Python Program to Retain first N Elements of a String and Replace the Remaining by K

In this article, we will learn how to retain the first N characters of a string and replace the remaining characters with a specified character K. Python provides several built-in functions like len(), slicing, replace(), and ljust() to accomplish this task efficiently.

Problem Overview

Given an input string, we need to keep the first N characters unchanged and replace all remaining characters with a replacement character K.

Example

Let's understand this with a simple example ?

# Input
input_string = 'tutorialspoint'
n = 9
k = "#"

# Expected output: 'tutorials#####'
print(f"Original: {input_string}")
print(f"Keep first {n} chars, replace rest with '{k}'")

Original: tutorialspoint
Keep first 9 chars, replace rest with '#'

In this example, the first 9 characters 'tutorials' are retained, and the remaining 5 characters are replaced with '#' symbols.

Method 1: Using Slicing and Multiplication Operator

This approach uses string slicing to extract the first N characters and the multiplication operator to create replacement characters ?

def retain_and_replace_v1(input_string, n, k):
    # Slice first N characters + multiply K by remaining length
    return input_string[:n] + k * (len(input_string) - n)

# Example usage
input_string = 'tutorialspoint'
n = 9
k = "#"

result = retain_and_replace_v1(input_string, n, k)
print(f"Input string: {input_string}")
print(f"Result: {result}")

Input string: tutorialspoint
Result: tutorials#####

Method 2: Using ljust() Function

The ljust() method left-aligns the string and pads it with a specified character ?

def retain_and_replace_v2(input_string, n, k):
    # Slice first N characters, then use ljust to pad with K
    return input_string[:n].ljust(len(input_string), k)

# Example usage
input_string = 'tutorialspoint'
n = 9
k = "#"

result = retain_and_replace_v2(input_string, n, k)
print(f"Input string: {input_string}")
print(f"Result: {result}")

Output: tutorialspoint
Result: tutorials#####

Method 3: Using replace() Function

This method uses replace() to substitute the remaining characters with the replacement character ?

def retain_and_replace_v3(input_string, n, k):
    # Replace the substring after N with K characters
    remaining_part = input_string[n:]
    replacement = k * len(remaining_part)
    return input_string.replace(remaining_part, replacement)

# Example usage
input_string = 'tutorialspoint'
n = 9
k = "#"

result = retain_and_replace_v3(input_string, n, k)
print(f"Input string: {input_string}")
print(f"Result: {result}")

Input string: tutorialspoint
Result: tutorials#####

Comparison of Methods

Complete Example with Error Handling

def retain_and_replace(input_string, n, k):
    """Retain first N characters and replace remaining with K"""
    if n >= len(input_string):
        return input_string  # No replacement needed
    
    return input_string[:n] + k * (len(input_string) - n)

# Test with different inputs
test_cases = [
    ('tutorialspoint', 9, '#'),
    ('python', 3, '*'),
    ('hello', 10, '-'),  # N greater than string length
]

for string, n, k in test_cases:
    result = retain_and_replace(string, n, k)
    print(f"'{string}' (N={n}, K='{k}') ? '{result}'")

'tutorialspoint' (N=9, K='#') ? 'tutorials#####'
'python' (N=3, K='*') ? 'pyt***'
'hello' (N=10, K='-') ? 'hello'

Conclusion

We explored three different methods to retain the first N characters of a string and replace the remaining with a specified character. The slicing with multiplication operator approach is the most efficient and readable solution for this task.