Access Array Elements | Indexing and Slicing
NumPy in a Nutshell

# Access Array Elements

In both lists and arrays, elements are accessed using square brackets. Let's review the distinction between indexing and slicing:

• To retrieve a single element, you simply need to specify the index of that element in square brackets (start counting from 0);
• If you want to obtain a sequence from the original array, you should use slices.

We'll start with simple indexing. Let's have a look at the following image:

Let's see how it works with examples.

Get the first element from the following array:

Retrieve the second element from the following array:

Retrieve the third and fourth elements from the following array and then add them together:

Now, it's time to explore slicing. First, let's examine the syntax of slicing: `array[start:end:step]`, where:

• `start` is the index from which slicing begins;
• `end` is the index where slicing stops (note that this index is not included);
• `step` is the parameter that specifies the intervals between the indices.

Let's have a look at the following image:

## Omitting start, end and step

As you can see, we can often omit the `start`, `end`, `step` or even all of them at the same time. `step`, for example, can be omitted when we want it to be equal to 1. `start` and `end` can be omitted in the following scenarios:

1. Omitting `start`:
• slicing from the first element (`step` is positive);
• slicing from the last element (`step` is negative).
2. Omitting `end`:
• slicing to the last element inclusive (`step` is positive);
• slicing to the first element inclusive (`step` is negative).

In the example above, `a[2:4]` has the `step` equal to 1. `a[-2:]` goes from the second to last element to the end of the array with `step` equal to 1. `a[::2]` goes from the first element to the end of the array with `step` equal to 2.

It's time to practice.

Retrieve the first and last elements from the following array `[13, 99, 11, 23, 5, 41]` and then multiply them. Please use positive indexing.

Everything was clear?

Section 3. Chapter 1

Course Content

NumPy in a Nutshell

# Access Array Elements

In both lists and arrays, elements are accessed using square brackets. Let's review the distinction between indexing and slicing:

• To retrieve a single element, you simply need to specify the index of that element in square brackets (start counting from 0);
• If you want to obtain a sequence from the original array, you should use slices.

We'll start with simple indexing. Let's have a look at the following image:

Let's see how it works with examples.

Get the first element from the following array:

Retrieve the second element from the following array:

Retrieve the third and fourth elements from the following array and then add them together:

Now, it's time to explore slicing. First, let's examine the syntax of slicing: `array[start:end:step]`, where:

• `start` is the index from which slicing begins;
• `end` is the index where slicing stops (note that this index is not included);
• `step` is the parameter that specifies the intervals between the indices.

Let's have a look at the following image:

## Omitting start, end and step

As you can see, we can often omit the `start`, `end`, `step` or even all of them at the same time. `step`, for example, can be omitted when we want it to be equal to 1. `start` and `end` can be omitted in the following scenarios:

1. Omitting `start`:
• slicing from the first element (`step` is positive);
• slicing from the last element (`step` is negative).
2. Omitting `end`:
• slicing to the last element inclusive (`step` is positive);
• slicing to the first element inclusive (`step` is negative).

In the example above, `a[2:4]` has the `step` equal to 1. `a[-2:]` goes from the second to last element to the end of the array with `step` equal to 1. `a[::2]` goes from the first element to the end of the array with `step` equal to 2.

It's time to practice.

Retrieve the first and last elements from the following array `[13, 99, 11, 23, 5, 41]` and then multiply them. Please use positive indexing.

Everything was clear?

Section 3. Chapter 1