Week2: Loops

Loops

Ability in Python and other programming languages, to do something again and again.

while Loops

• Runs for forever unless certain conditions are met.

An infinite loop (design flaw):

i = 3
while i != 0:
    print('meow')

• This code will run for forever unless interrupted by the user (Ctrl + C).
• i != 0 will always remain True, we change the value of i inside the loop on each run.

Well designed while loop:

i = 3
while i != 0:
    print('meow')
    i = i - 1

• On each run, i = i - 1 reduces the value of i by one digit. Until i != 0 condition is no longer true, and the loop stops.

for loop

In Python, a for loop is a control flow statement used to iterate over a sequence (such as a list, tuple, string etc.) or any other iterable object.

for i in [0, 2, 3]:
    print("meow")
#OR using range() function
for i in range(3):
    print("meow")

We are not using the i variable in the code, though it has a use case for holding the range value.

The more Pythonic approach is to use _, when you don’t care about the variable used or not use it later in the code.

for _ in range(3):
    print("meow")

List []

A list in Python, is a ordered, and mutable data structure used to store a collection of items in a single variable.

• Defined using square brackets []
• Each list element is separated by commas
• Lists are heterogeneous, meaning they can contain elements of different data types (such as integers, strings, booleans or else) within the same structure.
• Each element has an index starting from [0].

len() function

It returns number of items (length or size) in an object, such as strings, lists, tuples, dictionaries, sets, etc.

students = ["Hermione", "Harry", "Ron"]
for i in range(len(students)):
  print(i + 1, students[i])

List Methods

To add an item to a list:

list.append("name")

The list.append() can also add/append another list to the particular list.

To add multiple items at once to the list

list.extend(["Silly", "Donkey King"])

The same list.append(["Silly", "Donkey King"]) would have added them as a list to the existing list.

Remove an item’s first instance from a list: For a single item:

list.remove("Silly")

You can remove a nested list:

list.remove(["Silly", "Donkey King"])

Add an item at the particular place in the list

list.insert([index], "Marty")

Reverse the order of a list

list.reverse()

Remove the last item from a list

list.pop()

To clear the whole list

list.clear()

List Comprehension

List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list. — W3School

newlist = [expression for item in iterable if condition == True]

list = []
for i in range(20):
    if i%2 == 0:
        list.append(i)
print(list)

Using list comprehension, the whole for loop can be condensed to a single line:

list = []
even_list = [i for i in range(20) if i%2==0]
print(even_list)

Dictionaries dict

A dictionary (or dict) is a built-in mutable mapping type that stores data as a collection of unique, hashable key-value pairs.

• Since Python 3.7, dictionaries are ordered by insertion, meaning they preserve the sequence in which keys were added.

Key characteristics:

• Keys must be immutable, while values can be any data type.
• Dictionaries are created using curly braces {} with colon-separated key-value pairs.

Dictionary Methods

Python dictionary methods are built-in functions designed to manipulate, access, and manage the key-value pairs stored in Python dictionaries.

If you want to get the value of specific key and don’t know if that exists:

dict.get("name", "Unknown")

If “name” is not a valid key, it will return “Unknown” instead of throwing error if we have tried to get it via dict.["name"].

To update the dictionary.

dict["name"] = "David"

Another method to update a dictionary is via dict.update(). It takes another dictionary as an arguments adds to the current dictionary.

dic.update({"name": "Dawood", "name": "Amin"})

To return all keys in the dict:

dict.keys()

To return all values in the dict:

dict.values()

Delete a particular key from the dictionary:

dict.pop("key")

To clear the whole dictionary key-value pairs:

dict.clear()

Return the key-value pairs as tuples in the dictionary

dict.items()

It returns the both key and value pairs as tuples, which we can access as:

for key, value in dict.items()
    print(key, value)

Dictionary Comprehension

Dictionary comprehension is used to create a dictionary in a short and clear way. It allows keys and values to be generated from a loop in one line. This helps in building dictionaries directly without writing multiple statements. — GeeksforGeeks

dict_comp = {key: value for key, value in iterable if condition}

Normal dictionary creation with a starting empty dict.

dict = {}
for i in range(10):
    if i%2 == 0:
        dict[i] = i ** 2
print(dict)

The dict comprehension:

dict = {}
dict_comp = {i:i**2 for i in range(10) if i%2==0}
print(dict_comp)

None

None is special built-in constant that represents the absence of a value or a null value. It’s the sole instance of the NoneType class and acts as a singleton, meaning there is only one None object in any Python interpreter session.

• None is distinct from 0, False, or an empty string "", though it evaluates to False in boolean contexts.
• Functions that don’t explicitly return a value automatically return None.

Tuples

Tuples are used to store multiple items in a single variable.

A tuple is a collection which is ordered and unchangeable. —W3School

• Tuples are written within parenthesis ().
• A tuple can contain different data-types at once
• A one item tuple can be created with ("item1", ), otherwise Python will not recognize it.
• The () will create an empty tuple.
• The len() can be used to find out the number of items in the tuple.

Tuples Items

Tuple items are ordered, unchangeable, and allow duplicate values.

Tuples items are indexed, the first item has index[0], the second item has index[1] etc.