Lists

List functions

• Many built-in functions can operate on lists
• Examples include len, max, min, and sum

Example

mylist = [1, 2, 3, 4]

print(f"Number of values: {len(mylist)}")
print(f"Sum of values: {sum(mylist)}")
print(f"Smallest value: {min(mylist)}")
print(f"Largest value: {max(mylist)}")

Numeric lists

• Some functions will only work properly on lists with numeric elements
• sum won’t work properly with strings, for example

Example

mylist = ["a", "b", "c"]

print(sum(mylist))

List constructor

• The list constructor can be used to create a list from a string

Example

items = list("abcdef")

print(type(items))
print(items)

Lists and Strings

• A list is a sequence of arbitrary values
• A string is different data type that represents a sequence of characters
• They cannot always be used interchangeably

Example

items = "abc"

for letter in items:
    print(letter)

Example

items = ["a", "b", "c"]

items.split()

Objects and values

• Two values can hold the same contents without pointing to the same object
• Consider the following assignment:

a = 'banana'
b = 'banana'

is

• The is operator can be used to test if two values are the same object

Example

a = 'banana'
b = 'banana'

print(a is b)

Lists

• Because lists are mutable, a new list is a new object
• Two lists with the same values may not be the same object

Example

a = list('banana')
b = list('banana')

print(a is b)

List equivalence

• We can also use == to compare lists
• This will perform a deep compare to check if the two lists have identical values

Example

a = list('banana')
b = list('banana')

print(a == b)

Exercise

Write a Python function that will return True if and only if the two lists contain the same items but are not the same list.

def equal_but_not_same(a, b):


a = [1, 2]

assert(equal_but_not_same([1, 2], [1, 2]) == True)
assert(equal_but_not_same(a, [1, 2]) == True)
assert(equal_but_not_same([1], [2]) == False)
assert(equal_but_not_same(a, a) == False)

Multiple References

• Multiple variable can point to the same object

Example

a = [1, 2, 3]
b = a

print(a is b)

Aliasing

• An object with more than one reference to it is aliased
• Care must be taken when aliasing mutable objects
• A modification to one will affect the other

Example

scores = [100, 85, 75, 87]

letter_grades = scores

for i in range(len(scores)):
    if scores[i] >= 90:
        letter_grades[i] = 'A'
    elif scores[i] >= 80:
        letter_grades[i] = 'B'
    elif scores[i] >= 70:
        letter_grades[i] = 'C'

print(letter_grades)
print(scores)

Lists as arguments

• If a list is passed to a function, it will be aliased
• A modification inside the list will be seen by the caller

Example

def square_all(numbers):
    """ Return a list with the squares of all numbers"""

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

    return numbers

mynums = [1, 2, 3, 4]

print(mynums)
print(square_all(mynums))
print(mynums)

Exercise

Adjust the following program so that first_half does not modify the list it is passed

mylist = list(range(10))

def first_half(values):
    """ Returns the first half of a list of values """
    for i in range(len(values) // 2):
        values.pop(-1)

    return values

print(first_half(mylist))
print(mylist)