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From: smize@Starbase.NeoSoft.COM (Samuel Mize)
Newsgroups: comp.lang.ada
Subject: Re: "Subtract C, add Ada"
Date: 19 Jan 1995 22:43:52 -0600
Organization: NeoSoft Internet Services   +1 713 684 5969
Message-ID: <3fnf28$s3f@Starbase.NeoSoft.COM>
References: <3etund$hnr@miranda.gmrc.gecm.com>
 <3f4mbe$rud@cronkite.seas.gwu.edu> <3f5s92$3id@info.epfl.ch>
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Date: 1995-01-19T22:43:52-06:00
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In article <3f5s92$3id@info.epfl.ch>,
Laurent Gasser <gasser@masg1.epfl.ch> wrote:
>
>The problem is in the language definition, not in the choic of symbols.
>C is one of the rare language I use to allow having an assignment after 
>the if statement.  All other languages only allow for a boolean test.
>
>I never saw a good reason for allowing this.
>

It's a question of who optimizes your code.  In the 60s,
when C was developed, compilers *couldn't* optimize code,
so the programmer had to.  Many apparently-bizarre C
capabilities are there to support optimization level -1
(programmer does the optimizations).

Consider the common C idiom to copy a null-terminated
array, usually a string [syntax may not be quite right]:

     while (*a++=*b++);

Recall that C considers 0 false, a null character is integer
0, and an assignment returns what was assigned.  This loop
condition copies the character at address b to address a,
increments both, and exits when a null was copied.  There is
no statement inside the loop.

This is more efficient, with a non-optimizing compiler, than

  while (*a)
    { *a = *b; a=a+1; b=b+1}; /*or whatever is right syntax*/

because
* The operator ++ increments the value of while it's still
  in a register.  The second code reads a from memory to
  do the character assign, reads it again to increment it.

* ++ can use the assembler's increment instruction.  a=a+1
  could not.

* Using the assignment as the loop expression indirectly
  directs the compiler to keep the result of the assignment
  in a register for reuse in the expression, instead
  of writing it to memory, then reading it.

Thus, with a lot of man-hours, expertise and skull-sweat,
someone using a basic C compiler can come within an order
of magnitude of the efficiency of someone using a half-good
optimizing Ada compiler.  Often.

Of course, if you only *read* the code, and don't *run*
it, it looks like the C is more optimized, because you can
*see* the optimizations.

With Ada you write less-optimized-looking code, and trust
the compiler.  (But verify.)

>-- 
>Laurent Gasser (gasser@dma.epfl.ch)
>Computers do not solve problems, they execute solutions.
>
>I know very few ideas worth dying for, none is worth killing.

I dunno, I've killed quite a few ideas in my time :-)

Sam Mize - smize@starbase.neosoft.com