Unicon Book

1.10. PROCEDURES 27
Now move the n-1 disks from needle1 to other, move the biggest disk, and then move
the n-1 again. The needles are passed as additional parameters into the recursive calls.
They are always two distinct values out of the set {1, 2, 3}.
hanoi(n-1, needle1, other)
write("Move disk from ", needle1, " to ", needle2)
hanoi(n-1, other, needle2)
That’s it! You’re done. Listing 1-2 contains the complete program for you to try:
Listing 1-2 Towers of Hanoi
procedure main()
write("How many disks are on the towers of Hanoi?")
hanoi(read())
end
procedure hanoi(n:integer, needle1:1, needle2:2)
local other
if n = 1 then write("Move disk from ", needle1, " to ", needle2)
else {
other := 6 - needle1 - needle2
hanoi(n-1, needle1, other)
write("Move disk from ", needle1, " to ", needle2)
hanoi(n-1, other, needle2)
}
end
Turn on tracing see how this program works. To enable tracing, compile your program
with a -t option, or assign the keyword &trace a non-zero number giving the depth of calls
to trace. Setting &trace to -1 will turn on tracing to an infinite depth.
To move n disks, 2 n - 1 individual disk movements will be required. If the monks move
one disk a second, it will take 2 64 - 1 seconds, or about 60 trillion years. Wikipedia has
listed the age of the universe at around 13.75 billion years. It seems unlikely that we need
worry about the monks finishing their task!
Summary
In this chapter you have learned:
• Unicon is an expression-based language organized as a set of procedures starting from
a procedure called main().
• Unicon has four atomic types: arbitrary precision integers, real numbers, arbitrary
length strings of characters, and character sets.