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Python | Assign ids to each unique value in a list

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  • Last Updated : 29 Sep, 2022
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Sometimes, while working in web development domain or in competitive programming, we require to assign a unique id to each of the different values to track its occurrence for the count or any other required use case. Let’s discuss certain ways in which this task can be performed.

Method #1: Using defaultdict + lambda + list comprehension

The combination of the above functions can be used to accomplish this particular task. The defaultdict function performs the main task of assigning Ids using the lambda function, it assigns the current number of keys to every new key. The list comprehension is used in later stage to form the list. 

Python3




# Python3 code to demonstrate
# assigning ids to values
# using list comprehension + defaultdict + lambda
 
from collections import defaultdict
 
# initializing list
test_list = [5, 6, 7, 6, 5, 1]
 
# printing original list
print("The original list : " + str(test_list))
 
# using list comprehension + defaultdict + lambda
# assigning ids to values
temp = defaultdict(lambda: len(temp))
res = [temp[ele] for ele in test_list]
 
# print result
print("The ids of assigned values is : " + str(res))

Output

The original list : [5, 6, 7, 6, 5, 1]
The ids of assigned values is : [0, 1, 2, 1, 0, 3]

Method #2 : Using OrderedDict.fromkeys() + enumerate() + list comprehension

This method performs tasks similar to the above method. In this orderedDict.fromkeys function performs the function to remove the duplicates and enumerate function helps us get the indices of values to map them together. 

Python3




# Python3 code to demonstrate
# assigning ids to values using
# list comprehension + OrderedDict.fromkeys() + enumerate()
from collections import OrderedDict
 
# initializing list
test_list = [5, 6, 7, 6, 5, 1]
 
# printing original list
print("The original list : " + str(test_list))
 
# using list comprehension + OrderedDict.fromkeys() + enumerate()
# assigning ids to values
res = [{val: key for key, val in enumerate(
    OrderedDict.fromkeys(test_list))}
    [ele] for ele in test_list]
 
# print result
print("The ids of assigned values is : " + str(res))

Output

The original list : [5, 6, 7, 6, 5, 1]
The ids of assigned values is : [0, 1, 2, 1, 0, 3]

Method #3: Using in , not in operators and index() method

Python3




# Python3 code to demonstrate
# assigning ids to values
 
# initializing list
test_list = [5, 6, 7, 6, 5, 1]
 
# printing original list
print("The original list : " + str(test_list))
 
x = []
for i in test_list:
    if i not in x:
        x.append(i)
res = []
for i in test_list:
    res.append(x.index(i))
 
# print result
print("The ids of assigned values is : " + str(res))

Output

The original list : [5, 6, 7, 6, 5, 1]
The ids of assigned values is : [0, 1, 2, 1, 0, 3]

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