Python – Test if Kth character is digit in String

Given a String, check if Kth index is a digit.

Input : test_str = ‘neveropen9neveropen’, K = 5 
Output : True 
Explanation : 5th idx element is 9, a digit, hence True.
Input : test_str = ‘neveropen9neveropen’, K = 4 
Output : False 
Explanation : 4th idx element is s, not a digit, hence False. 
 

Method #1: Using in operator

In this, we create a string of numerics and then use in operator to check if Kth digit lies in that numeric string.

The original string is :neveropen4neveropen
Is Kth element String : True

Method #2 : Using isdigit()

In this, we use inbuilt Py. function to solve this problem, and check if Kth element is digit.

The original string is :neveropen4neveropen
Is Kth element String : True

The Time and Space Complexity for all the methods are the same:

Time Complexity: O(1)

Auxiliary Space: O(1)

Method #3 : Using ord() method

The original string is :neveropen4neveropen
Is Kth element String : True

Method #4 : Using isnumeric() method

The original string is :neveropen4neveropen
Is Kth element String : True

Time Complexity : O(1)

Auxiliary Space : O(1)

Method #5 : Using try-except block

Algorithm:

1. Initialize the test_str variable with a given string.
2. Print the original string.
3. Initialize the K variable with a given value.
4. Use try-except block to check if Kth element is a digit.
5. If the Kth element is a digit, then set res to True, else False.
6. Print the result.

The original string is :neveropen4neveropen
Is Kth element String : True

Time complexity: O(1)
The time complexity of this code is O(1) because it only performs a constant number of operations regardless of the length of the input string.

Auxiliary space: O(1)
The auxiliary space complexity of this code is O(1) because it only uses a constant amount of extra space to store the Boolean result.

Method 6 :  using regular expressions. 

 step-by-step approach

1. Import the re module to use regular expressions.
2. Define a regular expression pattern that matches a single digit. In regular expressions, the \d character class represents a digit. So, our pattern will be r”\d”.
3. Use the re.match() function to check if the Kth character matches the pattern. We can access the Kth character using string indexing, like test_str[K]. If the character matches the pattern, the match() function returns a match object, which is a truthy value in Python. If the character does not match the pattern, the function returns None, which is a falsy value in Python.
4. Based on the return value of the match() function, set the value of the res variable to True or False.
5. Print the result.

The original string is :neveropen4neveropen
Is Kth element String : True

Time complexity: The time complexity of this method is O(1), as it only involves matching a single character with a regular expression pattern, which takes constant time.

Auxiliary space: The auxiliary space used by this method is also O(1), as it only involves storing a regular expression pattern and a match object, both of which take constant space.