How to merge two lists with or without duplicates python with example

How to merge two lists with or without duplicates python with example

This tutorial explains how to concatenate two lists into a single list. Duplicates are allowed.

Python merges two lists into a list with duplicates

There are multiple ways we can do

The below approaches merge the lists into a new list, the order is insertion and iteration order, Duplicates are allowed, No sorting.

#1 using for in loop:

In this approach,

  • Create an empty list to store merged lists
  • Iterate each element from lists, and append to the merged list using the append method
  • The order is the iteration order of lists.
  • Duplicates are allowed
one = [5,2,4,41]
two = [41, 15, 61]

output =[]
for item in one:
output.append(item)
for elem in two:
output.append(item)
print(output)

[5, 2, 4, 41, 41, 41, 41]

#2 Concatenate operator:

operator is + or += used to copy two lists and appends the elements into new list Here is an example python plus operator

one = [5,2,4]
two = [41, 15, 61]

output = one + two
print(output)

Output

[5, 2, 4, 41, 15, 61]

To use +=, the variable must be defined to host the result of the merge, otherwise, throws NameError: name 'output1' is not defined with the below code.

output1 += (one+two)
print(outpu1)

The same code can be rewritten using variable declaration with empty initialization[]

# += operator
output1=[]
output1 += (one+two)
print(output1)

Output:

#3 use Unpack operator

unpack operator(*) allows you to iterate each element and copies to the list. This is introduced in Python.3.5 version.

output1 = [*one,*two]
print(output1)

Output:

[5, 2, 4, 41, 15, 61]

How to merge two lists with unique values

These examples concatenate two lists and duplicates are removed. Append the list using the + operator, and pass this result to the Set constructor. The set does not allow duplicates. Convert the set to a List using a list constructor.

Here is an example

one = [5,2,4,41]
two = [41, 15, 61]
#allow duplicates
output = one + two
print(output)

# //unique elements

output1 =list(set(one+two))
print(output1)

[5, 2, 4, 41, 41, 15, 61]
[2, 4, 5, 41, 15, 61]

How to concatenate an existing list with another list in python?

This example does not create a new list but appends elements to an end of an existing list.

The extend method is used to merge one into another

Here is an example

one = [5,2,4,41]
two = [41, 15, 61]
one.extend(two)
print(one)

Output

[5, 2, 4, 41, 41, 15, 61]