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Pointer to an array of structures

Hi there..

My name is Frank Münnich. I've got a question about pointers that
refer to an array of a structure.
How do I declare that type?

If I have declared a structure
struct mystruc {
int x,y,z;
char a,b,c;
};

and have furthermore declared

mystruc data[20];

and now I would like to have a pointer that refers to the array of
structures, how do I do this?

mystruc *mypointer[20] declares an array of 20 pointers, not a pointer
referring to an array of 20 structures.

I need this in order to pass the whole structure as a parameter in a
function, so that the function can alter the data in the field.

If anyone could help, it would be HIGHLY appreciated.
Sincerely yours,

Frank Münnich / TU Dresden
Nov 13 '05 #1
8 54422
Frank Münnich <To*******@gmx.de> wrote:
mystruc data[20];

and now I would like to have a pointer that refers to the array of
structures, how do I do this?


You can do:

mystruc *dataArray = &data[0];
mystruc *dataArray = data;

Also, you may want to read:

http://www.eskimo.com/~scs/C-faq/q6.2.html
http://www.eskimo.com/~scs/C-faq/q6.3.html
http://www.eskimo.com/~scs/C-faq/q6.8.html

--
== Eric Gorr ========= http://www.ericgorr.net ========= ICQ:9293199 ===
"Therefore the considerations of the intelligent always include both
benefit and harm." - Sun Tzu
== Insults, like violence, are the last refuge of the incompetent... ===
Nov 13 '05 #2
To*******@gmx.de (Frank Münnich) writes:
My name is Frank Münnich. I've got a question about pointers that
refer to an array of a structure.
How do I declare that type?


struct x (*p)[size];
Nov 13 '05 #3
"Frank Münnich" <To*******@gmx.de> wrote in message
news:3f*************@news.t-online.de...

I've got a question about pointers that refer to an array of a structure.
How do I declare that type?

If I have declared a structure
struct mystruc {
int x,y,z;
char a,b,c;
};

and have furthermore declared

mystruc data[20];
That would have to be

struct mystruc data[20];
and now I would like to have a pointer that refers to the array of
structures, how do I do this?
struct mystruc (*mypointer)[20];
mystruc *mypointer[20] declares an array of 20 pointers, not a pointer
referring to an array of 20 structures.
Yes, though that would be

struct mystruc *mypointer[20];
I need this in order to pass the whole structure as a parameter in a
function, so that the function can alter the data in the field.


If you just need to modify one structure, you could have your function take
a pointer to a structure:

void foo (struct mystruc *mypointer) { mypointer->x = 4; }

In fact, if you need to modify the contents of an array of structures, you
could just pass a pointer to the first element:

void foo (struct mystruc *mypointer, size_t array_length)
{
size_t i;
for (i = 0; i < array_length; ++i)
{
mypointer[i].x = 4;
}
}

Hope that helps.

--
Russell Hanneken
rh*******@pobox.com
Nov 13 '05 #4


"Frank Münnich" wrote:

Hi there..

My name is Frank Münnich. I've got a question about pointers that
refer to an array of a structure.
How do I declare that type?

If I have declared a structure
struct mystruc {
int x,y,z;
char a,b,c;
};

and have furthermore declared

mystruc data[20];
This is an illegal definition. Perhaps you meant

struct mystruc data[20];
If you are compile this as C++, stop now. You will continue to have more
problems.
and now I would like to have a pointer that refers to the array of
structures, how do I do this?
What book are you using that doesn't explain the array to pointer
conversion?
I need this in order to pass the whole structure as a parameter in a
function, so that the function can alter the data in the field.

Declare your function:

void func (struct mystruc *funcdata);
Then call it:

func (data);


Brian Rodenborn
Nov 13 '05 #5
On Fri, 11 Jul 2003 19:42:34 GMT, To*******@gmx.de (Frank Münnich)
wrote:
Hi there..

My name is Frank Münnich. I've got a question about pointers that
refer to an array of a structure.
How do I declare that type?

If I have declared a structure
struct mystruc {
int x,y,z;
char a,b,c;
};

and have furthermore declared

mystruc data[20];

and now I would like to have a pointer that refers to the array of
structures, how do I do this?

mystruc *mypointer[20] declares an array of 20 pointers, not a pointer
referring to an array of 20 structures.

I need this in order to pass the whole structure as a parameter in a
function, so that the function can alter the data in the field.

If anyone could help, it would be HIGHLY appreciated.
Sincerely yours,

Frank Münnich / TU Dresden


To all those helpers out there, THANK YOU.
You made my day, I really appreciate your work! Thanks!!
Sincerely yours,
Frank Münnich
Nov 13 '05 #6
To*******@gmx.de (Frank Münnich) wrote (11 Jul 2003) in
news:3f*************@news.t-online.de / comp.lang.c:
Hi there..

My name is Frank Münnich. I've got a question about pointers that
refer to an array of a structure.
How do I declare that type?

If I have declared a structure
struct mystruc {
int x,y,z;
char a,b,c;
};

and have furthermore declared

mystruc data[20];
This is either wrong and should be
struct mystruc data[20];
or it is C++ and you are in the wrong place (news:comp.lang.c++ will
serve better)

and now I would like to have a pointer that refers to the array of
structures, how do I do this?
struct mystruc *datap;
mystruc *mypointer[20] declares an array of 20 pointers, not a
pointer referring to an array of 20 structures.


If you actually want explicitly "a pointer to an array of 20
structures," use
struct mystruc (*datap2)[20];
But you may find member access a tiny bit trickier.


--
Martin Ambuhl
Returning soon to the
Fourth Largest City in America
Nov 13 '05 #7

"Martin Ambuhl" <ma*****@earthlink.net> wrote in message

If you actually want explicitly "a pointer to an array of 20
structures," use
struct mystruc (*datap2)[20];
But you may find member access a tiny bit trickier.

The point is, you almost certainly don't. I don't think I've ever used such
a construct in more than ten years of C programming.

Even if you know that your array is necessarily 20 items long, almost all C
programmers would take the address of the first element

struct mystruct *datap = array;
or
struct mystruct *datap = &array[0];

and then use a counter to access the members of the array;

/* set all the x members to 100 */
for(i=0;i<20;i++)
datap[i].x = 100;
Nov 13 '05 #8
To*******@gmx.de (Frank Münnich) writes:
Hi there..

My name is Frank Münnich. I've got a question about pointers that
refer to an array of a structure.
How do I declare that type?

If I have declared a structure
struct mystruc {
int x,y,z;
char a,b,c;
};

and have furthermore declared

mystruc data[20];

and now I would like to have a pointer that refers to the array of
structures, how do I do this?


Declaration mirrors usage. So think how you would access the root type
of struct mystruc once you had obtained such a type.

First you'd have to dereference the pointer:

*foo

Then, you could access element n of the resulting array:

(*foo)[n]

NOTE: Remember that postfix operators have higher precedence than any
others; thus, without the parens, it would be assumed that foo is an
array of pointers, not the other way around (as you have already
discovered). Now, you have the struct you needed! So to declare foo,
you just use:

struct mystruc (*data)[20];

Note that, since declaration mirrors usage, and the postfix []
operators bind closer than the unary *, the [] declarator
also binds closer than the * declarator.

HTH,
-Micah

Nov 13 '05 #9

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