Question

Write a python program to create a third list after adding two lists.

Answer 1

Explanation:

One of the easiest ways are by using the + operator.

1. Join two list: list1 = ["a", "b" , "c"] list2 = [1, 2, 3] list3 = list1 + list2. ...
2. Append list2 into list1: list1 = ["a", "b" , "c"] list2 = [1, 2, 3] for x in list2: ...
3. Use the extend() method to add list2 at the end of list1: list1 = ["a", "b" , "c"] list2 = [1, 2, 3]

Answer 2

Answer:

Ways to concatenate two lists in Python

Let’s see how to concatenate two lists using different methods in Python. This operation is useful when we have numbers of lists of elements which needs to be processed in a similar manner.

Method #1 : Using Naive Method

In this method, we traverse the second list and keep appending elements in the first list, so that first list would have all the elements in both lists and hence would perform the append.

# Python3 code to demonstrate list

# concatenation using naive method

# Initializing lists

test_list1 = [1, 4, 5, 6, 5]

test_list2 = [3, 5, 7, 2, 5]

# using naive method to concat

for i in test_list2 :

test_list1.append(i)

# Printing concatenated list

print ("Concatenated list using naive method : "

+ str(test_list1))

Output:

Concatenated list using naive method : [1, 4, 5, 6, 5, 3, 5, 7, 2, 5]

Method #2 : Using + operator

The most conventional method to perform the list concatenation, the use of “+” operator can easily add the whole of one list behind the other list and hence perform the concatenation.

# Python 3 code to demonstrate list

# concatenation using + operator

# Initializing lists

test_list3 = [1, 4, 5, 6, 5]

test_list4 = [3, 5, 7, 2, 5]

# using + operator to concat

test_list3 = test_list3 + test_list4

# Printing concatenated list

print ("Concatenated list using + : "

+ str(test_list3))

Output:

Concatenated list using + : [1, 4, 5, 6, 5, 3, 5, 7, 2, 5]

Method #3 : Using list comprehension

List comprehension can also accomplish this task of list concatenation. In this case, a new list is created, but this method is a one liner alternative to the loop method discussed above.

# Python3 code to demonstrate list

# concatenation using list comprehension

# Initializing lists

test_list1 = [1, 4, 5, 6, 5]

test_list2 = [3, 5, 7, 2, 5]

# using list comprehension to concat

res_list = [y for x in [test_list1, test_list2] for y in x]

# Printing concatenated list

print ("Concatenated list using list comprehension: "

+ str(res_list))

Output:

Concatenated list using list comprehension: [1, 4, 5, 6, 5, 3, 5, 7, 2, 5]

Method #4 : Using extend()

extend() is the function extended by lists in Python and hence can be used to perform this task. This function performs the inplace extension of first list.

# Python3 code to demonstrate list

# concatenation using list.extend()

# Initializing lists

test_list3 = [1, 4, 5, 6, 5]

test_list4 = [3, 5, 7, 2, 5]

# using list.extend() to concat

test_list3.extend(test_list4)

# Printing concatenated list

print ("Concatenated list using list.extend() : "

+ str(test_list3))

Output:

Concatenated list using list.extend() : [1, 4, 5, 6, 5, 3, 5, 7, 2, 5]

Method #5 : Using * operator

Using * operator, this method is the new addition to list concatenation and works only in Python 3.6+. Any no. of lists can be concatenated and returned in a new list using this operator.

# Python3 code to demonstrate list

# concatenation using * operator

# Initializing lists

test_list1 = [1, 4, 5, 6, 5]

test_list2 = [3, 5, 7, 2, 5]

# using * operator to concat

res_list = [*test_list1, *test_list2]

# Printing concatenated list

print ("Concatenated list using * operator : "

+ str(res_list))

Output:

Concatenated list using * operator : [1, 4, 5, 6, 5, 3, 5, 7, 2, 5]

Method #6 : Using itertools.chain()

itertools.chain() returns the iterable after chaining its arguments in one and hence does not require to store the concatenated list if only its initial iteration is required. This is useful when concatenated list has to be used just once.

# Python3 code to demonstrate list

# concatenation using itertools.chain()

import itertools

# Initializing lists

test_list1 = [1, 4, 5, 6, 5]

test_list2 = [3, 5, 7, 2, 5]

# using itertools.chain() to concat

res_list = list(itertools.chain(test_list1, test_list2))

# Printing concatenated list

print ("Concatenated list using itertools.chain() : "

+ str(res_list))

Output:

Concatenated list using itertools.chain() : [1, 4, 5, 6, 5, 3, 5, 7, 2, 5]

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