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Mystery program...

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In my high-school "electronic data processing" text book from 1974 I found an 
"explain what this program does" exercise that I don't quite grok now.

It's exercise 4.25 from page 72 of "EDB for videregående skoler" by Gram, Hope 
and Ommundsen, NKS-forlaget 1974.

Here's the program translated from its original Basic to JavaScript (I'm 
assuming the Windows Script Host for i/o, and if  you have Windows simply copy 
this text to a [.js] file and run it via e.g. 'cscript' command):


<code>
lineNum_ = 0

function input()
{
     WScript.stdout.write( "? " );
     var s = WScript.stdin.readLine();
     return parseFloat( s );
}

function print( v )
{
     WScript.stdout.writeLine( v );
}

function goto( new_n )
{
     lineNum_ = new_n - 1
}

function end()
{
     print( "FINISHED!" )
     WScript.Quit()
}

function onStatementExecuted( lineNum )
{
     //print( "lineNum="+lineNum + ", n="+n + ", m="+m + ", p="+p + ", p2="+p2 )
}

m = null
n = null
p = null
p2 = null

for( ;; )
{
     lineNum_ += 1
     if( lineNum_ > 99999 )
     {
         print( "!Program execution beyond last statement." );
         WScript.Quit( -1 );
     }

     var executedLineNum = lineNum_
     var didSmth = true
     switch( lineNum_ )
     {
     case -1:

     // BEGIN translation of original code:
     break; case  30:    m = 1
     break; case  40:    n = input()
     break; case  50:    p2 = n
     break; case  60:    p = p2 - 2*m
     break; case  70:    p2 = p
     break; case  80:    m = m + 1
     break; case  90:    if( p < m ) goto( 140 )
     break; case 100:    p = p - m
     break; case 110:    if( p > 0 ) goto( 100 )
     break; case 120:    if( p == 0 ) goto( 160 )
     break; case 130:    if( p2 > 2*m ) goto( 60 )
     break; case 140:    print( "n=" + n );
     break; case 150:    goto( 170 )
     break; case 160:    print( "m=" + m )
     break; case 170:    end()
     // END translation of original code

     break; default:
         didSmth = false
     }

     if( didSmth )
     {
         onStatementExecuted( executedLineNum )
     }
}
</code>

The exercise's part (a) recommends trying out the program "on paper" (noting 
down values of variables) for n = 3, 10 and 28, and for part (b) it recommends 
running the program and trying n = 2, 14, 22, 36 and 39.

OK, I can see that there's apparently a computation of remainder in there, but 
it's not exactly meaningful to me.

I'm probably just blind on both eyes, but any help appreciated.


Cheers,

- Alf

-- 
blog at <url: http://alfps.wordpress.com>

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Thread

Mystery program... "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-05-02 00:10 +0200
  Re: Mystery program... Paul N <gw7rib@aol.com> - 2011-05-04 13:23 -0700
    Re: Mystery program... "io_x" <a@b.c.invalid> - 2011-05-05 09:02 +0200
      Re: Mystery program... "io_x" <a@b.c.invalid> - 2011-05-06 17:22 +0200
        Re: Mystery program... "io_x" <a@b.c.invalid> - 2011-05-07 09:12 +0200
        Re: Mystery program... "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-05-12 04:14 +0200
  Re: Mystery program... tm <thomas.mertes@gmx.at> - 2011-05-11 23:42 -0700
    Re: Mystery program... "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-05-12 14:17 +0200

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