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).

Diagram of a list named c labeled with its element names

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: Diagram of the list c labeled with its 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

©1992–2020 by Pearson Education, Inc. All Rights Reserved. This content is based on Chapter 5 of the book Intro to Python for Computer Science and Data Science: Learning to Program with AI, Big Data and the Cloud.

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