Ok, these kinds of problem scream out to use a while loop, so let's see how it might work with that:
This works just like the odometer on your car, with three being the far right hand "wheel", and two being one wheel to the left of that, and etc.
pseudo-code
Code:
set char's one, two, and three to @ //one less than A
get a number from the user, or a file.
while(number > 0)
number--;
three++;
if(three > 'Z')
then increment two and set three = 'A'
Extend the three if statement, to include two, so if two > 'Z', the one
wheel, will be incremented, and two and three reset to 'A'
and loop back
if one equals '@' then one = 32
if two equals '@') then two = 32
print num was: %d, which is: %c%c%c num, one, two, three.
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When you're programming, you'll refer to it frequently.