Tuples

Tuples#

A tuple, like a list, is a sequence of items of any type.
Unlike lists, however, tuples are immutable.

Type	Collection of	Syntax	Ordered	Indexed	Mutable	Passed By	Duplicates Allowed
`strings`	characters	`" "`	✓	✓	✗	value	✓
`list`	any data type	`[ ]`	✓	✓	✓	reference	✓
`tuple`	any data type	`( )`	✓	✓	✗	value	✓

Syntactically, a tuple is a comma-separated sequence of values:

tup1 = 2, 4, 6, 8, 10, 2
print(tup1)

(2, 4, 6, 8, 10, 2)

Although it is not necessary, it is conventional to enclose tuples in parentheses:

tup2 = (2, 4, 6, 8, 10, 2)
print(tup2)

(2, 4, 6, 8, 10, 2)

To create a tuple with a single element, we have to include the final comma:

listt = [5]
print(type(listt), listt)

<class 'list'> [5]

tup = (5,)
print(type(tup), tup)

<class 'tuple'> (5,)

Without the comma, Python treats (5) as an integer in parentheses:

tup = (5)
print(type(tup), tup)

<class 'int'> 5

Syntax issues aside, tuples support the same sequence operations as strings and lists.
The index operator selects an element from a tuple.

tup = ('a', 'b', 'c', 'd', 'e')
tup[4]

'e'

And the slice operator selects a range of elements.

tup = ('a', 'b', 'c', 'd', 'e')

tup[2:4]

('c', 'd')

Concatenation Operator + works the same as with lists and strings

tup1 = ('X',) 
tup2 = ('a', 'b', 'c', 'd', 'e')
tup = tup1 + tup2
tup

('X', 'a', 'b', 'c', 'd', 'e')

When we try to use item assignment to modify one of the elements of the tuple, we get an error:

tup = (1, 2, 3)
tup[0] = 'X'

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Input In [9], in <cell line: 2>()
      1 tup = (1, 2, 3)
----> 2 tup[0] = 'X'

TypeError: 'tuple' object does not support item assignment

Tuple assignment#

Once in a while, it is useful to swap the values of two variables.
- With conventional assignment statements, we have to use a temporary variable.
For example, to swap a and b:

a = "a"
b = "b"
temp = a
a = b
b = temp
print(a, b)

b a

Python provides a form of tuple assignment that solves this problem neatly:

a = "a"
b = "b"
c = "c"
d = "d"

a, b, c, d = b, b, b, b

print(a, b, c, d)

b b b b

The left side is a tuple of variables; the right side is a tuple of values.
Each value is assigned to its respective variable.
This feature makes tuple assignment quite versatile.
The number of variables on the left and the number of values on the right have to be the same.

Tuples as return values#

Functions can return tuples as return values.

def find_min_max(numbers):
    

    min_val = 9999
    max_val = -9999

    i = 0
    while i < len(numbers):
        if numbers[i] < min_val:
            min_val = numbers[i]
        
        if numbers[i] > max_val:
            max_val = numbers[i]
            
        i = i + 1
        
    return min_val, max_val


minimum, maximum = find_min_max([3, 1, 5, 4, 2])
print(minimum, maximum)

1 5

Reminder: Tuples are passed to functions by value
Translation: Any changes you make inside a function, would NOT be reflected outside the function

def swap(inputs):
    x, y = inputs
    y, x = x, y
    
a, b = "a", "b"
swap((a, b))
print(a, b)

Exercises#

Question 1.#

Find below the exam score of three students Jane, John and Mary.

Compute the average exam score for the three.

student1 = ("Jane Doe", 80, "Chemistry")
student2 = ("John Doe", 75, "Computer Science")
student3 = ("Mary Sue", 100, "Maths")

avg = (student1[1] + student2[1] + student3[1])/3

print(avg)

85.0

Question 2.#

Write a function swap that accepts as input a tuple tup_in of length=2 and returns a tuple tup_out with values of tup_in swapped.

def swap(tup):
    
assert swap(("a", "b"))==("b", "a"), "Test case 1 failed"

Question 3.#

Write a function that accepts as input a list sequence and returns (first_item, last_item).

def get_first_and_last(sequence):
    return sequence[0], sequence[-1]
    
assert get_first_and_last([4, 5, 1, 3, 2])==(4, 2),    "Test case 1 passed"
assert get_first_and_last([42])           ==(42,42),   "Test case 2 passed"
assert get_first_and_last("apple")        ==("a","e"), "Test case 3 passed"

print("All test cases passed")