element.
• Provides a shortcut for reverse iteration.

This method can be particularly useful in scenarios where the size of the list isn't known, and accessing elements from the end is needed.

List Manipulation

List manipulation in Python encompasses a variety of operations that allow you to modify lists in diverse ways. This includes operations like adding, removing, or changing elements as well as more advanced manipulations such as slicing and sorting.

For example, you can append an item to a list using `list.append()`, remove an item with `list.remove()`, or change an item by assigning a new value to a specific index like so: `numbers[1] = 10`.

• Inserting elements can be done via `append()` and `insert()`.
• Removing elements can be done with `pop()`, `remove()`, or del keyword.

Mastering these operations allows you to effectively control the structure and content of your lists, making your programming more efficient and versatile.