\(\renewcommand\AA{\unicode{x212B}}\)
Sequence Data Types
- Python supports a range of types to store sequences. Here we will
explore lists, sets, tuples and dictionaries. The string type is also
considered a sequence but for our purposes here we shall consider it
as a simple list of characters.
- A list is essentially an ordered collection of elements where the
ordering is defined by the creator.
- Lists are created using square brackets to enclose the sequence of
elements
- Elements can be added using the
append()
function
- Access to a specific element is done again by the square bracket
operator by providing the required index of the element. Note that
in Python the first index is 0
- Removing an element can be done by the
del
command or the
.remove()
function
- Replacement is also done using the square-bracket operator
lottery_numbers = [1,2,3,4,5,6]
bonus = 7
lottery_numbers.append(bonus)
# print the first element
print(lottery_numbers[0])
# print the last element
print(lottery_numbers[6])
print(lottery_numbers)
lottery_numbers.remove(5) # Removes first occurrence of value 5 in list
del lottery_numbers[0] # Removes the 0th element of the list
print(lottery_numbers)
lottery_numbers[3] = 42
print(lottery_numbers)
Givest the output:
1
7
[1, 2, 3, 4, 5, 6, 7]
[2, 3, 4, 6, 7]
[2, 3, 4, 42, 7]
- The square bracket operator also provides an operation known as
slicing.
- Slicing allows contiguous portions of lists to be sectioned out by
using
[i:j]
syntax where i and j are indexes.
- In this case it is helpful to think of the indices of the sequence
slightly differently. Instead of thinking of each index as being
assigned to a specific element within the list, think of them as
being assigned to the boundaries of the elements, e.g.
- The output of a slice operation is then much more obvious as it
simple slices out the items within the boxes contained by the given
range.
- This also works with strings
my_list = ['M','A', 'N', 'T', 'I', 'D']
print(my_list[1:4])
my_string = 'MANTID'
print(my_string[1:4])
Gives the output:
- Lists can be sorted using the
sort()
function which modifies the
list in place.
my_list = [5,4,3,2,7]
print(my_list)
my_list.sort()
print(my_list)
Gives the output:
[5, 4, 3, 2, 7]
[2, 3, 4, 5, 7]
- The default sorting criterion is less-than where items lower in the
list are “less-than” items higher in the list. You can reverse this
with,
l = [5,4,3,2,7]
l.sort(reverse=True)
print(l) #prints list in descending order
Gives the output:
Category:Tested Examples