NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/7751

Text File | 1996-08-05 | 3.7 KB | 82 lines

Newsgroups: comp.lang.c
Path: news.sprintlink.net!eskimo!scs
From: [e-mail] (Steve Summit)
Subject: why arrays may seem like pointers (was: What is &Variable ... )
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Message-ID: <[e-mail]>
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References: <4gsdno$[e-mail]> <4gtab6$[e-mail]> <[e-mail]>
Date: Wed, 28 Feb 1996 17:47:49 GMT

In article <[e-mail]>, [e-mail]
(Mike Rubenstein) writes:
> Brenda <g051286> wrote:
>> [e-mail] (Jonas J. Schlein) wrote:
>>> Yes it could and yes it is...'myarray' is not a pointer, but &myarray is
>>> a pointer to 'myarray'.
>>
>> Um, that's not correct.  myarray is DEFINITELY a pointer! ...
>
> NO. NO. NO.  Where do people get this idea that arrays are pointers.
> Arrays are arrays and pointers are pointers.  In many, but not all,
> situations an array is converted to a pointer.

I know of at least two reasons why confusion here persists.
The first, of course, is that as formal parameters to functions,
what look like (that is, what may be declared as) arrays are
really pointers.  It's assumed that some programmers read more
into this facet of C's pointer/array "equivalence" than is the
case.  (It's been suggested that the pagination between pages 99
and 100 of K&R2 doesn't help any, although I sincerely hope that
no one tries to learn C by opening K&R2 at random and reading the
top of page 100 in isolation.)

I recently discovered a second reason.  I teach a small, informal
class on C programming, and one of my main reasons for teaching
it is so that *I* can learn where some of these perpetual
misconceptions and difficulties come from.  I've noticed that if
I say, "In blah blah blah, an array is converted into a pointer,"
some students hear that and think that the array itself has been
converted into a pointer, now and forever.  (Perhaps this notion
is reinforced by the fact that, in a few other situations, such
as function parameters, C does rewrite declarations for you.)
In other words, when you say "When an array reference appears in
an expression, it is converted into a pointer," what a student
may hear and learn is something like "When you reference an array
in an expression, the array is changed into a pointer."  It's a
small step from there to noticing that, since you can hardly use
an array without referencing it in some expression somewhere in
your program, by the time you're done, all your arrays will have
been changed into pointers, and you won't have any arrays left.

(In a similar vein, I've had a student ask whether code like

    int i = 1, j = 2;
    float f;
    f = (float)i / j;

changed i into a float permanently.  Would this make any sense at
all?  Not to those of us who know C already, but a student, by
definition, doesn't, and can form the most marvelously peculiar
models is left to his own devices.)

Having learned this, I'm now careful to say things like, "When
you make reference to an array in an expression, the compiler
automatically generates a pointer to its first element, just as
if you'd written &array[0]."  I try to use words like "generate,"
and *not* to use words like "convert."

(As a final note: I composed this article to answer Mike's
question, which is a good one, and not to poke any fun at him,
but you might be intrigued if you go back, as I just did, and
look at the words he happened to use.)

                    Steve Summit
                    [e-mail]
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