# 5.2 Lists¶

- Many of the capabilities shown in this section apply to all sequence types.

### Creating a List¶

- Lists typically store homogeneous data, but may store heterogeneous data.

In [1]:

    c = [-45, 6, 0, 72, 1543]

In [2]:

    c

Out[2]:

    [-45, 6, 0, 72, 1543]

### Accessing Elements of a List¶

- Reference a list element by writing the list’s name followed by the element’s index enclosed in `[]` (the subscription operator).

In [3]:

    c[0]

Out[3]:

    -45

In [4]:

    c[4]

Out[4]:

    1543

### Determining a List’s Length¶

In [5]:

    len(c)

Out[5]:

    5

### Accessing Elements from the End of the List with Negative Indices¶

- Lists can be accessed from the end by using negative indices:

In [6]:

    c[-1]

Out[6]:

    1543

In [7]:

    c[-5]

Out[7]:

    -45

### Indices Must Be Integers or Integer Expressions¶

In [8]:

    a = 1

In [9]:

    b = 2

In [10]:

    c[a + b]

Out[10]:

    72

### Lists Are Mutable¶

In [11]:

    c[4] = 17

In [12]:

    c

Out[12]:

    [-45, 6, 0, 72, 17]

### Some Sequences Are Immutable¶

- Python’s string and tuple sequences are immutable.

In [13]:

    s = 'hello'

In [14]:

    s[0]

Out[14]:

    'h'

In [15]:

    s[0] = 'H'

    ------------------------------------------------------------------------
    TypeError                              Traceback (most recent call last)
    <ipython-input-15-d6f3b193531a> in <module>
    ----> 1 s[0] = 'H'

    TypeError: 'str' object does not support item assignment

### Attempting to Access a Nonexistent Element¶

- Index values must be in range.

In [16]:

    c[100]

    ------------------------------------------------------------------------
    IndexError                             Traceback (most recent call last)
    <ipython-input-16-78fb4de297cc> in <module>
    ----> 1 c[100]

    IndexError: list index out of range

### Using List Elements in Expressions¶

In [17]:

    c[0] + c[1] + c[2]

Out[17]:

    -39

### Appending to a List with +=¶

- Lists can grow dynamically to accommodate new items.

In [18]:

    a_list = []

In [19]:

    for number in range(1, 6):
        a_list += [number]

In [20]:

    a_list

Out[20]:

    [1, 2, 3, 4, 5]

- When the left operand of `+=` is a list, the right operand must be an iterable; otherwise, a `TypeError` occurs.

In [21]:

    letters = []

In [22]:

    letters += 'Python'

In [23]:

    letters

Out[23]:

    ['P', 'y', 't', 'h', 'o', 'n']

### Concatenating Lists with +¶

- Can concatenate two lists, two tuples or two strings using `+` to create a new sequence of the same type.

In [24]:

    list1 = [10, 20, 30]

In [25]:

    list2 = [40, 50]

In [26]:

    concatenated_list = list1 + list2

In [27]:

    concatenated_list

Out[27]:

    [10, 20, 30, 40, 50]

### Using `for` and `range` to Access List Indices and Values¶

In [28]:

    for i in range(len(concatenated_list)):
        print(f'{i}: {concatenated_list[i]}')

    0: 10
    1: 20
    2: 30
    3: 40
    4: 50

- We’ll show a safer way to access element indices and values using built-in function `enumerate`.

### Comparison Operators¶

- Can compare entire lists element-by-element.

In [29]:

    a = [1, 2, 3]

In [30]:

    b = [1, 2, 3]

In [31]:

    c = [1, 2, 3, 4]

In [32]:

    a == b

Out[32]:

    True

In [33]:

    a == c

Out[33]:

    False

In [34]:

    a < c

Out[34]:

    True

In [35]:

    c >= b

Out[35]:

    True

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