Recursion Without Local Variables

Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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# 24.3. Recursion Without Local Variables

A function may recursively call itself even without use of local variables.

Example 24-16. The Fibonacci Sequence

       1 #!/bin/bash
       2 # fibo.sh : Fibonacci sequence (recursive)
       3 # Author: M. Cooper
       4 # License: GPL3
       5
       6 # ----------algorithm--------------
       7 # Fibo(0) = 0
       8 # Fibo(1) = 1
       9 # else
      10 #   Fibo(j) = Fibo(j-1) + Fibo(j-2)
      11 # ---------------------------------
      12
      13 MAXTERM=15       # Number of terms (+1) to generate.
      14 MINIDX=2         # If idx is less than 2, then Fibo(idx) = idx.
      15
      16 Fibonacci ()
      17 {
      18   idx=$1   # Doesn't need to be local. Why not?
      19   if [ "$idx" -lt "$MINIDX" ]
      20   then
      21     echo "$idx"  # First two terms are 0 1 ... see above.
      22   else
      23     (( --idx ))  # j-1
      24     term1=$( Fibonacci $idx )   #  Fibo(j-1)
      25
      26     (( --idx ))  # j-2
      27     term2=$( Fibonacci $idx )   #  Fibo(j-2)
      28
      29     echo $(( term1 + term2 ))
      30   fi
      31   #  An ugly, ugly kludge.
      32   #  The more elegant implementation of recursive fibo in C
      33   #+ is a straightforward translation of the algorithm in lines 7 - 10.
      34 }
      35
      36 for i in $(seq 0 $MAXTERM)
      37 do  # Calculate $MAXTERM+1 terms.
      38   FIBO=$(Fibonacci $i)
      39   echo -n "$FIBO "
      40 done
      41 # 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610
      42 # Takes a while, doesn't it? Recursion in a script is slow.
      43
      44 echo
      45
      46 exit 0

Example 24-17. The Towers of Hanoi

       1 #! /bin/bash
       2 #
       3 # The Towers Of Hanoi
       4 # Bash script
       5 # Copyright (C) 2000 Amit Singh. All Rights Reserved.
       6 # http://hanoi.kernelthread.com
       7 #
       8 # Tested under Bash version 2.05b.0(13)-release.
       9 # Also works under Bash version 3.x.
      10 #
      11 #  Used in "Advanced Bash Scripting Guide"
      12 #+ with permission of script author.
      13 #  Slightly modified and commented by ABS author.
      14
      15 #=================================================================#
      16 #  The Tower of Hanoi is a mathematical puzzle attributed to
      17 #+ Edouard Lucas, a nineteenth-century French mathematician.
      18 #
      19 #  There are three vertical posts set in a base.
      20 #  The first post has a set of annular rings stacked on it.
      21 #  These rings are disks with a hole drilled out of the center,
      22 #+ so they can slip over the posts and rest flat.
      23 #  The rings have different diameters, and they stack in ascending
      24 #+ order, according to size.
      25 #  The smallest ring is on top, and the largest on the bottom.
      26 #
      27 #  The task is to transfer the stack of rings
      28 #+ to one of the other posts.
      29 #  You can move only one ring at a time to another post.
      30 #  You are permitted to move rings back to the original post.
      31 #  You may place a smaller ring atop a larger one,
      32 #+ but *not* vice versa.
      33 #  Again, it is forbidden to place a larger ring atop a smaller one.
      34 #
      35 #  For a small number of rings, only a few moves are required.
      36 #+ For each additional ring,
      37 #+ the required number of moves approximately doubles,
      38 #+ and the "strategy" becomes increasingly complicated.
      39 #
      40 #  For more information, see http://hanoi.kernelthread.com
      41 #+ or pp. 186-92 of _The Armchair Universe_ by A.K. Dewdney.
      42 #
      43 #
      44 #         ...                   ...                    ...
      45 #         | |                   | |                    | |
      46 #        _|_|_                  | |                    | |
      47 #       |_____|                 | |                    | |
      48 #      |_______|                | |                    | |
      49 #     |_________|               | |                    | |
      50 #    |___________|              | |                    | |
      51 #   |             |             | |                    | |
      52 # .--------------------------------------------------------------.
      53 # |**************************************************************|
      54 #          #1                   #2                      #3
      55 #
      56 #=================================================================#
      57
      58
      59 E_NOPARAM=66  # No parameter passed to script.
      60 E_BADPARAM=67 # Illegal number of disks passed to script.
      61 Moves=        # Global variable holding number of moves.
      62               # Modification to original script.
      63
      64 dohanoi() {   # Recursive function.
      65     case $1 in
      66     0)
      67         ;;
      68     *)
      69         dohanoi "$(($1-1))" $2 $4 $3
      70         echo move $2 "-->" $3
      71         ((Moves++))          # Modification to original script.
      72         dohanoi "$(($1-1))" $4 $3 $2
      73         ;;
      74     esac
      75 }
      76
      77 case $# in
      78     1) case $(($1>0)) in     # Must have at least one disk.
      79        1)  # Nested case statement.
      80            dohanoi $1 1 3 2
      81            echo "Total moves = $Moves"   # 2^n - 1, where n = # of disks.
      82            exit 0;
      83            ;;
      84        *)
      85            echo "$0: illegal value for number of disks";
      86            exit $E_BADPARAM;
      87            ;;
      88        esac
      89     ;;
      90     *)
      91        echo "usage: $0 N"
      92        echo "       Where \"N\" is the number of disks."
      93        exit $E_NOPARAM;
      94        ;;
      95 esac
      96
      97 # Exercises:
      98 # ---------
      99 # 1) Would commands beyond this point ever be executed?
     100 #    Why not? (Easy)
     101 # 2) Explain the workings of the workings of the "dohanoi" function.
     102 #    (Difficult -- see the Dewdney reference, above.)

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