Python program to replace first ‘K’ elements by ‘N’
Last Updated :
21 Apr, 2023
Given a List, replace first K elements by N.
Input : test_list = [3, 4, 6, 8, 4, 2, 6, 9], K = 4, N = 3
Output : [3, 3, 3, 3, 4, 2, 6, 9]
Explanation : First 4 elements are replaced by 3.
Input : test_list = [3, 4, 6, 8, 4, 2, 6, 9], K = 2, N = 10
Output : [10, 10, 6, 8, 4, 2, 6, 9]
Explanation : First 2 elements are replaced by 10.
Method 1: Using a loop
This naive method iterates through the list ‘k‘ number of times and assigns ‘N‘ to each element encountered till the loop ends.
Python3
test_list = [3, 4, 6, 8, 4, 2, 6, 9]
print("The original list is : " + str(test_list))
K = 5
N = 6
for idx in range(K):
test_list[idx] = N
print("Elements after replacement : " + str(test_list))
|
Output
The original list is : [3, 4, 6, 8, 4, 2, 6, 9]
Elements after replacement : [6, 6, 6, 6, 6, 2, 6, 9]
Method 2: Using list slicing
In this, we slice off, just a few elements from the list and assign ‘N‘ to the sliced list.
Python3
test_list = [3, 4, 6, 8, 4, 2, 6, 9]
print("The original list is : " + str(test_list))
K = 5
N = 6
test_list[: K] = [N] * K
print("Elements after replacement : " + str(test_list))
|
Output
The original list is : [3, 4, 6, 8, 4, 2, 6, 9]
Elements after replacement : [6, 6, 6, 6, 6, 2, 6, 9]
Method 3: Using list(),map(),join() and replace()
Initially convert each element of list to string and then join the list using empty string.Replace first K characters with N in that joined list(which is a string) ,later convert each element of a string to integer and make it a list.
Python3
test_list = [3, 4, 6, 8, 4, 2, 6, 9]
print("The original list is : " + str(test_list))
K = 5
N = 6
a=list(map(str,test_list))
b="".join(a)
b=b.replace(b[:K],str(N)*K)
b=list(map(int,b))
print("Elements after replacement : " + str(b))
|
Output
The original list is : [3, 4, 6, 8, 4, 2, 6, 9]
Elements after replacement : [6, 6, 6, 6, 6, 2, 6, 9]
Method 4:Using list comprehension
Algorithm
- Initialize the list test_list with some values.
- Initialize the value of K and N.
- Iterate over the first K elements of the list using a for loop and list comprehension.
- If the index is less than K, replace the value at that index with N, else keep the original value.
- Print the updated list test_list.
Python3
test_list = [3, 4, 6, 8, 4, 2, 6, 9]
print("The original list is : " + str(test_list))
K = 5
N = 6
test_list = [N if idx < K else val for idx, val in enumerate(test_list)]
print("Elements after replacement : " + str(test_list))
|
Output
The original list is : [3, 4, 6, 8, 4, 2, 6, 9]
Elements after replacement : [6, 6, 6, 6, 6, 2, 6, 9]
Time complexity:
Initializing the list takes O(n) time, where n is the length of the list.
Initializing K and N takes constant time, i.e., O(1).
The for loop that iterates over the first K elements takes O(K) time.
The list comprehension takes O(K) time.
Printing the updated list takes O(n) time.
Therefore, the overall time complexity is O(n + K).
Space complexity:
The space required to store the input list test_list is O(n).
The space required to store the variables K and N is constant, i.e., O(1).
The space required by the list comprehension is also O(n).
Therefore, the overall space complexity is O(n).
Method 5 : use the extend() method of lists
Steps:
Initialize the list test_list.
Initialize the values of K and N.
Use list slicing to select the sublist of test_list up to index K.
Replace the selected sublist with a list of K copies of N using the = assignment operator and the * operator.
Print the resulting list.
Python3
test_list = [3, 4, 6, 8, 4, 2, 6, 9]
K = 5
N = 6
test_list[:K] = [N] * K
print("Elements after replacement : " + str(test_list))
|
Output
Elements after replacement : [6, 6, 6, 6, 6, 2, 6, 9]
Time complexity: O(K)
Auxiliary space: O(K)
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