You will often need to group several elements in your program in order to process them as a single object.For this you will need to use different methods in collections.One of the most useful methods in collections module, in Python is a list.

Table Of Content

Lists In Python
Characteristics Of Lists
Methods In lists
Find the length of list using naive method
Find the length using len method
Find the length of list of lists
TIme COmplexity of running len method

Lists In Python

A list contains is a collection of multiple elements or items or arbitrary objects (in the programming sense).They are like arrays in the other languages but more versatile and flexible. 



Characteristics Of Lists

Lists are ordered i.e., their order is reserved after they are defined(unlike dictionaries or sets).
Lists are mutable i.e., their elements can be changed,or resized(unlike tuple or string).
Lists can contain elements with different data types with a finiteness.
Lists items can be concatenated
You can use logical operators with lists to test conditions.

Methods In Lists

Any element can be accessed using square brackets[] notation or .index(x) method.

{Note that the indexing starts at 0}

Elements can be appended to lists with the .append(x) , .insert(i,x) or .extend(iterable) method.
Elements can be removed from the lists with the .remove(x) or .pop() or .clear() element.
Elements can be changed by accessing and then assigning using = operator.
Elements can be concatenated and returned as a single object with .join() method.
A string can be converted to a list with the list() method.

There are many more useful methods in lists and you can grab insights of them, dynamically slicing lists and their powerful applications including real-time projects with live assistance, in Python Online Training by FITA.

Program 1: Finding the length of list using len method

len is a built-in method in python which returns the size of the object passed to it, irrespective of the index of last element.When a string is passed as an argument it returns the number of characters in the string.It is often used technique in programs these days.



print(‘length of our list: ‘,len(my_list))


Length of our list: 3

Program 2: Finding the length of list with a custom method.



for _ in my_list:


print(‘size of list with my method: ‘,counter)



size of list with my method: 6

Program 3: Find the length of the nested list

Nested lists have lists as an element in them. You can consider a nested list as a matrix..We can find the length of such lists, The length of the list can be using len method and the length of a sub list can be found by passing the sub list as argument to len method.

my_list=[[‘list 1’,‘element 1’,’element 2’],[‘list 2’, ‘element 1’, ‘element 2’]]

print(‘length of the list: ’,len(my_list)) 

print(‘length of the first sub list: ’,len(my_list[0]))



length of the list: 2

length of the first sub list: 3

The other ways to find the length of the list includes using length_hint() and __len__ method. Now you can also write your own unique program to find the length of the nested list using for or while loops.

Time Complexity Of len function

len() function actually calls the __len__ method internally to return the size of the object and every object (list, tuple, set, dictionary, set, array.array) has its own __len__ method, so all it does is return self.length.

So the time complexity of the len() method is O(1) (in big-O notation) or constant time.

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