Slicing lists

3.6.3. Slicing lists#

We already learned how to index a list by using an integer within square brackets [] to extract one element from a list. Sometimes, we need to extract not only one element, but a sub-list containing multiple elements. For instance, we may want to retain a certain portion of a list and discard the rest. This is done using slicing.

Note

Everything on this page also applies to tuples.

3.6.3.1. Slicing syntax#

The syntax for slicing is similar to indexing, the main difference being the use of a colon operator : inside the brackets. To extract a single element from a list, such as the 3rd element (at index 2), we would use:

list_of_strings = [
    "string 0",
    "string 1",
    "string 2",
    "string 3",
    "string 4",
    "string 5",
    "string 6",
    "string 7",
    "string 8",
    "string 9",
]

list_of_strings[2]

'string 2'

To extract a list containing the elements at indexes 2, 3, 4, 5, and 6, we would instead use:

list_of_strings[2:7]  # From index 2 (inclusive) to index 7 (exclusive)

['string 2', 'string 3', 'string 4', 'string 5', 'string 6']

This means “return a list that includes elements from index 2 up to (but excluding) index 7”.

Tip

It can be counter-intuitive that the first index is inclusive while the second is not. Here is a logical way to remember it:

  • The first index is the first element that we want.

  • The second index is the first element that we don’t want.

3.6.3.2. Slicing step#

We can extract every other element using a third integer that specifies the step to use in navigating the list. For instance, to extract a list containing the elements at indexes 2, 4, and 6:

# From index 2 (inclusive) to index 7 (exclusive), by steps of 2
list_of_strings[2:7:2]

['string 2', 'string 4', 'string 6']

To navigate a list backward, we would use a step of -1. For instance, to extract a list containing elements at indexes 6, 5, 4, 3, and 2, we would use:

# From index 6 (inclusive) to index 1 (exclusive), by steps of -1
list_of_strings[6:1:-1]

['string 6', 'string 5', 'string 4', 'string 3', 'string 2']

Tip

If this last example was harder to understand, let’s reuse the previous tip.

  • The first index is the first element that we want. This is index 6.

  • The second index is the first element that we don’t want. This is index 1.

3.6.3.3. Single bounds#

We can slice a list using only one bound. For example, to get every element from index 2 up to the end, we simply omit the “up to” index:

# From index 2 (inclusive)
list_of_strings[2:]

['string 2',
 'string 3',
 'string 4',
 'string 5',
 'string 6',
 'string 7',
 'string 8',
 'string 9']

Similarly:

# Up to index 8 (exclusive)
list_of_strings[:8]

['string 0',
 'string 1',
 'string 2',
 'string 3',
 'string 4',
 'string 5',
 'string 6',
 'string 7']

means “return a list that containing the elements up to index 8 (exclusive)”. This also works with variable increments:

# From index 2 (inclusive), by steps of 2
list_of_strings[2::2]

['string 2', 'string 4', 'string 6', 'string 8']