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Groups > comp.lang.c > #163946
| From | Meredith Montgomery <mmontgomery@levado.to> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: on understanding & and pointer arithmetic |
| Date | 2021-12-18 11:32 -0300 |
| Organization | Aioe.org NNTP Server |
| Message-ID | <86v8zm7zmd.fsf@levado.to> (permalink) |
| References | <86a6h0eiy9.fsf@levado.to> <spfllt$vkn$1@dont-email.me> <8635msbbcu.fsf@levado.to> <spg6b8$on0$1@dont-email.me> |
David Brown <david.brown@hesbynett.no> writes:
> On 16/12/2021 20:25, Meredith Montgomery wrote:
>> David Brown <david.brown@hesbynett.no> writes:
>>
>>> On 16/12/2021 15:13, Meredith Montgomery wrote:
>>>> I'm investigating the syntax array[index] and I'm getting surprised at
>>>> some places. My intuition says that a[3] is the same as a + 3. I also
>>>> know that /&a/ is the same as /a/. I first expected the following
>>>> program to print ``lo, world\n'' three times, but it does not.
>>>
>>> I can see why you are confused - you are close, but a bit mixed up.
>>>
>>> "a[3]" is the same as "*(a + 3)". The dereferencing is crucial.
>>
>> Thank you!
>>
>>> In the code below, "a" is an array of 13 char - and that is its type.
>>> When you take its address, "&a", you have a pointer to an array of 13
>>> char. This will have the same /value/ as &a[0], which is a pointer to
>>> the first element of the array - a pointer to a char.
>>
>> Yes, so that corrects me at least once. I didn't think &a would be of a
>> different type. So the type of &a is array of 13 char. I get that.
>
> /No/. "a" is of type "array of 13 char". "&a" is of type "pointer to
> array of 13 char". These are different.
Yes --- thanks. That's what I meant, but didn't write.
>> The value of &a is the same /value/ as &a[0]. I also get that. I'm
>> good here.
>
> Correct. As Ben said (and Ben is very good at explaining things
> accurately), a pointer has a type and has an address as it's value. So
> the values of pointers here are the same, but the pointers themselves
> are different because they have different types.
Yes, I think I will never make that mistake again because that corrected
my intuition now. I was thinking of a pointer as just a variable
holding an integer, but there is a type associated to it which totally
changes arithmetic with it. That's my new intuition. Thanks for
helping me get there --- and for checking my work in your previous post.
[...]
> Once you feel you have got the hang of this, repeat the whole thing with
> an array of "int" rather than an array of "char". That will help you
> appreciate where the scalings come in. (Note that while an "int" is 4
> bytes, or 4 chars, on your platform, it is not necessarily the case on
> other C implementations, especially for very small devices.)
I got the hang of it. Let's see. I'll write the program with my
predictions in comments --- and run it.
--8<---------------cut here---------------start------------->8---
#include <stdio.h>
int main(void) {
int a[] = {1, 2, 3};
printf("%lu\n", (unsigned long) a); // some address
printf("%lu\n", (unsigned long) &a); // same address
printf("%lu bytes\n", (unsigned long) sizeof a); // 12 bytes
printf("%lu bytes\n", (unsigned long) sizeof &a); // 8 bytes
// this will jump 12 bytes to the right
printf("%lu\n", (unsigned long) (&a + 1));
printf("Jumped %lu bytes to the right\n",
(unsigned long) (&a + 1) - (unsigned long) &a);
// this will hump 36 bytes to the right
printf("%lu\n", (unsigned long) (&a + 3));
printf("Jumped %lu bytes to the right\n",
(unsigned long) (&a + 3) - (unsigned long) &a);
}
--8<---------------cut here---------------end--------------->8---
%make arrint
cc -Wall -x c -g -std=c99 -pedantic-errors arrint.c -o arrint
%./arrint.exe
4294954036
4294954036
12 bytes
8 bytes
4294954048
Jumped 12 bytes to the right
4294954072
Jumped 36 bytes to the right
%
That looks good! Thanks very much!
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on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 11:13 -0300
Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-16 14:23 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 11:57 -0300
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 17:42 +0000
Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-16 18:14 +0000
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 21:14 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 16:42 -0300
Re: on understanding & and pointer arithmetic Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-16 12:44 -0800
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:06 -0300
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 21:34 +0000
Re: on understanding & and pointer arithmetic Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-16 14:14 -0800
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 23:44 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:09 -0300
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-16 16:23 +0100
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-16 16:25 -0300
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-16 21:08 +0100
Re: on understanding & and pointer arithmetic Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-16 12:54 -0800
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-17 10:53 +0100
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 11:39 -0300
Re: on understanding & and pointer arithmetic scott@slp53.sl.home (Scott Lurndal) - 2021-12-18 16:31 +0000
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-18 18:56 +0100
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-25 21:29 -0300
Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-18 18:29 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 11:32 -0300
Re: on understanding & and pointer arithmetic Manfred <noname@add.invalid> - 2021-12-16 21:34 +0100
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 22:20 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:02 -0300
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:34 -0300
Re: on understanding & and pointer arithmetic Manfred <noname@add.invalid> - 2021-12-18 18:36 +0100
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-18 21:17 +0000
Re: on understanding & and pointer arithmetic James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-12-16 18:59 -0500
Re: on understanding & and pointer arithmetic James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-12-16 18:38 -0500
Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-16 23:50 +0000
Re: on understanding & and pointer arithmetic "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2021-12-17 07:34 -0800
Re: on understanding & and pointer arithmetic Bart <bc@freeuk.com> - 2021-12-17 17:48 +0000
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-16 23:56 +0000
Re: on understanding & and pointer arithmetic James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-12-16 19:06 -0500
Re: on understanding & and pointer arithmetic Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-12-17 00:10 +0000
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:32 -0300
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:30 -0300
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-17 11:08 +0100
Re: on understanding & and pointer arithmetic Manfred <noname@add.invalid> - 2021-12-17 16:24 +0100
Re: on understanding & and pointer arithmetic David Brown <david.brown@hesbynett.no> - 2021-12-17 18:17 +0100
Re: on understanding & and pointer arithmetic Meredith Montgomery <mmontgomery@levado.to> - 2021-12-18 12:11 -0300
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