Lists

list

• A list object is a sequence of values
• Values may be of any type
• Values may have different types

Creating a list

• Lists can be created using square brackets enclosing expressions separated by commas
• [1, 2, 3]
• ["a", "b", "c"]

Example

mylist = [1, 2, 3]

Empty lists

• An empty list is a valid data structure
• This is created using square brackets with nothing inside: []
• Empty lists evaluate to False in Boolean expressions

Example

if []:
    print("This won't print")

Falsy Values

• Most Python values evaluate as True in a Boolean expression
• Exceptions include empty containers and 0-valued numerics
• More info

Mutability

• Unlike strings, lists are mutable
• We can change an item inside a list

Example

mylist = [1, 2, 3]

mylist[0] = 7

print(mylist)

Mapping

• One way to reason about a list is as a mapping from numbers to elements
• Language of one value “mapping to” another is also useful in later data structures

Indices

• Any integer may be an index
• Indices outside the list will raise an IndexError
• Negative numbers index from the back

Example

mylist = [1, 2, 3]

mylist[-4] = 7

in operator

• The in operator is supported on lists
• It will return True if a given value is found in the list

Example

mylist = [1,2,3]

if 2 in mylist:
    print("The list has a 2")

List Traversal

• A list is an iterable datatype
• We can use for to iterate over the items in a list

Example

mylist = [1,2,3]

for i in mylist:
    print(i)

Writing List Elements

• for is useful for reading, but doesn’t allow modification directly
• If we need to write, we need access to an index
• This can be achieved using len and range

Example

mylist = [1,2,3]

for i in range(len(mylist)):
    mylist[i] *= mylist[i]

print(mylist)

Exercise

Create a Python program that will add 1 to every element in a list.

Nested Lists

• Lists can be used as list values
• Nested lists only count as 1 element when computing len

Example

mylist = [1, 2, [3, 4, 5], 6]

print(len(mylist))

Exercise

Create a Python program that will count all individual elements in nested lists.