Python - Every Kth Strings Uppercase

Given a String list, change every Kth string to uppercase.

Input : test_list = ["gfg", "is", "best", "for", "geeks"], K = 3 
Output : ['GFG', 'is', 'best', 'FOR', 'geeks'] 
Explanation : All Kth strings are uppercased. 

Input : test_list = ["gfg", "is", "best", "for", "geeks"], K = 4 
Output : ['GFG', 'is', 'best', 'for', 'GEEKS'] 
Explanation : All Kth strings are uppercased.

Method #1 : Using loop + upper()

In this, we iterate for all strings using loop and upper is used to perform uppercase, Kth index is detected using modulo operator.

# Python3 code to demonstrate working of 
# Every Kth Strings Uppercase
# Using loop + upper()

# initializing list
test_list = ["gfg", "is", "best", "for", "geeks", "and", "CS"] 

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

# initializing K 
K = 3

res = []
for idx in range(len(test_list)):
    
    # checking for Kth index
    if idx % K == 0:
        res.append(test_list[idx].upper())
    else :
        res.append(test_list[idx])

# printing result 
print("The resultant String list : " + str(res))

The original list is : ['gfg', 'is', 'best', 'for', 'geeks', 'and', 'CS']
The resultant String list : ['GFG', 'is', 'best', 'FOR', 'geeks', 'and', 'CS']

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

Auxiliary Space: O(n), where n is length of res list to store the result.

Method #2: Using list comprehension

This is yet another way in which this task can be performed. In this we use list comprehension as shorthand, performs tasks similar to the above method.

# Python3 code to demonstrate working of 
# Every Kth Strings Uppercase
# Using list comprehension

# initializing list
test_list = ["gfg", "is", "best", "for", "geeks", "and", "CS"] 

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

# initializing K 
K = 3

# shorthand to perform this task
res = [test_list[idx].upper() if idx % K == 0 else test_list[idx]
       for idx in range(len(test_list))]

# printing result 
print("The resultant String list : " + str(res))

The original list is : ['gfg', 'is', 'best', 'for', 'geeks', 'and', 'CS']
The resultant String list : ['GFG', 'is', 'best', 'FOR', 'geeks', 'and', 'CS']

Time Complexity: O(n)

Auxiliary Space: O(n)

Method #3: Using reduce():
Algorithm:

1. Initialize the list test_list with the given strings.
2. Print the original list test_list.
3. Initialize the value of K to 3.
4. Define a lambda function to_upper that takes an index and an element as input and returns the uppercase
5. version of the element if the index is a multiple of K, otherwise returns the original element.
6. Use the reduce method from the functools module to apply the lambda function to each element of the
7. test_list and accumulate the results in a list called res.
8. Print the resulting uppercase string list res.

# Python3 code to demonstrate working of
# Every Kth Strings Uppercase
# Using reduce method

# import reduce from functools module
from functools import reduce

# initializing list
test_list = ["gfg", "is", "best", "for", "geeks", "and", "CS"]

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

# initializing K
K = 3

# define lambda function to convert strings to uppercase
to_upper = lambda idx, elem: elem.upper() if idx % K == 0 else elem

# apply lambda function to each element of the list using reduce method
res = reduce(lambda acc, elem: acc + [to_upper(len(acc), elem)], test_list, [])

# printing result
print("The resultant String list : " + str(res))
#This code is contributed by Vinay pinjala.

The original list is : ['gfg', 'is', 'best', 'for', 'geeks', 'and', 'CS']
The resultant String list : ['GFG', 'is', 'best', 'FOR', 'geeks', 'and', 'CS']

Time Complexity: O(n), where n is the length of the input list test_list. The reduce method applies the to_upper lambda function to each element of the test_list exactly once, so the time complexity of the reduce method is O(n).

Space Complexity:  O(n). The test_list and res lists both have a size of n, where n is the length of the input list. The to_upper lambda function requires constant space, so its space complexity is O(1).