# MLE - Week 2

Summary
-------
- Collections: *List* (mentioned in [Week 1](https://hackmd.io/@qhoang/S1bHJG7V_)), Tuple, Set, Dictionary
- Collection comparisons
- Function
- *while* Loop (mentioned in [Week 1](https://hackmd.io/@qhoang/S1bHJG7V_))
- Others

Tuple
-----

:point_right: Tuples are **immutable** sequences, typically used to store collections of *heterogeneous data* [[2]]

In another word, the *tuples* are useful when we need an immutable object to be stored in the [Set](#Set) or [Dictionary](#Dictionary) keys.

>Tuples may be constructed in a number of ways [[2]]:
>- Using a pair of parentheses to denote the empty tuple: **()**
>- Using a trailing comma for a singleton tuple:**a**, or **(a,)**
>- Separating items with commas: **a, b, c** or **(a, b, c)**
>- Using the tuple() built-in: **tuple()** or **tuple(iterable)**

Example:

>>> type(('hello',))
<class 'tuple'>
>>> type(('hello'))
<class 'str'>

:penguin: **Allow duplicates**: since tuple are indexed, tuples can have items with the same value. [[3]]

thistuple = ("apple", "banana", "cherry", "apple", "cherry")

| Method | Description |
| ------- | --------------------------------------------------------------------------------------- |
| count() | Returns the number of times a specified value occurs in a tuple |
| index() | Searches the tuple for a specified value and returns the position of where it was found |

:penguin: [Python Tuple Methods](https://www.w3schools.com/python/python_ref_tuple.asp)

Set
---
> :point_right: A set object is an **unordered** collection of distinct [hashable](#Hashable) objects.

:penguin: `set` is mutable. `frozenset` is an [immutable](#Immutable) set.

> Sets can be created by several means [[8]]:
> - Use a comma-separated list of elements within braces: `{'jack', 'sjoerd'}`
> - Use a set comprehension: `{c for c in 'abracadabra' if c not in 'abc'}`
> - Use the type constructor: `set()`, `set('foobar')`, `set(['a', 'b', 'foo'])`

### Methods
| Method | Description |
| ----------------------------- | ------------------------------------------------------------------------------ |
| add() | Adds an element to the set |
| clear() | Removes all the elements from the set |
| discard() | Remove the specified **item** |
| remove() | Removes the specified **element** |
| pop() | Removes an element from the set |
| copy() | Returns a copy of the set |
| difference() | Returns a set containing the difference between two or more sets |
| intersection() | Returns a set, that is the intersection of two other sets |
| issubset() | Returns whether another set contains this set or not |
| issuperset() | Returns whether this set contains another set or not |
| union() | Return a set containing the union of sets |

:penguin: [Python Set Methods](https://www.w3schools.com/python/python_ref_set.asp)

Dictionary
----------
:point_right: Like a *list*, a dictionary is a mutable collection of many values (key-value pair).

:penguin: Dictionaries can still **use integer values as keys**, just like lists use integers for indexes, but they do not have to start at 0 and **can be any number**. [[5]]

>>> spam = {12345: 'Luggage Combination', 42: 'The Answer'}

### Dictionary view objects
> :point_right: There are three dictionary methods that will return list-like values of the dictionary’s keys, values, or both keys and values: `keys()`, `values()`, and `items()`. [[5]]

:exclamation: The values returned by these methods (`dict_keys`, `dict_values`, and `dict_items`, respectively) are view objects and **not true lists**: they **cannot** be modified and do not have an `append()` method.

Any [hashable](#Hashable) objects can be the dictionary keys.

### Methods
| Method | Description |
| -------- | --------------------------------------------------------- |
| items() | Returns a list containing a tuple for each key value pair |
| keys() | Returns a list containing the dictionary's keys |
| values() | Returns a list of all the values in the dictionary |
| clear() | Removes all the elements from the dictionary |
| pop() | Removes the element with the specified key |

:penguin: [Python Dictionary Methods](https://www.w3schools.com/python/python_ref_dictionary.asp)

Comparison
----------
:penguin: Goups of data types:
- `Sequence Types` — list, tuple, range
- `Text Sequence Type` — str
- `Set Types` — set, frozenset
- `Mapping Types` — dict

| Operation / Characteristic | List | Tuple | Set | Dictionary | Remarks|
| ---------- | ------------------ | ------------------ | ------------------ | ------------------ |---|
| `x` in `s` | :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: | :heavy_check_mark: |Check if `x` in `s`|
| `x + t`|:heavy_check_mark:|:heavy_check_mark:|:x:|:x:|Concatenation|
|`s * n`|:heavy_check_mark:|:heavy_check_mark:|:x:|:x:|Replication|
|`s[i]`|:heavy_check_mark:|:heavy_check_mark:|:x:|:x:|Indexing|
|`s[i:j:k]`|:heavy_check_mark:|:heavy_check_mark:|:x:|:x:|Slicing|
|`len(s)`|:heavy_check_mark:|:heavy_check_mark:|:heavy_check_mark:|:heavy_check_mark:|Get length|
|`s.count(x)`|:heavy_check_mark:|:heavy_check_mark:|:x:|:x:|Count element `x`|
|Immutable|:x:|:heavy_check_mark:|:x:|:white_check_mark:| For `Dictionary`, only keys are immutable|
|Ordered|:heavy_check_mark:|:heavy_check_mark:|:x:|:x:| Since Python`3.7`, `Dictionary` order is guaranteed to be insertion order [[4]]. The dictionaries are still unordered, as you can’t access items in them using integer indexes.|
|Allow duplicates|:heavy_check_mark:|:heavy_check_mark:|:x:|:x:||
|Any data types|:heavy_check_mark:|:x:|:x:|:x:|`Tuple`, `Set` and `Dictionary` keys allow only hashable data types.|

Function
--------
:point_right: Function objects are created by function definitions.
:point_right: Methods are functions that are called using the attribute notation.

:exclamation: Function has to be defined before using.

Others
------

### *for* Loop vs *while* Loop
Use *for* loop when you know exactly the number of the loops.

:point_right: Should use *for* loop in most of the cases. Only use the *while* loop when the condition is unknown and determined at the runtime.

### Hashable
:point_right: An object is **hashable** if it has a hash value which **never changes** during its lifetime.[[6]]

| Data types | Hashable | Remarks |
| ---------------------------------------------------- | ------------------ | -------------------------------------------- |
| Immutable built-in objects | :heavy_check_mark: | |
| Mutable containers (such as lists or dictionaries) | :x: | |
| Immutable containers (such as tuples and frozensets) | :heavy_check_mark: | Only hashable if their elements are hashable |
| Objects which are instances of user-defined classes | :heavy_check_mark: | |

### Immutable
:point_right: An object with a fixed value. Immutable objects include numbers, strings and tuples. [[7]]

:penguin: [Glossary](https://docs.python.org/3/glossary.html#glossary)

### Enumerate()
:point_right: The enumerate() method adds counter to the iterable. The returned object is a enumerate object. [[1]]

Example:

days= { 'Mon', 'Tue', 'Wed','Thu'}
enum_days = enumerate(days)
# enumearte using loop
for enum_days in enumerate(days):
   print(enum_days)

for count,enum_days in enumerate(days,5):
   print(count,enum_days)

Output:

(0, 'Thu')
(1, 'Tue')
(2, 'Wed')
(3, 'Mon')
5 Thu
6 Tue
7 Wed
8 Mon

[1]: https://www.tutorialspoint.com/enumerate-in-python
[2]: https://docs.python.org/3/library/stdtypes.html#tuples
[3]: https://www.w3schools.com/python/python_tuples.asp
[4]: https://docs.python.org/3/library/stdtypes.html#mapping-types-dict
[5]: https://automatetheboringstuff.com/2e/chapter5/
[6]: https://docs.python.org/3/glossary.html#term-hashable
[7]: https://docs.python.org/3/glossary.html#term-immutable
[8]: https://docs.python.org/3/library/stdtypes.html#set-types-set-frozenset

###### tags: `mle` `week2` `basics` `dictionary` `function` `set` `tuple`