A function may recursively call itself even without use of local variables.
Example 23-13. The Towers of Hanoi
#! /bin/bash
#
# The Towers Of Hanoi
# Bash script
# Copyright (C) 2000 Amit Singh. All Rights Reserved.
# http://hanoi.kernelthread.com
#
# Last tested under bash version 2.05b.0(13)-release
#
# Used in "Advanced Bash Scripting Guide"
#+ with permission of script author.
# Slightly modified and commented by ABS author.
#=================================================================#
# The Tower of Hanoi is an old mathematical puzzle.
# There are three vertical posts set in a base.
# The first post has a set of annular rings stacked on it.
# These rings are flat disks with a hole drilled out of the center,
#+ so they can slip over the posts.
# The rings have different diameters, and they stack in descending
#+ order, according to size.
# The smallest ring is on top, and the largest on the bottom.
#
# The task is to transfer the stack of rings
#+ to one of the other posts.
# You can move only one ring at a time to another post.
# You are permitted to move rings back to the original post.
# You may place a smaller ring atop a larger one,
#+ but *not* vice versa.
# Again, it is forbidden to place a larger ring atop a smaller one.
#
# For a small number of rings, only a few moves are required.
#+ For each additional ring,
#+ the required number of moves approximately doubles,
#+ and the "strategy" becomes increasingly complicated.
#
# For more information, see http://hanoi.kernelthread.com.
#
#
# ... ... ...
# | | | | | |
# _|_|_ | | | |
# |_____| | | | |
# |_______| | | | |
# |_________| | | | |
# |___________| | | | |
# | | | | | |
# .--------------------------------------------------------------.
# |**************************************************************|
# #1 #2 #3
#
#=================================================================#
E_NOPARAM=66 # No parameter passed to script.
E_BADPARAM=67 # Illegal number of disks passed to script.
Moves= # Global variable holding number of moves.
# Modifications to original script.
dohanoi() { # Recursive function.
case $1 in
0)
;;
*)
dohanoi "$(($1-1))" $2 $4 $3
echo move $2 "-->" $3
let "Moves += 1" # Modification to original script.
dohanoi "$(($1-1))" $4 $3 $2
;;
esac
}
case $# in
1)
case $(($1>0)) in # Must have at least one disk.
1)
dohanoi $1 1 3 2
echo "Total moves = $Moves"
exit 0;
;;
*)
echo "$0: illegal value for number of disks";
exit $E_BADPARAM;
;;
esac
;;
*)
echo "usage: $0 N"
echo " Where \"N\" is the number of disks."
exit $E_NOPARAM;
;;
esac
# Exercises:
# ---------
# 1) Would commands beyond this point ever be executed?
# Why not? (Easy)
# 2) Explain the workings of the workings of the "dohanoi" function.
# (Difficult) |