>>However, you make various assumptions that I don't appreciate.

My apologies if I offended you. I was just trying be be as thorough as possible just for the sake of clarity. It was not my intention to sound condescending.

>>To my knowledge, also, var++ employs the current value of var, and then increments it before moving on.

This is exactly right. However on your original code you need to reset the "letter" variable after it reaches 23. That's why on my last post, I removed the letter++ from within the parenthesis and then added the following:
letter = ++letter%24;