Python Program to replace elements of a list based on the comparison with a number

Given a list, the task here is to write a Python program to replace its elements after comparing them with another number here described using K.

For the example depicted in this article, any number greater than K will be replaced with the value given in high and any number less than or equal to K will be replaced with the value given in low.

Input : test_list = [7, 4, 3, 2, 6, 8, 9, 1], low = 2, high = 9, K = 5
Output : [9, 2, 2, 2, 9, 9, 9, 2]
Explanation : Elements less than K substituted by 2, rest are by 9.

Input : test_list = [7, 4, 3, 2, 6, 8, 9, 1], low =2, high = 8, K = 5
Output : [8, 2, 2, 2, 8, 8, 8, 2]
Explanation : Elements less than K substituted by 2, rest are by 8.

Method 1 : Using loop

In this, we perform replacements using conditional statements and iteration is performed using loop.

Program:

Python3

# initializing list
test_list = [7, 4, 3, 2, 6, 8, 9, 1]

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

# initializing K
K = 5

# initializing low and high Replacement Replacement
low, high = 2, 9

res = []
for ele in test_list:

    # conditional tests
    if ele > K:
        res.append(high)
    else:
        res.append(low)

# printing result
print("List after replacement ? : " + str(res))

Output:

The original list is : [7, 4, 3, 2, 6, 8, 9, 1]

List after replacement ? : [9, 2, 2, 2, 9, 9, 9, 2]

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

Method 2: Using list comprehension

Similar to the method above, only difference is this is a one liner solution and a compact alternative using list comprehension.

Program:

Python3

# initializing list
test_list = [7, 4, 3, 2, 6, 8, 9, 1]

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

# initializing K
K = 5

# initializing low and high Replacement Replacement
low, high = 2, 9

# list comprehension for shorthand solution
res = [high if ele > K else low for ele in test_list]

# printing result
print("List after replacement ? : " + str(res))

Output:

The original list is : [7, 4, 3, 2, 6, 8, 9, 1]

List after replacement ? : [9, 2, 2, 2, 9, 9, 9, 2]

Time Complexity: O(n), where n is the number of elements in the list “test_list”.
Auxiliary Space: O(n), where n is the number of elements in the list “test_list”.

Method 3: Use the map() function along with a lambda function. 

Step-by-step approach:

• Initialize low and high replacement values: low, high
• Define a lambda function to perform the replacement: replace = lambda x: high if x > K else low.
• Use the map() function to apply the replace function to each element in test_list: res = list(map(replace, test_list)).
• Print the result: print(“List after replacement: ” + str(res)).

Python3

# initializing list
test_list = [7, 4, 3, 2, 6, 8, 9, 1]

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

# initializing K
K = 5

# initializing low and high Replacement Replacement
low, high = 2, 9

# define lambda function for replacement
replace = lambda x: high if x > K else low

# apply map function with replace to test_list and convert to list
res = list(map(replace, test_list))

# printing result
print("List after replacement: " + str(res))

Output

The original list is : [7, 4, 3, 2, 6, 8, 9, 1]
List after replacement: [9, 2, 2, 2, 9, 9, 9, 2]

Time Complexity: O(n), where n is the number of elements in the list “test_list”.
Auxiliary Space: O(n), where n is the number of elements in the list “test_list”.