He is correct, that the value 16 given to MID$ as a 'start' will yield an "F" because the 'start' argument to the MID$ function is one-based.

This is the string:

     A$ = "0123456789ABCDEF"

And calling the function will give you: MID$ (A$,1,1) => "0" ... MID$ (A$,15,1) => "E" MID$ (A$,16,1) => "F"

MID$ (

Right, the reason it won't ever emit F is because RND(1) will never return 1, it's implementation will return 0<=X<1, and INT() is a truncation, so INT(RND(1) + 15) will return an integer between 0 and 14. Adding one gives 1 to 15, and since F is in the 16th position of the string (Apple BASIC indexed from 1, not 0) it will never get selected.

But they're using MID to extract a character from the string. Seems to me that string indexes are 1-based in Apple BASIC as evidenced by the " ... + 1" and the fact that there are 0's on the page. So you select from characters 1 to 16 in the string.

Edit: seems I was slower than all the other replies...