Python - Sequences
This lesson covers the following topics:
Sequences
In Python, a sequence is the generic term for an ordered set. There are several types of sequences in Python, the most important being strings, tuples, and lists. The location of each element within a sequence is called its index.
Furthermore, some sequences are immutable (strings & tuples) and others are mutable (lists).
- Immutable - means you can't change individual elements within the sequence.
- Mutable - means you can change individual elements within the sequence.
Strings
Strings are a special type of immutable sequence within Python that can only store characters. You can think of a string as a tuple where every element is a single character. Notice that we use quotes (double or single) to define a string.
myString = "hello world"
In memory, the string above is a sequence of characters that looks like:
Tuples
Tuples are a type of immutable sequence within Python that can store any data-type as its elements. However, since they are immutable, once they are defined, you cannot change any of the individual elements within that sequence. Notice that we use parenthesis to define a tuple.
myTuple = (1, 2, 3, 123, "Hello", True)
In memory, the tuple above is a sequence of elements that looks like:
Lists
Lists are a type of mutable sequence within Python that can store any data-type as its elements. Additionally, since they are mutable, you can change any of the individual elements within the sequence, even after it is initially defined. Notice that we use box-brackets to define a list.
myList = [1, 2, 3, 123, "Hello", True]
In memory, the list above is a sequence of elements that looks like:
| Operation | Notes | Example | Output |
|---|---|---|---|
| + | Concatenates (joins) two sequences. | (1, 2) + (3, 4) | (1, 2, 3, 4) |
| * | Repeats a sequence a number of times. | 3 * (1, 2) | (1, 2, 1, 2, 1, 2) |
| in | Returns True/False if an element is in a sequence. | 'a' in ['a', 'b', 'c'] | True |
| mySeq[i] | Returns the element of the sequence at index i. *Left to right, the first element starts at index 0. *Right to left, the last element starts at index -1. |
mySeq = ['x', 'y', 'z'] mySeq[1] mySeq[-1] |
'y' 'z' |
| mySeq[i:j] | Returns a slice of the sequence, starting at element i and ending at element j (not including j). *If i is left blank, it will start at the beginning. *If j is left blank, it will go until the end. |
mySeq = [9, 8, 7, 6, 5, 4] mySeq[2:5] mySeq[:2] mySeq[:] |
[7, 6, 5] [9, 8] [9, 8, 7, 6, 5, 4] |
| len(mySeq) | Returns the length of a sequence (# of elements). | mySeq = ['a', 'b', 'c'] len(mySeq) |
3 |
Below are some operations that are specific to lists, due to them be mutable, which means you can change elements within the list.
(strings and tuples are immutable, so they lack this functionality)
| Operation | Notes | Example | Output |
|---|---|---|---|
| myList[i] = e | Sets the value at index i to the value e. | myList = ['a', 'b', 'c'] myList[0] = 1 print(myList) |
[1, 'b', 'c'] |
| myList.append(e) | Adds element e to the end of the list. | myList = ['a', 'b', 'c'] myList.append('d') print(myList) |
['a', 'b', 'c', 'd'] |
| myList.clear() | Removes all elements from a list. | myList = ['a', 'b', 'c'] myList.clear() print(myList) |
[] |
| myList.index(e) | Returns the index of the first occurence of e. | myList = ['a', 'b', 'c', 'b'] myList.index('b') |
1 |
| myList.insert(e, i) | Inserts element e at index i. All other elements get pushed back one position. |
myList = ['a', 'b', 'c'] myList.insert(True, 1) print(myList) |
['a', True, 'b', 'c'] |
| myList.pop(i) | Removes (and returns) the element at index i. | myList = ['a', 'b', 'c'] myList.pop(0) print(myList) |
'a' ['b', 'c'] |
| myList.remove(e) | Removes first occurrence of element e. | myList = ['a', 'b', 'c', 'b'] myList.remove('b') print(myList) |
['a', 'c', 'b'] |
| myList.reverse() | Reverses the order of a list. | myList = ['a', 'b', 'c', 'd'] myList.reverse() print(myList) |
['d', 'c', 'b', 'a'] |
| myList.sort() | Sorts a list (ascending by default). | myList = [3, 1, 4, 2] myList.sort() print(myList) |
[1, 2, 3, 4] |
# Here is an example of using the index to iterate through a sequence.
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
myString = "Simplicity is the ultimate sophistication. – Leonardo da Vinci"
for i in range(len(myString)):
if myString[i] == 'i':
print(f"Found an 'i' at index {i}.")
print("The loop has terminated.")
# Here is an example of an "enhanced for loop", where the iterator variable will be the VALUE of each element,
# as opposed to the INDEX. (we lose a little information with this method)
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
myTuple = (0, 5, 7, 5, 7, 1, 5, 0, 9, 2, 7, 9, 6, 2, 7, 3, 6, 7, 7, 5)
for value in myTuple:
if value == 2:
print("I found a 2, but I can't tell you what index it's at.")
print("The loop has terminated.")
# This example traverses a famous quote and counts the total words and composite words in the quote.
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
quote = "One day the people that don’t even believe in you will tell everyone how they met you."
author = "Johnny Depp"
words = 1
composite = 0
for letter in quote:
if letter == " ":
words += 1
if letter == "’":
composite += 1
print(f"This quote has {words} total word(s) in it and {composite} composite word(s).")
# This example creates a list of 20 random numbers in the range from 0 to 100 (inclusive),
# then looks for the value of 42 within the list.
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
import random
randomNumbers = []
for i in range(20):
num = random.randrange(1, 101)
randomNumbers.append(num)
print(randomNumbers)
for i in range(len(randomNumbers)):
if randomNumbers[i] == 42:
print(f"Found a 42 at index {i}.")
# This example takes a tuple of numeric codes that represent items in a game
# and builds a new tuple based on the values.
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gameCodes = (3, 1, 0, 2, 0, 0, 2, 3, 0, 0, 2, 0, 3, 3)
gameSprites = ()
for i in range(len(gameCodes)):
if gameCodes[i] == 0:
gameSprites += ("Grass",)
elif gameCodes[i] == 1:
gameSprites += ("Hero",)
elif gameCodes[i] == 2:
gameSprites += ("Enemy",)
elif gameCodes[i] == 3:
gameSprites += ("Tree",)
print(gameSprites)