Re: Just curiosity about some constructs

"Jordan Abel" <random832@xxxxxxxxx> wrote in message
news:slrndu1qar.crm.random832@xxxxxxxxxxxxxxxx
> On 2006-02-01, Rod Pemberton <dont_have@xxxxxxxxxxxxx> wrote:
> >
> > "Keith Thompson" <kst-u@xxxxxxx> wrote in message
> > news:lnacdb26n0.fsf@xxxxxxxxxxxxxxxxxx
> >> "Rod Pemberton" <dont_have@xxxxxxxxxxxxx> writes:
> >> [...]
> >> > The ultimate example of this can be found in "C: A Reference Manual"
by
> >> > Samuel Harbison and Guy Steele, Tartan Laboratories, 3rd. edition, on
> > page
> >> > 231:
> >> >
> >> > switch (x)
> >> >     default:
> >> >     if (prime(x))
> >> >         case 2: case 3: case 5: case 7:
> >> >             process_prime(x);
> >> >     else
> >> >         case 4: case 6: case 8: case 9: case 10:
> >> >             process_composite(x);
> >> >
> >> > The only other situation where I've seen an unstructured switch was
to
> >> > select between two while's at the end of a do-while loop.  Otherwise,
> >> > structured programming of switch is the way to go (using {}).  I
think
> > the
> >> > reason behind unstructured elements in C, like 'goto' and 'switch',
was
> > to
> >>
> >> See also Duff's Device, <http://www.c-faq.com/misc/duff.html>.
> >>
> >
> > Cool!  I don't recall seeing that one, although I'm sure I read
Stroustrup's
> > book, just not as many times as Harbison's and Steele's, I guess.  But
on
> > the web, Tom Duff, regarding his "Duff's Device" said: "(Actually, I
have
> > another revolting way to use switches to implement interrupt driven
state
> > machines but it's too horrid to go into.)"  So, we are still missing the
> > _ULTIMATE_ unstructured switch.  :(  The worst (or best?) part was I
> > understood Duff's Device before reading the description.  :(
>
> Check this out:
> http://www.chiark.greenend.org.uk/~sgtatham/coroutines.html
>
> Update, 2005-03-07: Tom Duff confirms this in a blog comment. The
> "revolting way to use switches to implement interrupt driven state
> machines" of which he speaks in his original email is indeed the same
> trick as I describe here.

Yes!  Thank you.

He calls it 'continue and return,' or similar to Pascal's 'with' (I have no
idea what that means).  But, it is a nice way to 'ping-pong' between two
communicating/coordinated routines.  It essentially allows a single
procedure with one entry point to have as many entry points as needed.  Very
cool.  Usefulness, questionable...  That brings up another unstructured
element of C, besides 'goto' and 'switch', which is 'return'.  A truly
structured procedure would only have or allow a single entry point and a
single exit point, i.e., 'return'.  Instead , C allows multiple exit points,
via 'return', for procedures but has no multiple entry points by default.

Rod Pemberton


.

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