Python | Selective casing in String

Sometimes, while working with Python, we can have a problem in which we need to perform the case change of certain characters in string. This kind of problem can come in many types of applications. Let’s discuss certain ways in which this problem can be solved.

Method #1 : Using enumerate() + loop + upper() This problem can be solved using set of above functionalities. This is brute force way to perform this task, in this we iterate through each element in string and change to uppercase if it’s present in case change list. 

The original string : gfg is best
String after Selective casing : GFG iS beSt

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

Method #2 : Using list comprehension + upper() + join() This is shorthand version in which this problem can be solved. In this, we perform similar task in similar way as above method but in one-liner way using list comprehension. 

The original string : gfg is best
String after Selective casing : GFG iS beSt

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

Method #3 : Using replace() method 

The original string : gfg is best
String after Selective casing : GFG iS beSt

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

Method #4 : Using replace() and index() methods

The original string : gfg is best
String after Selective casing : GFG iS beSt

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

Method #5: Using Regular Expressions

This approach can be useful when you need to change case of multiple characters in a string and you know the pattern of those characters. You can use the re module in Python to perform this task.

String after Selective casing : GFG iS beSt

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

Method #6: using regex()

1.Define the input string ‘test_str’.
2.Define a list of characters to change uppercase called ‘chg_list’.
3.Use the join() function to concatenate the elements of ‘chg_list’ using ‘|’ as the separator and store the resulting string in a variable called ‘uppercased_chars’.
4.Use the re.sub() function to substitute each character in ‘uppercased_chars’ with its uppercase equivalent in ‘test_str’.
a. The first argument to re.sub() is a regular expression pattern that matches the characters to be substituted.
b. The second argument to re.sub() is a lambda function that takes a single argument (a match object) and returns the replacement string.
c. In the lambda function, use the group() method of the match object to get the matched string, and the upper() method to convert it to uppercase.
5.Store the resulting string in a variable called ‘result’.
6.Print the original string using the print() function.
7.Print the resulting string using the print() function.

The original string : gfg is best
String after selective casing: GFG iS beSt

Time Complexity:

The join() method has a time complexity of O(n), where n is the number of elements in the list of characters to be uppercased.
The re.sub() method has a time complexity of O(m), where m is the length of the input string.
In the lambda function, the group() method and the upper() method both have a time complexity of O(1).
Therefore, the overall time complexity of the code is O(m+n).
Space Complexity:

The input string ‘test_str’ occupies O(m) space, where m is the length of the input string.
The ‘chg_list’ list occupies O(n) space, where n is the number of characters to be uppercased.
The ‘uppercased_chars’ variable occupies O(n) space.
The re.sub() function and the lambda function do not create any new data structures, and hence their space complexity is O(1).
The resulting string stored in the ‘result’ variable occupies O(m) space.
Therefore, the overall space complexity of the code is O(m+n).