Python – Append given number with every element of the list

Given a list and a number, write a Python program to append the number with every element of the list.

Examples:

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

key = 5 (number to be appended)

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

The following are ways to achieve this functionality:

Method 1: Using for loop

Python

input = [1, 2, 3, 4, 5]
key = 7

result = []
for ele in input:
    result.append(ele)
    result.append(key)

print(result)

Output:

[1, 7, 2, 7, 3, 7, 4, 7, 5, 7]

Time complexity: O(n)
Space complexity: O(n)

Method 2: Using list comprehension and itertools.chain

Python

import itertools

input = [1, 2, 3, 4, 5]
key = 7

result = list(itertools.chain(*[[ele, key] for ele in input]))

print(result)

Output:

[1, 7, 2, 7, 3, 7, 4, 7, 5, 7]

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

Method 3: Using zip and for loop

Python

input = [1, 2, 3, 4, 5]
key = 7

result = []
for x, y in zip(input, [key]*len(input)):
    result.extend([x, y])

print(result)

Output:

[1, 7, 2, 7, 3, 7, 4, 7, 5, 7]

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

Method 4: Using list(),map(),str(),join() and split()

Python3

input = [1, 2, 3, 4, 5]
key = 7

l=list(map(str,input))
p="*"+str(key)+"*"
x=p.join(l)
a=x.split("*")
res=list(map(int,a))
res.append(key)

print(res)

Output

[1, 7, 2, 7, 3, 7, 4, 7, 5, 7]

Time complexity: O(n) 
Space complexity: O(n)

Method 5: Using pandas

Python3

import pandas as pd

input = [1, 2, 3, 4, 5]
key = 7

df = pd.DataFrame({'col': input})
result = df['col'].apply(lambda x: [x, key]).sum()

print(result)
#This code is contributed by Vinay pinjala.

Output:

[1, 7, 2, 7, 3, 7, 4, 7, 5, 7]

Time complexity: O(n)

Space complexity: O(n)

Method#6: Using Recursive method.

Algorithm:

The recursive_method function takes two inputs, an input_list of integers and a key integer. It first checks if the input_list is empty or not. If it is empty, it returns an empty list. If it is not empty, it takes the first element of the input_list, adds it to the key, and creates a new list with these two elements. It then recursively calls the recursive_method function on the rest of the input_list (i.e., input_list[1:]) and concatenates the result of this recursive call to the new list it created earlier. This process continues until the entire input_list has been processed.

Python3

def recursive_method(input_list, key):
    if not input_list:
        return []
    else:
        return [input_list[0], key] + recursive_method(input_list[1:], key)
input_list = [1, 2, 3, 4, 5]
key = 7
result = recursive_method(input_list, key)
print(result)

Output

[1, 7, 2, 7, 3, 7, 4, 7, 5, 7]

The time complexity of this recursive_method function is O(n), where n is the length of the input_list. This is because for each element in the input_list, the function performs a constant amount of work (i.e., creating a list with two elements and making a recursive call on a sublist of length n-1). Therefore, the total number of operations is proportional to the length of the input_list.

The auxiliary space of this function is also O(n), because at each recursive call, a new list is created and stored in memory. The maximum number of recursive calls that can be made is equal to the length of the input_list, so the total amount of space used is proportional to the length of the input_list