Python – Random insertion of elements K times

Given 2 list, insert random elements from List 2 to List 1, K times at random position.

Input : test_list = [5, 7, 4, 2, 8, 1], add_list = [“Gfg”, “Best”, “CS”], K = 2 Output : [5, 7, 4, 2, 8, 1, ‘Best’, ‘Gfg’] Explanation : Random elements from List 2 are added 2 times. Input : test_list = [5, 7, 4, 2, 8, 1], add_list = [“Gfg”, “Best”, “CS”], K = 1 Output : [5, 7, 4, 2, 8, 1, ‘Gfg’] Explanation : Random elements from List 2 are added 1 times.

Method : Using randint() + list slicing + loop + choice()

In this, choice() is used to get random elements from list 2 to insert into list 1 and, randint() is used to get index at which this needs to be inserted. Then list slicing is used to remake list according to newer order.

Python3

# Python3 code to demonstrate working of
# Random insertion of elements K times
# Using randint() + list slicing + loop + choice()
import random

# initializing list
test_list = [5, 7, 4, 2, 8, 1]

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

# initializing add list
add_list = ["Gfg", "Best", "CS"]

# initializing K
K = 3

for idx in range(K):

    # choosing index to enter element
    index = random.randint(0, len(test_list))

    # reforming list and getting random element to add
    test_list = test_list[:index] + [random.choice(add_list)] + test_list[index:]

# printing result
print("The created List : " + str(test_list))

Output

The original list : [5, 7, 4, 2, 8, 1]
The created List : ['CS', 'CS', 5, 7, 4, 'CS', 2, 8, 1]

Time complexity: O(n*n), where n is the length of the test_list. The randint() + list slicing + loop + choice() takes O(n*n) time
Auxiliary Space: O(n), extra space of size n is required

Method #2: Using random.choice()

step-by-step approach :

1. Define the function insert_random_elements which takes three arguments: test_list, add_list, and K.
2. Create a range object range_obj that contains the indices where new elements can be inserted. The range object should span from 0 to the length of test_list plus K.
3. Loop over K times.
4. Generate a random index rand_index using random.choice(range_obj) to randomly select an index from range_obj.
5. Insert a random element from add_list into test_list at the randomly chosen index.
6. Return the modified test_list.

Python3

import random

def insert_random_elements(test_list, add_list, K):
    # Create range object for indices where new elements can be inserted
    range_obj = range(len(test_list) + K)

    # Insert elements K times
    for i in range(K):
        # Generate random index
        rand_index = random.choice(range_obj)

        # Insert random element at random index
        test_list.insert(rand_index, random.choice(add_list))

    return test_list
test_list = [5, 7, 4, 2, 8, 1]
add_list = ["Gfg", "Best", "CS"]
K = 3
print(insert_random_elements(test_list, add_list, K))

Output

[5, 7, 4, 'Best', 'Best', 2, 8, 1, 'Best']

time complexity: O(K^2) 

space complexity: O(1)

Method #3: Using numpy:

Algorithm:

1. Create a for loop to iterate K times.
2. Generate a random index between 0 and len(test_list) – 1.
3. Save the element at the random index to a variable called old_element.
4. Choose a random element from add_list and set it as the element at the random index in test_list.
5. Insert old_element after the new element at the random index.
6. Return the modified test_list with randomly inserted elements.

Python3

import random
import numpy as np

def insert_random_elements(test_list, add_list, K):
    for i in range(K):
        # Generate random index
        rand_index = random.randint(0, len(test_list) - 1)

        # Insert random element at random index and remove old element
        old_element = test_list[rand_index]
        new_element = np.random.choice(add_list)
        test_list[rand_index] = new_element
        test_list.insert(rand_index + 1, old_element)

    return test_list

test_list = [5, 7, 4, 2, 8, 1]
add_list = ["Gfg", "Best", "CS"]
K = 3
print(insert_random_elements(test_list, add_list, K))
#This code is contributed by Jyothi Pinjala.

Output:

[5, 7, 4, ‘Gfg’, ‘CS’, ‘CS’, 2, 8, 1]

Time complexity:

Generating a random index using random.randint(0, len(test_list) – 1) takes O(1) time.
Choosing a random element from add_list using np.random.choice(add_list) takes O(1) time.
Inserting a new element into test_list at a given index using insert() takes O(n) time, where n is the length of test_list.
Therefore, the overall time complexity of the insert_random_elements() function is O(K * n), where K is the number of random elements to insert and n is the length of test_list.

Auxiliary Space:

The function uses O(K) additional space to store the old_element variable for each insertion.
The function also modifies the input test_list in place, so the space complexity for that is O(1).
Therefore, the overall space complexity of the insert_random_elements() function is O(K).

Approach#4: Using lambda+map

Create a list of random elements from add_list using the map() function and lambda function. Append the generated list to the original test_list using the + operator. Return the resultant list.

Algorithm

1. Initialize the test_list, add_list and K values.
2. Generate a list of K random elements from the add_list using map() function and lambda function.
3. Append the generated list to the test_list using + operator.
4. Return the resultant list.

Python3

import random

test_list = [5, 7, 4, 2, 8, 1]
add_list = ["Gfg", "Best", "CS"]
K = 2

result_list = test_list + list(map(lambda x: random.choice(add_list), range(K)))
print(result_list)

Output

[5, 7, 4, 2, 8, 1, 'Best', 'CS']

Time Complexity: O(K), as the time complexity of generating a list of K random elements using map() function and lambda function is O(K) and appending two lists using + operator takes O(n) time where n is the length of the list.

Auxiliary Space: O(K), as the space complexity of generating a list of K random elements using map() function and lambda function is O(K) and the space complexity of storing the resultant list is also O(K).