Indexing a list

3.6.2. Indexing a list#

Every element of a list is accessible using an index. The first element is at index 0, the second at index 1, etc.

Note

Zero-based addressing (first index is 0) is common in generic programming languages such as C, C++, Java, Go, PHP, Ruby, and Rust. Being a generic programming language itself, Python also uses zero-addressing. Other languages that are usually more aimed to mathematics, such as Matlab, R, Julia, and Mathematica, use one-based addressing (first index is 1). No convention is inherently better than another, but for non-programmers, it may take a bit of time to adapt to zero-based addressing.

We index a list using square brackets []:

list_of_strings = [
    "first element",
    "second element",
    "third element",
    "fourth element",
    "fifth element",
    "sixth element",
    "seventh element",
    "eighth element",
    "ninth element",
    "last element",
]

list_of_strings[0]

'first element'

list_of_strings[1]

'second element'

Tip

Until you get used to zero-based addressing, you may sometimes get this error:

# Get the last element
list_of_strings[10]

Results in:

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Cell In[3], line 2
      1 # Get the last element
----> 2 list_of_strings[10]

IndexError: list index out of range

In this case, this error happened because while the list is indeed 10-element long, the last element is at index 9, not 10.

3.6.2.1. Negative indexing#

We can address a list from its last element, using negative indexing. The last element is available at index -1:

list_of_strings[-1]

'last element'

the previous at index -2, etc.:

list_of_strings[-2]

'ninth element'

3.6.2.2. Indexing using a variable#

In the examples above, we indexed a list using a literal constant. We can also index it using a variable:

i = 3
list_of_strings[i]

'fourth element'

In any case, the variable must be an integer. Indeed, a list element could not be halfway between two elements.

Tip

If the index is calculated using a division, then the result is a float, not an integer. In this case, this statement would generate an error:

some_value = 6
some_string = list_of_strings[some_value / 2]

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[6], line 2
      1 some_value = 6
----> 2 list_of_strings[some_value / 2]

TypeError: list indices must be integers or slices, not float

This code would correct this error:

some_value = 6
some_string = list_of_strings[int(some_value / 2)]

3.6.2.3. Nested lists#

As seen in section Creating lists and tuples, lists can be nested, which means that a list can contain another list. To access a specific element of the inner list, we start by indexing the outer list, then the inner list. For example, to access the first element of the second list of list_of_lists:

list_of_lists = [
    [1, 2, 3],
    ["one", "two", "three"],  # <--- "one"
    [1, 2, 3, 4, 5],
]

we first access the second list of list_of_list:

temp = list_of_lists[1]  # Second element of list_of_lists

temp

['one', 'two', 'three']

then we access the first element of this result:

temp[0]  # First element of the second element of list_of_lists

'one'

or on one line:

list_of_lists[1][0]

'one'

where list_of_lists[1] returns the list ['one', 'two', 'three'], and [0] indexes this new list.

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