Text 41213, 113 rader
Skriven 2006-10-13 11:37:45 av mark lewis (1:3634/12.0)
  Kommentar till text 41210 av Michiel van der Vlist (2:280/5555.1)
Ärende: smart compilers (was: pi)
=================================
 MvdV> Ok, now that we got that out of the way...

good... :)

 ml> what would your compiler do with each of the following?

 ml>   if (2+2==5) or (3+3==7) then print "egads!";

 MvdV> I have done quit a bit in Pascal programming, but that was a long
 MvdV> time ago. Before the  IBM PC (clones) dominated the market. My
 MvdV> compiler went to the museum together with my home brew 8 bit
 MvdV> system running 6809 FLEX. Haven't written a line of Pascal in
 MvdV> over fifteen years. So maybe someone will point out that it will
 MvdV> not complie at all because something is missing...

don't look at "the language"... look at the code... t'is a mismash of at least
three...

 ml>   if (2+2==5) and (3+3==7) then print "EGADS!";

 ml>   if (2+2==4) or (3+3==7) then print "whew!";

 ml>   if (2+2==5) or (3+3==6) then print "Whew!";

 ml>   if (2+2==4) and (3+3==6) then print "WHEW!";

 MvdV> All these examples are variations of one ande the same thing. A
 MvdV> constant between the if and the then. The constant is either TRUE
 MvdV> or it is FALSE. If it is true the print statement will be
 MvdV> compiled and executed when the object code is run. If it is
 MvdV> FALSEe, the smart compiler will skip it so nothing will ever be
 MvdV> printed.

ahhh... you see one constant where i see three... a large one made of two
seperate smaller ones... the large one being made of the "and" and "or" between
the two smaller ones...

 MvdV> You want me to do the leg work? Ok...

 MvdV> EGADS! : skipped
 MvdV> whew!  : printed
 MvdV> Whew!  : printed
 MvdV> WHEW!  : printed

hey! you didn't take the short-circut like you seemed to be doing before...
otherwise, you'd have skipped the "Whew!" because the evaluation of the first
was false and short-circut boolean evaluation would have jumped out right there
assuming the whole thing was false...

 ml> now, try these...

 ml>   a=2; b=2; c=5; d=3; e=3; f=7;
 ml>   if (a+b==c) or (d+e==f) then print "egads!";

 MvdV> Ah, that is a bit more tough. If it is a *very* smart optimising
 MvdV> compiler it will be aware that although the terms in the if
 MvdV> statement are written as variables, the actual values are already
 MvdV> known at compile time and so the compiler will handle it the same
 MvdV> as when the actual values were substituted: 

yeah, but we're not studying compiler design so please... don't let that bother
your noggin', eh? each of those variables could have been grabbed by some
randomizing function... but i had to have some way of presenting the scenerios
to you...

 MvdV> if (2+2==5) or (3+3==7) then print "egads!"

 MvdV> The compiler will skip it and nothing will be printed

 MvdV> If it is a less smart compiler, it will treat the terms in the if
 MvdV> statement as true variables - like the ones that are only entered
 MvdV> at run time - and generate the complete object code needed to
 MvdV> handle the general case.

yes...

 MvdV> Still, when the object code is run, it will evaluate, the
 MvdV> condition as FALSE (2+2 is not equal to 5 and 3+3 inot equal to
 MvdV> 7) and it will not look what is after the then and just skip to
 MvdV> the statement following the print "egads!" 

and yes...

 ml>   if (a+b==c) and (d+e==f) then print "EGADS!";

 MvdV> In this example, when the object code is run, is will not
 MvdV> evaluate the (d+e==f) part as the condition has already failed on
 MvdV> the first part.  Whatever is after the and can never make the
 MvdV> condition come TRUE, so no need to waste time evaluating it.

you're sure? there's several assumptions being made... one is that the first
statement is on the left ;)

 ml>   c=4;
 ml>   if (a+b==c) or (d+e==f) then print "whew!";

 ml>   c=5; f=6;
 ml>   if (a+b==c) or (d+e==f) then print "Whew!";

 ml>   c=4; f=6;
 ml>   if (a+b==c) and (d+e==f) then print "WHEW!";

 MvdV> I will leave it to the reader to work out the rest, I think I
 MvdV> have explained the basics.

yes, the basics as well as some shortcuts that can bite at not so good times...
witness bill gates' numerous demonstrations of various windows platforms in
front of hundreds of people :0 :) :) :)  witness, also, how many bugs have lead
to some very severe security holes in those same windows platforms...

)\/(ark

 * Origin:  (1:3634/12)