An array is a fundamental data structure used to store collections of elements that can be accessed by indexes. Arrays are widely used in programming due to their efficiency and simplicity.

• Arrays are of course ordered, and in most programming languages, the index starts at 0.

• Arrays use a contiguous block of memory. This allows efficient random access with indexes.

• In many programming languages, you have to specify the size of an array at initialization time, so it can reserve the appropriate amount of memory from the start. (but not in Python)

• In many programming languages (again, not in Python) you can store only one type of objects inside.

In Python, arrays are implemented using a versatile data structure called lists.

Counter(iterable) is a subclass of Python dict built-in.

The Counter class is a variation of dictionary that makes counting of elements in iterables way easier.

Counter is designed for counting hashable items in an iterable. It provides an easy way to count elements in lists, strings, and other iterables, allowing you to access counts by calling the keys.

Counter has method most_common(n=None). It lists the n most common elements and their counts, (from high to low) in tuples. If you don't provide n, it lists all elements counts.

And Counter also has method elements(). It creates an iterator with elements repeating as many times as their count.

It also acts similar to defaultdict in some cases, when you will call a key that don't exist, Counter will return a 0.

To use Counter, you must import it from the collections module.

defaultdict(default_factory, **kwargs)is a subclass of Python standard dictionary (dict).

Python creators added several "versions" of standard dict implementation to answer some of the most common user problems.

If you call a non existent key in a standard, normal dict in Python you will get a KeyError. You can avoid this with get() method but if you would like to provide a default value for all keys that you will call in your dict, then defaultdict is a way to go.

To use defaultdict, you must import it from the collections module.

default_factory needs to be a callable (such as int, list, lambda, etc.) used to calculate the default value. It cannot accept arguments and must return the value to be used as the default.

Note: If defaultdict is initialized without a default_factory, it behaves like a regular dictionary and will raise a KeyError for missing keys.

Dict Comprehensions in Python lets you quickly generate a dictionary from another iterable, similar to list comprehensions.

[<key_1>: <value_1> for <variable> in <iterable> if <expression_2>]

While doing so, you can optionally perform operations on keys and values going into your dict and also filter them based on conditions.

List Comprehensions in Python lets you quickly generate a list from another iterable.

[<expression_1> for <variable> in <iterable> if <expression_2>]

While doing so, you can optionally perform operations on arguments going into your list and also filter them.

Remember that even though we are calling this "List Comprehensions" we can simply transform them into:

• Tuple Comprehensions - simply replace [] with (). Important - Even though we are calling it a "tuple" comprehension, this will create a generator.

• Set Comprehensions - simply replace [] with {}. As you guessed, Set Comprehensions will create set so it will delete duplicates.