On today's blog, we’ll look at more examples of string manipulation without using any built-in Python methods. The key is to understand the problem and break it down step-by-step.

Problem: Swap Letter Cases

Task

Given a string input_string, write a function that transforms all lowercase letters to uppercase and all uppercase letters to lowercase. If the character is not a letter, leave it unchanged.

Understanding the Problem

What we’re asked to do is swap the case of each letter in the string:

  • a becomes A
  • Z becomes z
  • Non-letter characters remain unchanged

Step-by-Step Breakdown

  • Start by creating an empty string to hold the result: 
store = ''
  • Loop through each character in input_string: 
for char in input_string:
  • Use ASCII values to switch cases:
    • Lowercase to uppercase → ord(char) - 32
    • Uppercase to lowercase → ord(char) + 32

Code Implementation 

def solution(input_string):
    store = ''

    for char in input_string:
        if 'a' <= char <= 'z':
            store += chr(ord(char) - 32)
        elif 'A' <= char <= 'Z':
            store += chr(ord(char) + 32)
        else:
            store += char

    return store

Why subtract or add 32?

In the ASCII table, lowercase and uppercase letters are separated by 32. For example, 'A' is 65 and 'a' is 97 → 65 + 32 = 97.

Problem: Replace All Occurrences of a Character

Task

Given a string input_string, return a new string where all occurrences of character c1 are replaced by character c2.

Do not use any built-in string methods like .replace()

Understanding the Problem

We simply need to replace every instance of c1 with c2, and leave the rest of the string unchanged.

Step-by-Step Breakdown

  • Create an empty string to store the result: 
store = ''
  • Loop through each character:
    • If it's equal to c1, append c2 to the store.
    • Otherwise, append the original character.

Code Implementation 

def replace_character(input_string, c1, c2):
    store = ''

    for char in input_string:
        if char == c1:
            store += c2
        else:
            store += char

    return store

That’s it for Part 2!
In Part 3, we’ll step into more advanced problems—like counting character frequencies and tracking patterns in strings—all without built-in methods. Stay tuned!