Python | Get unique values from a list

Given a list, print all the unique numbers in any order.

Examples:

Input : 10 20 10 30 40 40
Output : 10 20 30 40 

Input : 1 2 1 1 3 4 3 3 5 
Output : 1 2 3 4 5  

Method 1: Traversal of the list

Using traversal, we can traverse for every element in the list and check if the element is in the unique_list already if it is not over there, then we can append it to the unique_list. This is done using one for loop and another if statement which checks if the value is in the unique list or not which is equivalent to another for a loop.  

Python

# Python program to check if two
# to get unique values from list
# using traversal

# function to get unique values


def unique(list1):

    # initialize a null list
    unique_list = []

    # traverse for all elements
    for x in list1:
        # check if exists in unique_list or not
        if x not in unique_list:
            unique_list.append(x)
    # print list
    for x in unique_list:
        print x,


# driver code
list1 = [10, 20, 10, 30, 40, 40]
print("the unique values from 1st list is")
unique(list1)


list2 = [1, 2, 1, 1, 3, 4, 3, 3, 5]
print("\nthe unique values from 2nd list is")
unique(list2)

Output

the unique values from 1st list is
10 20 30 40 
the unique values from 2nd list is
1 2 3 4 5

Time Complexity: O(n*n)
Auxiliary Space: O(n)

Method 2: Using Set

Using set() property of Python, we can easily check for the unique values. Insert the values of the list in a set. Set only stores a value once even if it is inserted more than once. After inserting all the values in the set by list_set=set(list1), convert this set to a list to print it. 

Python

# Python program to check if two
# to get unique values from list
# using set

# function to get unique values


def unique(list1):

    # insert the list to the set
    list_set = set(list1)
    # convert the set to the list
    unique_list = (list(list_set))
    for x in unique_list:
        print x,


# driver code
list1 = [10, 20, 10, 30, 40, 40]
print("the unique values from 1st list is")
unique(list1)


list2 = [1, 2, 1, 1, 3, 4, 3, 3, 5]
print("\nthe unique values from 2nd list is")
unique(list2)

Output

the unique values from 1st list is
40 10 20 30 
the unique values from 2nd list is
1 2 3 4 5

Time complexity: O(n), where n is length of list.
Auxiliary Space: O(n), where n is length of list.

Method 3: Using numpy.unique

Using Python’s import numpy, the unique elements in the array are also obtained. In the first step convert the list to x=numpy.array(list) and then use numpy.unique(x) function to get the unique values from the list. numpy.unique() returns only the unique values in the list. 

Python3

# Python program to check if two
# to get unique values from list
# using numpy.unique
import numpy as np

# function to get unique values


def unique(list1):
    x = np.array(list1)
    print(np.unique(x))


# driver code
list1 = [10, 20, 10, 30, 40, 40]
print("the unique values from 1st list is")
unique(list1)


list2 = [1, 2, 1, 1, 3, 4, 3, 3, 5]
print("\nthe unique values from 2nd list is")
unique(list2)

Output: 

the unique values from 1st list is
[10 20 30 40]

the unique values from 2nd list is
[1 2 3 4 5]

Time complexity: O(nlogn) due to the use of the sorting algorithm used by the numpy.unique() function. 
Auxiliary space: O(n) because numpy.unique() function creates a copy of the input array and then sorts it before returning the unique elements. 

Method #4: Using collections.Counter()

Using python import Counter() from collections print all the keys of Counter elements or we print directly by using the “*” symbol. Below is the implementation of above approach.

Python3

# Python program to check if two
# to get unique values from list
# importing counter from collections

from collections import Counter

# Function to get unique values


def unique(list1):

    # Print directly by using * symbol
    print(*Counter(list1))


# driver code
list1 = [10, 20, 10, 30, 40, 40]
print("the unique values from 1st list is")
unique(list1)


list2 = [1, 2, 1, 1, 3, 4, 3, 3, 5]
print("\nthe unique values from 2nd list is")
unique(list2)

Output

the unique values from 1st list is
10 20 30 40

the unique values from 2nd list is
1 2 3 4 5

Time Complexity: O(n), where n is the number of elements in the input list.
Auxiliary Space : O(n)

Method #5: Using reduce()

Using python import reduce() from functools and iterate over all element and checks if the element is a duplicate or unique value. Below is the implementation of the above approach.

Python

# Python program to check if two
# to get unique values from list

from functools import reduce

# Function to get unique values


def unique(list1):

    # Print directly by using * symbol
    ans = reduce(lambda re, x: re+[x] if x not in re else re, list1, [])
    print(ans)


# driver code
list1 = [10, 20, 10, 30, 40, 40]
print("the unique values from 1st list is")
unique(list1)


list2 = [1, 2, 1, 1, 3, 4, 3, 3, 5]
print("\nthe unique values from 2nd list is")
unique(list2)

Output

the unique values from 1st list is
[10, 20, 30, 40]

the unique values from 2nd list is
[1, 2, 3, 4, 5]

Method #6:Using Operator.countOf() method

Python3

# Python program to check if two
# to get unique values from list
# using traversal
import operator as op
# function to get unique values


def unique(list1):

    # initialize a null list
    unique_list = []

    # traverse for all elements
    for x in list1:
        # check if exists in unique_list or not
        if op.countOf(unique_list, x) == 0:
            unique_list.append(x)
    # print list
    for x in unique_list:
        print(x)


# driver code
list1 = [10, 20, 10, 30, 40, 40]
print("the unique values from 1st list is")
unique(list1)


list2 = [1, 2, 1, 1, 3, 4, 3, 3, 5]
print("\nthe unique values from 2nd list is")
unique(list2)

Output

the unique values from 1st list is
10
20
30
40

the unique values from 2nd list is
1
2
3
4
5

Time Complexity:O(N)
Auxiliary Space: O(N)

Method#7: Using pandas

Python3

import pandas as pd

# function to get unique values
def unique(list1):
    unique_list = pd.Series(list1).drop_duplicates().tolist()
    for x in unique_list:
        print(x)

# driver code
list1 = [10, 20, 10, 30, 40, 40]
print("the unique values from 1st list is")
unique(list1)

list2 = [1, 2, 1, 1, 3, 4, 3, 3, 5]
print("\nthe unique values from 2nd list is")
unique(list2)
#This code is contributed by Vinay Pinjala.

Output:

the unique values from 1st list is
10
20
30
40

the unique values from 2nd list is
1
2
3
4
5

Time Complexity:O(N)
Auxiliary Space: O(N)

Method #8: Using dict.fromkeys() –

Using the fromkeys() method of dictionary data structure we can fetch the unique elements.

Step – 1: Firstly we need to define a list which consists of duplicate elements.

Step – 2 : Then we need to use a variable in which we will store the result after using the fromkeys() method.

Step – 3 : We need to convert that result into list, as the fromkeys() method is part of the dictionary so by default it returns a dictionary with all the unique keys and None as their values.

Step – 4 : We will print the final result.

Python3

# python code to fetch
# unique values from list
# using dict.fromkeys() method


# defining a list which consists
# duplicate values
list1 = [10, 20, 10, 30, 40, 40]

list2 = [1, 2, 1, 1, 3, 4, 3, 3, 5]

# storing the result of the fromkeys()
# operation and converting it into list
unique_list_1 = list(dict.fromkeys(list1))

unique_list_2 = list(dict.fromkeys(list2))

# Printing the final result
print(unique_list_1,unique_list_2,sep="\n")

Output

[10, 20, 30, 40]
[1, 2, 3, 4, 5]

Time Complexity – O(n)
Space Complexity – O(n)