How come C allow structure members to be addressed like an array ?

[pereges]

#include <stdio.h>

typedef struct
{
double x, y, z;
}vector;

int main(void)
{
int i;
vector v;
double *cord;

v.x = 10;
v.y = 1;
v.z = 2;

cord = &v.x;

for(i = 0; i < 3; i++)
{
printf("%f\n", cord[i]);
}
return 0;
}

here's the output i get:
10.000000
1.000000
2.000000

which is the same as v

i don't how it happens as i was just trying some random ideas but
great stuff really. helped me to reduce some of my code to almost
1/3rd its size.

[Ben Bacarisse]

Ian Collins <ia******@hotma il.comwrites:

Ben Bacarisse wrote:

>Ian Collins <ia******@hotma il.comwrites:

>>viza wrote:
Hi

On Sun, 15 Jun 2008 10:51:28 +1200, Ian Collins wrote:
Tomás Ó hÉilidhe wrote:
> if (99 == (&v.x)[0] &&
> 56 == (&v.x)[1] &&
> 42 == (&v.x)[2]) return;
> puts("ERROR: There's padding in the Vector struct.");
> exit(EXIT_FAILU RE);
A very verbose way of writing
assert( sizeof(vector)= =sizeof(double) *3);
The two are not equivalent and neither is perfect. Better is:

assert( offsetof(vector ,y) == sizeof(double)
&& offsetof(vector ,z) == sizeof(double)* 2 );

Why?

Tomás's code is over complex (one test is enough) and could fail in
unusual cases (see my other posting) and your test will be thrown if
there is padding at the end of the struct.

OK, make it

assert( sizeof(vector[2])==sizeof(doubl e)*6 );

Or even a compile time assert:

const int n = 1/( sizeof(vector[2])==sizeof(doubl e)*6 );

I don't think any of the proposed sizeof tests work due to possible
padding at the end of the struct. I know this has been covered by
later messages, but it seems worth summarising.

>>Anyway, if offsetof(vector ,z) == sizeof(double)* 2, how can
offsetof(vect or,y) be anything other than sizeof(double)?

Agreed. I still prefer my address test, though.

Fair enough, but the simplified offsetof (or sizeof) test has the
(small) advantage of not requiring an instance of vector to test.

Good point. That gives the offsetof version the edge, I'd say.

--
Ben.

[Antoninus Twink]

On 14 Jun 2008 at 22:34, Tomás Ó hÉilidhe wrote:

Vector const v = { 99, 56, 42 };

if (99 == (&v.x)[0] &&
56 == (&v.x)[1] &&
42 == (&v.x)[2]) return;

Holy crap, are you trying to follow the CBF anti-style rules?

(I know that floating-point arithmetic isn't exact but as far as I
know it's exact for integer values... but I'm open to correction!)

It's nothing to do with integer values. Just passing around a
floating-point value won't magically change its bits.

In

double a=42;
double b=42.422242;
double c=1./3;
assert(a == 42);
assert(b == 42.422242);
assert(c == 1./3);

all the asserts will succeed.

Problems only arise when you start doing calculations and get
incremental rounding errors, either from separate steps in the
calculation, or from single steps where the compiled code uses a
temporary, or from type conversions (e.g. float promoted to double).

[Antoninus Twink]

On 15 Jun 2008 at 3:56, vi******@gmail. com wrote:

Thomas: Please change your name to ASCII.

Probably he cares more about making a nationalist point than making life
simpler for people using non-unicode aware terminals/newsreaders. It's
interesting that most Russian, Chinese and Japanese posters are able to
swallow their pride enough to Latinize their names.

[Ben Bacarisse]

Antoninus Twink <no****@nospam. invalidwrites:

On 15 Jun 2008 at 3:56, vi******@gmail. com wrote:

>Thomas: Please change your name to ASCII.

Probably he cares more about making a nationalist point than making life
simpler for people using non-unicode aware terminals/newsreaders. It's
interesting that most Russian, Chinese and Japanese posters are able to
swallow their pride enough to Latinize their names.

Well, that helps me get a clearer picture of how you see the world.

I think it is quite reasonable that people should be able to spell
their names correctly, but that argument will be lost on someone who
does not even dare let their name be know.

--
Ben.

[Joachim Schmitz]

Ben Bacarisse wrote:

Antoninus Twink <no****@nospam. invalidwrites:

>On 15 Jun 2008 at 3:56, vi******@gmail. com wrote:

>>Thomas: Please change your name to ASCII.

Probably he cares more about making a nationalist point than making
life simpler for people using non-unicode aware
terminals/newsreaders. It's interesting that most Russian, Chinese
and Japanese posters are able to swallow their pride enough to
Latinize their names.

Well, that helps me get a clearer picture of how you see the world.

I think it is quite reasonable that people should be able to spell
their names correctly, but that argument will be lost on someone who
does not even dare let their name be know.

Just curious: what makes you believe that Antonius Twink is not a real name?

There are quite a few 'regulars' here that obviously don't use real names,
Default User for example and quite a few where it's hard to tell whether
it's a nick, a first or a last name. To me Antonius Twink /apperas/ to be a
real name.

Bye, Jojo

[santosh]

Joachim Schmitz wrote:

Ben Bacarisse wrote:

>Antoninus Twink <no****@nospam. invalidwrites:

>>On 15 Jun 2008 at 3:56, vi******@gmail. com wrote:
Thomas: Please change your name to ASCII.

Probably he cares more about making a nationalist point than making
life simpler for people using non-unicode aware
terminals/newsreaders. It's interesting that most Russian, Chinese
and Japanese posters are able to swallow their pride enough to
Latinize their names.

Well, that helps me get a clearer picture of how you see the world.

I think it is quite reasonable that people should be able to spell
their names correctly, but that argument will be lost on someone who
does not even dare let their name be know.

Just curious: what makes you believe that Antonius Twink is not a real
name?

There are quite a few 'regulars' here that obviously don't use real
names, Default User for example and quite a few where it's hard to
tell whether it's a nick, a first or a last name. To me Antonius Twink
/apperas/ to be a real name.

I suppose it's the Twink part that leads people to think he is a pseudo.
Also Antoninus isn't IME a very common name either. Besides he used to
(still does?) troll clc for a long time. It's unlikely that he is
stupid enough to troll under his real name.

[Richard]

Ben Bacarisse <be********@bsb .me.ukwrites:

Antoninus Twink <no****@nospam. invalidwrites:

>On 15 Jun 2008 at 3:56, vi******@gmail. com wrote:

>>Thomas: Please change your name to ASCII.

Probably he cares more about making a nationalist point than making life
simpler for people using non-unicode aware terminals/newsreaders. It's
interesting that most Russian, Chinese and Japanese posters are able to
swallow their pride enough to Latinize their names.

Well, that helps me get a clearer picture of how you see the world.

I think it is quite reasonable that people should be able to spell
their names correctly, but that argument will be lost on someone who
does not even dare let their name be know.

You appear to have totally missed the point. If you re-read Twinks reply
then all the necessary points are therein.

Hint : Most usenet posting Germans convert the German characters to
"oe", "ae" and "ss" etc as appropriate.

[Bart]

"pereges" <Br*****@gmail. comwrote in message news:
30************* *************** **...legroups .com...

For something more robust you can try helper functions like get() and
set(), or work with pointers using getp():

#include <stdio.h>

typedef struct
{
double x, y, z;
}vector;

double *getp(vector *p,int i){
if (i==0) return &(p->x);
if (i==1) return &(p->y);
return &(p->z);
}

double get(vector *p,int i){
if (i==0) return p->x;
if (i==1) return p->y;
return p->z;
}

void set(vector *p,int i,double a){
if (i==0) {p->x=a; return;}
if (i==1) {p->y=a; return;}
p->z=a;
}

int main(void)
{
int i;
vector v;

for (i=0; i<3; ++i)
set(&v,i,(i+1)* 10);

for(i = 0; i < 3; i++)
printf("%f\n", get(&v,i));

return 0;
}

Where you are sure the struct is laid out like an array, the contents
of get/set/getp can be simplied to a single line (possibly inlined).
But your main code doesn't change.

--
Bartc

[Richard Tobin]

In article <sl************ *******@nospam. invalid>,
Antoninus Twink <no****@nospam. invalidwrote:

>Probably he cares more about making a nationalist point than making life
simpler for people using non-unicode aware terminals/newsreaders. It's
interesting that most Russian, Chinese and Japanese posters are able to
swallow their pride enough to Latinize their names.

His name is perfectly Latinised. It just isn't Anglicised. Unless
you take "Latin" to be what the ancient Romans used, in which case
your own name isn't Latinised.

-- Richard
--
In the selection of the two characters immediately succeeding the numeral 9,
consideration shall be given to their replacement by the graphics 10 and 11 to
facilitate the adoption of the code in the sterling monetary area. (X3.4-1963)

[Richard Tobin]

In article <g3**********@o nline.de>,
Joachim Schmitz <jo**@schmitz-digital.dewrote :

>Antoninus Twink <no****@nospam. invalidwrites:

[...]

>Just curious: what makes you believe that Antonius Twink is not a real name?

The fact that you've misread it might be taken as a clue.

-- Richard
--
In the selection of the two characters immediately succeeding the numeral 9,
consideration shall be given to their replacement by the graphics 10 and 11 to
facilitate the adoption of the code in the sterling monetary area. (X3.4-1963)