Assume use of 8 bit characters. Declare a constant 256 entry array pre-filled with all False except for the five (or six) vowel characters. This is baked into the code and not initialized at runtime.
Now for each character c in the input string, simply do an array index and see if it is true (a vowel) or not. This avoids either five conditionals, or a loop over the string 'aeiou'. The vowel test is constant time regardless of the character value.
Now for each character c in the input string, simply do an array index and see if it is true (a vowel) or not. This avoids either five conditionals, or a loop over the string 'aeiou'. The vowel test is constant time regardless of the character value.