The `alpha_index` function - kevshouse/exam_quest GitHub Wiki

The alpha_index function determines the 1-based position of a character in the alphabet if it's a letter (case-insensitive), or returns 0 for non-alphabetic characters. Here's a detailed breakdown:

Behavior:

1. Lowercase Letters ('a' to 'z'):

   • Returns the position in the alphabet starting from 1.
   • Example:
     alpha_index('a') → 1
alpha_index('z') → 26

2. Uppercase Letters ('A' to 'Z'):

   • Same as lowercase: returns the 1-based position.
   • Example:
     alpha_index('A') → 1
alpha_index('Z') → 26

3. Non-Alphabetic Characters:

   • Returns 0 for any character outside a-z or A-Z.
   • Examples:
     alpha_index('!') → 0
alpha_index('5') → 0
alpha_index('\0') → 0

How It Works:

• Lowercase Handling:

  if (c >= 'a' && c <= 'z') 
      return c - 'a' + 1;
  

  • Computes offset from 'a' (e.g., 'a' - 'a' = 0), then adds 1 to make it 1-based.

• Uppercase Handling:

  if (c >= 'A' && c <= 'Z') 
      return c - 'A' + 1;
  

  • Same logic as lowercase, using 'A' as the base.

• Non-Letter Handling:

  return 0;  // Default case
  

Key Notes:

• Case Insensitivity: Treats 'a' and 'A' the same (both return 1).
• ASCII Dependency: Relies on contiguous alphabetic ranges in ASCII (safe for most systems).
• Return Values:
  • Letters: 1 to 26.
  • Non-letters: 0.

Example Outputs:

Input	Output	Explanation
'd'	4	4th letter (lowercase)
'M'	13	13th letter (uppercase)
'@'	0	Not a letter
' '	0	Space (non-letter)

This function is useful for mapping alphabetic characters to numeric positions while ignoring case and filtering out non-letters.