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## Defining Sequences in Python

A sequence in Python represents an arrangement of items in a specific order. It's a data structure with elements indexed by their position, allowing you to access and manipulate individual elements based on their index. Sequences can store a collection of items from various data types like integers, strings, and even user-defined data types.

### Sequence Types in Python

Python offers several built-in sequence types to work with, including:

**Lists:**Mutable and ordered collection of items, which can be of any data type. Syntax:`[item1, item2, item3]`

**Tuples:**Immutable and ordered collection of items, which can be of any data type. Syntax:`(item1, item2, item3)`

**Strings:**Immutable and ordered collection of characters. Syntax:`'string'`

or`"string"`

**Ranges:**Ordered and immutable sequence of numbers, commonly used for looping a specific number of times. Syntax:`range(start, stop, step)`

**Mutable** means that the elements within a sequence can be changed after its creation, while **Immutable** sequences cannot be modified once they are created.

### Python Sequences of Numbers

The `range()`

function in Python creates a sequence of numbers, which is helpful for performing tasks with loops. By default, the function starts counting from 0 and increments by 1 with each loop. The `range()`

function takes three arguments:

**start:**The starting value of the range (optional).**stop:**The ending value of the range (not included).**step:**The interval between each number (optional).

When you create a range sequence, you can use a for loop to iterate through its elements:

```
for number in range(5, 15, 2):
print(number)
```

In the example above, the loop will start at 5, end before 15 and increment by 2 each time, so it prints these numbers: 5, 7, 9, 11, and 13.

#### Manipulating Python Sequences

Sequences offer various built-in methods and operations to modify and manipulate their elements. Here are some common operations:

Operation | Description | Example |

Indexing | Access individual elements in a sequence by their position. | `sequence[index]` |

Slicing | Extract a portion of a sequence by specifying start, stop, and step values. | `sequence[start : stop : step]` |

Concatenation | Combine two sequences together. | `sequence1 + sequence2` |

Repetition | Create a sequence that repeats the original sequence a given number of times. | `sequence * n` |

Length | Determine the number of elements in a sequence. | `len(sequence)` |

Search | Find the first occurrence of an item in a sequence. | `sequence.index(item)` |

Count | Count the number of times an item appears in a sequence. | `sequence.count(item)` |

Keep in mind that some sequence operations only apply to mutable sequences, such as lists, while others can be used with both mutable and immutable sequences. For example, slicing and concatenation operations are applicable to lists, tuples, and strings, while methods like `append()`

or `remove()`

only work with mutable sequences, such as lists.

## Exploring Fibonacci Sequence with Python

Fibonacci sequence is a sequence of numbers where each number is the sum of the two preceding ones, starting from 0 and 1. It looks like this:

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...

In this section, we will explore multiple ways of implementing Fibonacci Sequence in Python, including recursive, iterative, and optimised approaches with memoization.

### Recursive Fibonacci in Python

A common method to compute the nth Fibonacci number is by using a recursive function. In this approach, we define a function that computes the Fibonacci number as a sum of the previous two numbers in the sequence, and we use the function recursively to return the nth Fibonacci number. Here's the code:

```
def recursive_fibonacci(n):
if n == 0:
return 0
elif n == 1:
return 1
else:
return recursive_fibonacci(n - 1) + recursive_fibonacci(n - 2)
nth_fibonacci_number = recursive_fibonacci(10)
print(nth_fibonacci_number)
```

The recursive implementation of the Fibonacci sequence is straightforward to understand, but it's not efficient for larger values of n. Its time complexity is \(O(2^n)\), which makes it relatively slow for larger inputs.

### Iterative Fibonacci in Python

The iterative approach to computing the Fibonacci sequence is more efficient than the recursive one. In this method, we use a loop to compute the Fibonacci numbers up to the nth term. Here's the code:

```
def iterative_fibonacci(n):
if n == 0:
return 0
elif n == 1:
return 1
else:
a, b = 0, 1
for _ in range(n - 1):
a, b = b, a + b
return b
nth_fibonacci_number = iterative_fibonacci(10)
print(nth_fibonacci_number)
```

The iterative implementation of the Fibonacci sequence has a time complexity of \(O(n)\), which is a significant improvement over the recursive approach. It's more efficient for larger inputs and considered a better solution in most cases.

### Optimising Fibonacci Sequence with Memoization

Memoization is a technique used to optimise recursive algorithms by caching and reusing the results of expensive function calls. In Python, we can implement memoization using a dictionary or the built-in `functools.lru_cache`

decorator. Here's the code:

```
from functools import lru_cache
@lru_cache(maxsize=None)
def memoized_fibonacci(n):
if n == 0:
return 0
elif n == 1:
return 1
else:
return memoized_fibonacci(n - 1) + memoized_fibonacci(n - 2)
nth_fibonacci_number = memoized_fibonacci(10)
print(nth_fibonacci_number)
```

The memoized implementation of the Fibonacci sequence stores calculated Fibonacci numbers in a cache, so they don't have to be recomputed. This method reduces the time complexity to \(O(n)\), just like the iterative approach, but with the elegance of the recursive algorithm.

In conclusion, implementing Fibonacci Sequence in Python can be done in multiple ways, including recursive, iterative, and memoized approaches. While the recursive method is more intuitive, it's not efficient for larger inputs. In contrast, the iterative and memoized methods offer better performance, making them more suitable for practical applications.

## Mastering Escape Sequences in Python

Escape sequences are an essential tool when working with Python strings, as they enable you to utilise special characters and formatting that would otherwise be impossible or complicated to include. In this section, we will explore what escape sequences are, their common uses, and how to utilise them effectively in Python strings.

### Introduction to Escape Sequences

Escape sequences are character combinations that are used to represent special characters or formatting instructions in programming languages, including Python. They help to include characters that have a specific meaning in a programming language, or that cannot be easily typed using a keyboard.

#### What is an Escape Sequence in Python?

In Python, escape sequences are character sequences starting with a backslash \(\textbackslash\), followed by one or more characters that specify the desired special character or formatting instruction. They are interpreted by the Python interpreter as a single unit, representing the actual character or instruction that the sequence was intended to convey.

#### Commonly Used Escape Sequences in Python

There are several escape sequences available in Python, making it easy to include special characters and apply different formatting options within your Python strings. Some commonly used escape sequences in Python include:

`\n`

: Inserts a newline character, causing the text to start a new line.`\t`

: Inserts a horizontal tab character, providing a specific amount of space between characters.`\\`

: Inserts a literal backslash character into the string.`\'`

: Inserts a single quote character within the string, without ending the string input.`\"`

: Inserts a double quote character within the string, without ending the string input.`\xHH`

: Represents a character with the specified two-digit hexadecimal code`HH`

.`\uHHHH`

: Represents a Unicode character with the specified four-digit hexadecimal code`HHHH`

.`\UHHHHHHHH`

: Represents a Unicode character with the specified eight-digit hexadecimal code`HHHHHHHH`

.

```
print("Hello, World!")
print("Hello,\\nWorld!")
print("Hello,\\tWorld!")
print('I\\\'m learning Python!')
```

In the example above, the strings printed contain various escape sequences, including `\n`

for newline, `\t`

for a tab, and `\'`

to insert a single quote without ending the string.

### Utilising Escape Sequences in Python Strings

Using escape sequences in Python strings is straightforward. Whenever you want to incorporate a special character or instruction in your string, just insert the appropriate escape sequence where it is needed.

```
print("This is a string with a newline\\nAnd this is a new line.")
print("Here is a tab\\tthat creates some space in the middle.")
print("This string contains both single \\' and double \\\" quotes.")
print("\\u03C0 represents the Greek letter Pi.")
```

In the above examples, we have effectively used escape sequences in different scenarios to include newline characters, horizontal tabs, single and double quotes, and Unicode characters within our Python strings.

It is important to remember that escape sequences should be used with care, especially when working with user-generated input or data from external sources, as they can affect the behaviour and security of your Python code. Always ensure that you properly handle and validate user input and data by using appropriate string manipulation and sanitisation techniques before incorporating them into your code with escape sequences.

## Python Sequence - Key takeaways

Python Sequence: Represents an arrangement of items in a specific order and it's a data structure with elements indexed by their position; it can store various data types like integers, strings, and user-defined data types.

Sequence Types: Python offers built-in sequence types including Lists (mutable), Tuples (immutable), Strings (immutable), and Ranges (immutable, sequences of numbers).

Fibonacci Sequence Python: A sequence of numbers where each number is the sum of the preceding two, starting from 0 and 1; can be implemented using recursive, iterative, and optimising methods with memorization.

Escape Sequence in Python: Character sequences starting with a backslash (\), followed by one or more characters that specify a special character or formatting instruction.

Python Sequences of Numbers: The

`range()`

function creates a sequence of numbers for operations like looping; it takes three arguments (start, stop, step).

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