Python | Selective value selection in list of tuples

Last Updated : 02 May, 2023

Sometimes, while using list of tuples, we come across a problem in which we have a certain list of keys and we just need the value of those keys from list of tuples. This has a utility in rating or summation of specific entities. Let's discuss certain ways in which this can be done.

Method #1 : Using dict() + get() + list comprehension

We can perform this particular task by first, converting the list into the dictionary and then employing list comprehension to get the value of specific keys using get function.

Python3

# Python3 code to demonstrate
# Selective Value selection in list of tuples
# using dict() + get() + list comprehension

# initializing list of tuples
test_list = [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]

# initializing selection list
select_list = ['Nikhil', 'Akshat']

# printing original list
print("The original list is : " + str(test_list))

# printing selection list
print("The selection list is : " + str(select_list))

# using dict() + get() + list comprehension
# Selective Value selection in list of tuples
temp = dict(test_list)
res = [temp.get(i, 0) for i in select_list]

# printing result
print("The selective values of keys : " + str(res))

Output

The original list is : [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]
The selection list is : ['Nikhil', 'Akshat']
The selective values of keys : [1, 3]

Time Complexity: O(n), where n is the length of the test_list.
Auxiliary Space: O(n), where n is the length of the test_list.

Method #2: Using next() + list comprehension

This particular problem can be solved using the next function which performs the iteration using the iterators and hence more efficient way to achieve a possible solution.

Python3

# Python3 code to demonstrate
# Selective Value selection in list of tuples
# using next() + list comprehension

# initializing list of tuples
test_list = [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]

# initializing selection list
select_list = ['Nikhil', 'Akshat']

# printing original list
print("The original list is : " + str(test_list))

# printing selection list
print("The selection list is : " + str(select_list))

# using next() + list comprehension
# Selective Value selection in list of tuples
res = [next((sub[1] for sub in test_list
             if sub[0] == i), 0) for i in select_list]

# printing result
print("The selective values of keys : " + str(res))

Output

The original list is : [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]
The selection list is : ['Nikhil', 'Akshat']
The selective values of keys : [1, 3]

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

Method #3 : Using for loop

Python3

# Python3 code to demonstrate
# Selective Value selection in list of tuples

# initializing list of tuples
test_list = [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]

# initializing selection list
select_list = ['Nikhil', 'Akshat']

# printing original list
print("The original list is : " + str(test_list))

# printing selection list
print("The selection list is : " + str(select_list))
res = []
for i in select_list:
    for j in range(0, len(test_list)):
        if(test_list[j][0] == i):
            res.append(test_list[j][1])
# printing result
print("The selective values of keys : " + str(res))

Output

The original list is : [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]
The selection list is : ['Nikhil', 'Akshat']
The selective values of keys : [1, 3]

Time complexity: O(n * m), where n is the length of the select_list and m is the length of the test_list.
Auxiliary space: O(k), where k is the length of the res list.

Method #4 : Using list comprehension without dict()

Here is an example of how you can use list comprehension to selectively select values from a list of tuples:

Python3

# Initialize the list of tuples
test_list = [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]

# Initialize the selection list
select_list = ['Nikhil', 'Akshat']

# Selectively select the values using list comprehension
res = [tuple[1] for tuple in test_list if tuple[0] in select_list]

print(res)  # Output: [1, 3]
# This code is contributed by Edula Vinay Kumar Reddy

Output

[1, 3]

Time complexity: O(n), where n is the number of tuples in the test_list.
Auxiliary space: O(m), where m is the number of selected names in the select_list.

Method #5: Using filter() + map() function

Initialize the list of tuples
Initialize the selection list
Define a lambda function that takes in a tuple and returns True if the first element of the tuple is in the selection list, otherwise False
Use the filter() function to create a new list of tuples that match the selection criteria
Use the map() function to extract the second element of each tuple in the filtered list
Convert the map object to a list
Print the resulting list of values

Python3

# initializing list of tuples
test_list = [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]

# initializing selection list
select_list = ['Nikhil', 'Akshat']

# printing original list
print("The original list is : " + str(test_list))

# printing selection list
print("The selection list is : " + str(select_list))

# using filter() + map() function
# Selective Value selection in list of tuples
res = list(map(lambda x: x[1], filter(lambda x: x[0] in select_list, test_list)))

# printing result
print("The selective values of keys : " + str(res))

Output

The original list is : [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]
The selection list is : ['Nikhil', 'Akshat']
The selective values of keys : [1, 3]

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

Method #6: Using a dictionary comprehension

Create a dictionary comprehension that filters the original list of tuples by checking if the first element of each tuple is in the selection list, and returns a dictionary with keys equal to the first element of the tuples and values equal to the second element of the tuples.
Use a list comprehension to create a list of values from the resulting dictionary.

Python3

# Python3 code to demonstrate
# Selective Value selection in list of tuples
# using a dictionary comprehension

# initializing list of tuples
test_list = [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]

# initializing selection list
select_list = ['Nikhil', 'Akshat']

# printing original list
print("The original list is : " + str(test_list))

# printing selection list
print("The selection list is : " + str(select_list))

# using a dictionary comprehension
# Selective Value selection in list of tuples
res_dict = {k: v for k, v in test_list if k in select_list}
res = [res_dict[k] for k in select_list]

# printing result
print("The selective values of keys : " + str(res))

Output

The original list is : [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]
The selection list is : ['Nikhil', 'Akshat']
The selective values of keys : [1, 3]

Time complexity: O(n)
Auxiliary space: O(n)

Approach using Numpy:

Note: Install numpy module using command "pip install numpy"

Algorithm:

Convert the list of tuples into a NumPy array.
Extract the first and second columns of the array separately as two 1D arrays.
Use np.where() function to get the indices where the first column matches the values in select_list.
Use the above indices to get the corresponding values from the second array.
Return the resulting array.

Python3

import numpy as np

# initializing list of tuples
test_list = [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]

# initializing selection list
select_list = ['Nikhil', 'Akshat']

# printing original list
print("The original list is : " + str(test_list))

# printing selection list
print("The selection list is : " + str(select_list))

# converting the list of tuples to a NumPy array
arr = np.array(test_list)

# extracting the first and second columns separately
col1 = arr[:, 0]
col2 = arr[:, 1]

# getting the indices where the first column matches the values in select_list
indices = np.where(np.isin(col1, select_list))

# using the indices to get the corresponding values from the second column
res = list(map(int,col2[indices]))

# printing the resulting array
print("The selective values of keys : " + str(list(res)))

Output:
The original list is : [('Nikhil', 1), ('Akash', 2), ('Akshat', 3), ('Manjeet', 4)]
The selection list is : ['Nikhil', 'Akshat']
The selective values of keys : [1, 3]
Time Complexity: O(n), where n is the length of the test_list.
Auxiliary Space: O(n), where n is the length of the test_list.

Note: The time complexity of the NumPy solution is the same as the dict() solution, but it may have better performance due to NumPy's efficient array operations.