List 1, 2, 3, 4 if (list) % 2 == 0 print(list)

9. Lists¶

A list is an ordered set of values, where each value is identified by an index. The values that make up a list are called its elements. Lists are similar to strings, which are ordered sets of characters, except that the elements of a list can have any type. Lists and strings — and other things that behave like ordered sets — are called sequences.

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Python Lists
Print lists in Python (4 Different Ways)

Python Lists

Lists are one of the four built-in data structures in Python. Other data structures that you might know are tuples, dictionaries, and sets. A list in Python is different from, for example, int or bool, in the sense that it's a compound data type: you can group values in lists. These values don't need to be of the same type: they can be a combination of boolean, String, integer, float values.

List literals are a collection of data surrounded by brackets, and the elements are separated by a comma. The list is capable of holding various data types inside it, unlike arrays.

For example, let's say you want to build a list of courses then you could have:

courses = ['statistics', 'python', 'linear algebra']

Note that lists are ordered collections of items or objects. This makes lists in Python "sequence types", as they behave like a sequence. This means that they can be iterated using for loops. Other examples of sequences are Strings, tuples, or sets.

Lists are similar in spirit to strings you can use the len() function and square brackets [ ] to access the data, with the first element indexed at 0.

Tip: if you'd like to know more, test, or practice your knowledge of Python lists, you can do so by going through the most common questions on Python lists here.

Now, on a practical note: you build up a list with two square brackets (start bracket and end bracket). Inside these brackets, you'll use commas to separate your values. You can then assign your list to a variable. The values that you put in a Python list can be of any data type, even lists!

Take a look at the following example of a list:

# Assign integer values to `a` and `b` a = 4 b = 9 # Create a list with the variables `a` and `b` count_list = [1,2,3,a,5,6,7,8,b,10] count_list [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Note that the values of a and b have been updated in the list count_list.

Creation of List

A list is created by placing all the items inside a square bracket [] separated by commas. It can have an infinite number of elements having various data types like string, integer, float, etc.

list1 = [1,2,3,4,5,6] #with same data type list1 [1, 2, 3, 4, 5, 6] list2 = [1, 'Aditya', 2.5] #with mixed data type list2 [1, 'Aditya', 2.5]

Accessing Elements from a List

Elements from the list can be accessed in various ways:

Index based: You can use the index operator to access the element from a list. In python, the indexing starts from 0 and ends at n-1, where n is the number of elements in the list.

Indexing can be further categorized into positive and negative indexing.

index = [1,2,3,4,5] index[0], index[4] #positive indexing (1, 5) index[5] #this will give an error since there is no element at index 5 in the list --------------------------------------------------------------------------- IndexError Traceback (most recent call last) in ----> 1 index[5] #this will give an error since there is no element at index 5 in the list IndexError: list index out of range index[-1], index[-2] #negative indexing, here -1 means select first element from the last (5, 4)

Slice based: This is helpful when you want to access a sequence/range of elements from the list. The semicolon (:) is used as a slice operator to access the elements.

index[:3], index[2:] ([1, 2, 3], [3, 4, 5])

List Methods

Some of the most commonly used list methods are :

Print lists in Python (4 Different Ways)

Printing a list in python can be done is following ways:

Using for loop : Traverse from 0 to len(list) and print all elements of the list one by one using a for loop, this is the standard practice of doing it.
Python

# Python program to print list

# using for loop

a = [1, 2, 3, 4, 5]

# printing the list using loop

for x in range(len(a)):

print a[x],

Output: 1 2 3 4 5

Without using loops: * symbol is use to print the list elements in a single line with space. To print all elements in new lines or separated by space use sep=”\n” or sep=”, ” respectively.

Python

# Python program to print list 
# without using loop 

a = [1, 2, 3, 4, 5]

# printing the list using * operator separated 
# by space 

print(*a)

# printing the list using * and sep operator 

print("printing lists separated by commas")

print(*a, sep = ", ")

# print in new line 

print("printing lists in new line")

print(*a, sep = "\n")

Output: 1 2 3 4 5 printing lists separated by commas 1, 2, 3, 4, 5 printing lists in new line 1 2 3 4 5

Convert a list to a string for display : If it is a list of strings we can simply join them using join() function, but if the list contains integers then convert it into string and then use join() function to join them to a string and print the string.

Python

# Python program to print list 
# by Converting a list to a 
# string for display 

a =["Geeks", "for", "Geeks"]

# print the list using join function() 

print(' '.join(a))

# print the list by converting a list of 
# integers to string 

a = [1, 2, 3, 4, 5]

print str(a)[1:-1]

Output: Geeks for Geeks 1, 2, 3, 4, 5

Using map : Use map() to convert each item in the list to a string if list is not a string, and then join them:

Python

# Python program to print list 
# print the list by converting a list of 
# integers to string using map 

a = [1, 2, 3, 4, 5]

print(' '.join(map(str, a)))

print"in new line"

print('\n'.join(map(str, a)))