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Hello Francois!

Replying to a message of Francois Thunus to Steven Cox:

 FT> Hello Steven!

 FT> 24 Sep 95 22:36, Steven Cox wrote to All:

 SC>> I am trying to learn Rexx.  I am studying Rexx in 21 days by William
 SC>> & Esther Schindler.  I got to this example of recursion & cannot
 SC>> figure it out. This program figures the factorial of a number. Would
 SC>> someone please explain how this works.

 FT> what is it exactly that you want to know or don't understand ? the
 FT> rexx syntax or the recursion phenomenon? recursion is easy to explain
 FT> and hard to understand: it's using the hammer you are making to built
 FT> a hammer. as for the rexx syntax, I can hardly comment on 20 lines of
 FT> code...

I understand what recursion does and somewhat how it does it.  It is this
particular procedure I do not understand. (I think...)

[C:\rexx_in_21]list1406
     5 *-*   Say 'Enter a number:';
       >>>     "Enter a number:"
Enter a number:
     6 *-*   Pull number;
5
       >>>     "5"
     7 *-*   Say number || '! is' Factorial(number);
    10 *-*     Factorial:
    10 *-*     Procedure;
    11 *-*     num = Arg(1);
       >>>       "5"
    12 *-*     If num = 1;
       >>>       "0"
    14 *-*     Return Factorial(num - 1) * num;     /* if num = 5 then
5-1=4*5=20  This is were i loose it!!*/    /* I'm not sure I understand
this return instruction....It is returning to itself as I see it*/
    10 *-*       Factorial:
    10 *-*       Procedure;
    11 *-*       num = Arg(1);
       >>>         "4"
    12 *-*       If num = 1;
       >>>         "0"
   14 *-*           Return Factorial(num - 1) * num;
    10 *-*             Factorial:
    10 *-*             Procedure;
    11 *-*             num = Arg(1);
       >>>               "1"
    12 *-*             If num = 1;
       >>>               "1"
    12 *-*             Then;
    13 *-*               Return num;
       >>>                 "1"
       >>>             "2"       /* Now where did this
come from?? */
       >>>           "6"
       >>>         "24"
       >>>       "120"
       >>>     "5! is 120"
5! is 120
     8 *-*   Exit;

/* this it the program.*/
/* list1406.cmd */
/* Calculating factorial using a recursive function. */
TRACE RESULT

SAY 'Enter a number:'
PULL number
SAY number || '! is' Factorial( number)
EXIT

Factorial: PROCEDURE
        num = Arg( 1 )
        IF num = 1 THEN
                RETURN num
        RETURN Factorial( num - 1 ) * num


Thanks for your help,
Steven

PS. Sorry I haven't responded sooner, I have been real busy & this had
to be put on the side. So much for 21 days.

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