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])

dict Dictionaries

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.