Python - Collections - list

 Class  notes: 2/26/2021

Python Collections:

- When we want to treat some data as a group, then it is not good to store the data in individual variables.

- We need to use a collection to store the data a a group

- For eg, An university offers different courses to her students. So, all the courses should be stored as a group in a collection.

Note: list is mutable which allows us to change its contents. 

Say in restaurant, they can remove one item from their one item.

say, fish is not available, they can remove from menu for short time and

they may add some non-veg item. so this way contents can be changed.

- In python, we have following collection types

list

tuple

set

frozenset

dict

List

* List can be used to store the group of elements in a sequence.

* a list can store both homogeneous and hetrogeneous elements

   what is that mean?

   -> homegeneour -> same type

   -> hetrogeneous -> different type

* A list is represented with [] (square braces)

  within square braces, we can seperate elements with comma ,

* list is a mutable object.

* a list can store duplicate elements

* each element in a list has a positive & negative index.

important points

sequance

mutable

duplicate 

For eg

phone_numbers=[1234,2345, 3423,2123]

these are list of phone numbers. list of homogenous elements because these are all integers.

airlines=["AI",10,"JA",6]

if you look at this, there are two strings and two integers.

so, this is a list of hetrogeneous. because elements are not of same types.

You can have float value as well.

We can also create empty list as well. For eg,

sample=[ ]  # empty list

lst1=[ ]

lst2=[10,'x',20.15,True]

we have list containing 4 elements

This is list with known size and known elements.

Lets look at another eg

lst3=[None]*5

none is unknown but list will create element with 5 elements but we don't know. We will append later

we create a list with 5 element with known size and unknow elements

length of a list:

airlines=["AI","JA","LT", "VA"]

print(len(airlines))

output: 4

the output is the length of elements

courses=['Python', 'Data Science', 'Big Data']

index   0     1 2

negative index  -3    -2 -1

for last element, so do not use comma

print(courses[1])  => output is Data Science

print(courses[3])  => output is an "error"

because list has 3 items with index 0, 1, and 2.

print(Courses[-3]) => output? its python

because negative index starts with -1.