Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!willett!ForthNet
From: ForthNet@willett.UUCP (ForthNet articles from GEnie)
Newsgroups: comp.lang.forth
Subject: F-PC Forth Tutorial
Message-ID: <759.UUL1.3#5129@willett.UUCP>
Date: 11 Apr 90 01:57:07 GMT
Organization: Latest link in the ForthNet chain.  (Pgh, PA)
Lines: 78


 Date: 04-07-90 (12:21)              Number: 510 (Echo)
   To: PAUL BRANNON                  Refer#: 509
 From: JACK BROWN                      Read: 04-08-90 (21:31)
 Subj: RELEARNING FORTH              Status: PUBLIC MESSAGE

 >I was starting the lessons last spring when my son was born, and time
 >has been short since then. I finally have time again to learn Forth. I 
 >hope you don't mind if I resume where I left off, which was near the
 >end of lesson 2. The word below is my attempt to calculate the area of
 >a circle. It works except for printing the decimal point. Also I could

 I noticed in the output that the decimal point did not appear.  This may
 not be your problem as is versions of F-PC from 2.15 on up the phrase
 8 EMIT would not back up the cursor by one as was intended.

 Instead of using 8 EMIT  you mut use 1 BACKSPACES.  

 I am curently updating the tutorials to F-PC 3.5 but have not yet
 uploaded any of the changed lessons yet.  The problem you are working on
 is 2.17 in the revised lessons and here is the problem and your solution
 using 1 BACKSPACES.  First the corrected triangle program.

 : TRIANGLE_AREA ( b h --)
        2DUP SWAP
        CR ." Base =   "  .
           ." Height = "  .
           * 2 /MOD
           ." Area = " .
           1 BACKSPACES ( 8  EMIT  does not back up cursor in F-PC)
           ASCII . EMIT
           IF    ASCII 5 EMIT
           ELSE  ASCII 0 EMIT
           THEN ;

 COMMENT:
 Problem 2.17.
 a) What do you observe when you use 5 and 3 for the base and height?
 b) What do you observe when you use 5 and 4 for the base and height?
 c) Explain in detail why you get the result you see for question 1.
 d) Could you modify the idea contained in the above to compute the Area
 of a circle to three decimal places?  Hint:  It can be done quite easily
 and if you can do it you have the potential to become a Forth Super 
 Star!

 Solution to Problem 2.17

 a) What do you observe for output of triangle program
    with  5 3 AREA?  Answer:
 Base =   5 Height = 3 Area = 7.5

 b) What about  5 4 AREA ?  Answer:
 Base =   5 Height = 4 Area = 10.0

 c) Why the decimal result for question one when we only have
    integer math?  Answer:  Because if the remainder from the
    /MOD operation is 1 we can display the .5, if the remainder
    is 0 then we can display the required .0

 d) Computer area of a circle to three decimal places.

 COMMENT;

 \ Compute area of a circle to three decimal places.
 : CIRCLE_AREA ( r -- )
         DUP * 355 * 113 /MOD
         ." Area = " . ( 8 EMIT ) 1 BACKSPACES ASCII . EMIT
         5 0 DO
             10 * 113 /MOD . ( 8 EMIT ) 1 BACKSPACES
             LOOP DROP ;

 : TEST_CIRC ( -- )
        8 0 DO CR I CIRCLE_AREA LOOP ;

 NET/Mail : British Columbia Forth Board - Burnaby BC - (604)434-5886   
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