Python Dictionaries

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Dictionaries

In Python, a dictionary is a collection of key-value pairs. It is an unordered, mutable, and indexed collection of elements. Dictionaries are also sometimes referred to as “maps”, “hash maps”, or “associative arrays” in other programming languages.

my_dict = {"key1": "value1", "key2": "value2", "key3": "value3"}

The main characteristics of a list are:

1. Defined by curly braces {}, and colons are used to separate keys and values.
2. Iterable object: can be divided in smaller parts, and you can use loops
3. Mutable Object: which means that can be modified
4. Python Unordered Objects: can’t be sliced or indexed
5. Collection of element (values) associated to keys

Nota:

• keys can only be immutable objects, such as: integers, floats, strings, tuples
• Values can be any type of object, such as integers, floats, strings, booleans, as well as more complex data types such as lists, tuples, and even other dictionaries.

Example:

my_dict = {"name": "John", "age": 30, "is_student": True, "interests": ["reading", "sports"], "grades": {"math": 90, "science": 85}}

Key can't be repeated

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Concatenation and Replication

Python Operators

Concatenation Operator (+): This operator is used to concatenate two or more dictionaries into a one. For example:

Repetition Operator ( * ) : This operator is used to repeat a dictionary a specified number of times. For example:

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Comparison Operators

Here are some examples:

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Built-in Functions

Python offers numerous built-in functions for working with lists. Here are some of the most commonly used functions:

Getting a list length

len()

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> len(furniture)
# 4

Adding Values

append()

append adds an element to the end of a list:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> furniture.append('bed')
>>> furniture
# ['table', 'chair', 'rack', 'shelf', 'bed']

insert()

insert adds an element to a list at a given position:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> furniture.insert(1, 'bed')
>>> furniture
# ['table', 'bed', 'chair', 'rack', 'shelf']

Removing Values

del()

del removes an item using the index:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> del furniture[2]
>>> furniture
# ['table', 'chair', 'shelf']
 
>>> del furniture[2]
>>> furniture
# ['table', 'chair']

remove()

remove removes an item with using actual value of it:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> furniture.remove('chair')
>>> furniture
# ['table', 'rack', 'shelf']

Note:

If the value appears multiple times in the list, only the first instance of the value will be removed.

pop()

By default, pop will remove and return the last item of the list. You can also pass the index of the element as an optional parameter:

>>> animals = ['cat', 'bat', 'rat', 'elephant']
 
>>> animals.pop()
'elephant'
 
>>> animals
['cat', 'bat', 'rat']
 
>>> animals.pop(0)
'cat'
 
>>> animals
['bat', 'rat']

Sorting values

sort()

>>> numbers = [2, 5, 3.14, 1, -7]
>>> numbers.sort()
>>> numbers
# [-7, 1, 2, 3.14, 5]
 
furniture = ['table', 'chair', 'rack', 'shelf']
furniture.sort()
furniture
# ['chair', 'rack', 'shelf', 'table']

You can also pass True for the reverse keyword argument to have sort() sort the values in reverse order:

>>> furniture.sort(reverse=True)
>>> furniture
# ['table', 'shelf', 'rack', 'chair']

If you need to sort the values in regular alphabetical order, pass str.lower for the key keyword argument in the sort() method call:

>>> letters = ['a', 'z', 'A', 'Z']
>>> letters.sort(key=str.lower)
>>> letters
# ['a', 'A', 'z', 'Z']

You can use the built-in function sorted to return a new list:

>>> furniture = ['table', 'chair', 'rack', 'shelf']
>>> sorted(furniture)
# ['chair', 'rack', 'shelf', 'table']

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Slicing of a List

Note:

Also, slicing a list does not modify the original list - it returns a new list that contains only the specified portion of the original list.

You can slice a list in Python using the same syntax list[start:end:step] as you would use for slicing a string, where:

• start is the index of the first element you want to include in the slice
• end is the index of the first element you want to exclude from the slice
• step is the number of elements to skip between each element included in the slice

Here are some examples:

my_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
 
# slice the entire list
print(my_list[:])           # Output: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
 
# slice from index 0 to index 5 (excluding 5)
print(my_list[0:5])         # Output: [1, 2, 3, 4, 5]
 
# slice from index 7 to the end
print(my_list[7:])          # Output: [8, 9, 10]
 
# slice from index 2 to index 9, skipping every other element
print(my_list[2:9:2])       # Output: [3, 5, 7, 9]
 
# slice from the beginning to the end, skipping every other element
print(my_list[::2])         # Output: [1, 3, 5, 7, 9]
 
# slice from the end of the list to the beginning
print(my_list[::-1])        # Output: [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
 
# slice from the end of the list to index 2 (excluding 2), skipping every other element
print(my_list[-1:1:-2])     # Output: [10, 8, 6, 4]
 
# slice from index -5 (5th element from the end) to index -1 (excluding -1), skipping every other element
print(my_list[-5:-1:2])     # Output: [6, 8]
 
# slice from index -8 (8th element from the end) to index -5 (excluding -5)
print(my_list[-8:-5])       # Output: [3, 4, 5]
 
# slice from index -3 (3rd element from the end) to the end of the list
print(my_list[-3:])         # Output: [8, 9, 10]
 

Note:

Like with strings, you can mix positive and negative indices in a single slice, and you can omit any of the three parameters (start, end, and step) as needed.

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Quartz Index Example of a List

In Python, a list is a collection of elements, and you can access individual elements in the list by their position or index within the sequence.

• The index tarts at 0,
• The second character has an index of 1, and so on.

To access a specific element in a string, you can use its index within square brackets [], like this:

my_list = ["apple", "banana", "cherry"]
print(my_list[0])   # Output: "apple"
print(my_list[1])   # Output: "banana"
print(my_list[2])   # Output: "cherry"

Note:

The index must be an integer, and it must be within the range of valid indices for the string (from 0 to len(lits) - 1). Otherwise, a IndexError will be raised.

You can also use negative indices to access elements from the end of the list:

my_list = ["apple", "banana", "cherry"]
print(my_list[-1])   # Output: "cherry"
print(my_list[-2])   # Output: "banana"
print(my_list[-3])   # Output: "apple"

To find the index of a particular element in a list, you can use the index() method, just like with strings:

my_list = ["apple", "banana", "cherry"]
element = "banana"
index = my_list.index(element)
print(index)   # Output: 1

In this example, we define a list my_list and an element element, and then use the index() method to find the index of the first occurrence of element within my_list. The result is 1, which is the index of the second element in the list.

Note:

If the element element is not found in the list, a ValueError will be raised.

Modify an element using the index

you can also use the index operator [] to modify an element in a list. Here’s an example:

my_list = ["apple", "banana", "cherry"]
my_list[1] = "orange"
print(my_list)   # Output: ["apple", "orange", "cherry"]

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Loops in a List

In Python, a loop is a programming construct that allows you to iterate over a sequence of items, such as the characters in a string.

There are two types of loops in Python: the for loop and the while loop.

• we use a for loop to iterate over each character in the string.

  string = "hello"
  for char in string:
      print(char)

In Python, you can use loops to iterate over the elements in a list. There are two main types of loops you can use: for loops and while loops.

For Loops

• we use a for loop to iterate over each element in the list

  my_list = [1, 2, 3, 4, 5]
  for element in my_list:
      print(element)
   
  # This code will output alle the elements inside the list

  • You can also use the range() function to iterate over a range of indices in the list:

    my_list = [1, 2, 3, 4, 5]
    for i in range(len(2)):
        print(my_list[i])
     
    # This code will output the first 3 elemets (from o to 2)

While Loops

• while loop is designed to repeat a block of code while a specific condition is true

  my_list = [1, 2, 3, 4, 5]
  index = 0
  while index < len(my_list):
      print(my_list[index])
      index += 1

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