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Date: Mon, 06 Jun 2011 08:21:39 -0400
From: pete <pfiland@mindspring.com>
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Subject: Re: efficient or clever way
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.c:5557

dr.oktopus wrote:
> 
> Hello,
> walking an array is a common construct in programming.
> In C language, I see two common practises.
> 
> 1:
> 
> for (p = array, pend = array + size ; p < pend ; ++p)
> /* do something */
> 
> 2:
> 
> count = size;
> p = array;
> while (count--) {
> /* do something here */
> ++p;
> }
> 
> In complex contests, keeping a count variable could
> be more readable (IMO) than having a variable acting
> as a sentinel for the array bound (I'm speking of pieces
> of codes where array navigation is not from the start
> to the end, or inside Duff's devices, for example).
> My question is: is this code really inefficient than
> first approach (since it has to dec a variable every
> cycle, more than test it) or current cpus could handle a sort
> of decrement and test instr that do it all at the same time?
> Thanks,
>                   wily

It all depends on the cpu.
Comparing two variables against each other 
may or may not be slower 
than a decrement plus a comparison against constant zero.

A decrement plus a comparison against constant zero,
is a single instruction in Microchip assembly.

-- 
pete