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rougie
Joined: 14 Jun 2006
Posts: 31
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| Assigning the base address of a 2 dim array to a pointer |
 Posted: Wed Jul 29, 2009 2:04 pm |
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Hello,
I am using PCWH version 4.095 with PIC 18FL4685 and the programming language is C.
I would like to assign the base address of a two dimensional array to a pointer, and then use that pointer to access informations from the array using the double subscript notation format as:
subscript[0][0];
So basically here is the code:
| Code: | #include <Dv4685.h> // DEVICE INNITIALIZATIONS
#include <stdlib.h> // CCS LIBRARY STANDARD LIBRARY
#include <stdlibm.h> // CCS LIBRARY (HEAP) MALLOC MODULE
int main()
{
long *h, t;
long pmr[2][5] = { {195, 989, 193, 111, 111 },
{194, 195, 112, 222, 111 } };
h = pmr; // Assign the array's base address to a pointer
// Instead of accessing the array's data like this:
//t = *h;
//t = *(h+1);
//t = *(h+2);
// I would like to access the array's data using this format using the pointer:
t = *(h)[0][0];
t = *(h)[0][1];
t = *(h)[0][2];
return 0;
} |
I know that I can use pointer arithmetic, but I am very curious if it can be done using normal accesing method as shown in example above?
All feedback appreciated!
Robert |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed Jul 29, 2009 2:09 pm |
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| Quote: |
I am very curious if it can be done using normal accesing method as
shown in example. |
Test it, and tell us if it works or not. |
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Guest
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| Posted: Wed Jul 29, 2009 2:51 pm |
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I did test it and I get the following error:
too many subscripts?
I don't know how else this would work? |
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FvM
Joined: 27 Aug 2008
Posts: 2337
Location: Germany
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| Posted: Wed Jul 29, 2009 3:20 pm |
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You can't dereference h to a two-dimensional array, because the size of the first dimension is unknown to the compiler. Also *h[0][0] is a wrong level of indirection.
You can access the array elements by mapping them to a one-dimensional array, or you would have to define a long[2][5] typed pointer explicitely.
| Code: | h = pmr; // Assign the array's base address to a pointer
t = h[0*5 + 0]; //h[0][0];
t = h[0*5 + 1]; //h[0][1];
t = h[0*5 + 2]; //h[0][2];
t = h[1*5 + 0]; //h[1][0]; |
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rougie
Joined: 14 Jun 2006
Posts: 31
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| Posted: Wed Jul 29, 2009 5:18 pm |
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Hi Fvm,
| Quote: |
You can access the array elements by mapping them to a one-dimensional array
h = pmr; // Assign the array's base address to a pointer
t = h[0*5 + 0]; //h[0][0];
t = h[0*5 + 1]; //h[0][1];
t = h[0*5 + 2]; //h[0][2];
t = h[1*5 + 0]; //h[1][0];
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Okay, that I am aware of!
But then, does CCS PCWH compiler allow us to do the following:
| Quote: | | or you would have to define a long[2][5] typed pointer |
and if so, how?
I am confused!
Regards Robert |
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FvM
Joined: 27 Aug 2008
Posts: 2337
Location: Germany
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| Posted: Thu Jul 30, 2009 1:14 am |
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I see that the common C syntax for typed pointers to an array isn't accepted by PCH, e.g.:
| Code: | long (*h1)[5];
long (*h2)[2][5]; |
but multi-dimensional arrays can be constructed by hierarchical structs, as far as I remember. In the present case, the suggested index arithmetic is the most simple solution, I think. |
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rougie
Joined: 14 Jun 2006
Posts: 31
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| Posted: Thu Jul 30, 2009 9:58 am |
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Hello FvM,
| Quote: | I see that the common C syntax for typed pointers to an array isn't accepted by PCH, e.g.: "
Code:
long (*h1)[5];
long (*h2)[2][5]; |
I thought so! I was able to create an explicit type pointer of that sort:
In a .c file but compiled in VC++ environment (VC++ accepts C programs) and it did work fine but as you say I could not get this to work in PCWH.
Well I think I will stick to pointer arithmetic.
FvM, there is something else which I don't understand, if you may permit me to express my question!
Please view the following snippet of code:
| Code: | #include <Dv4685.h> // DEVICE INNITIALIZATIONS
#include <stdlib.h> // CCS LIBRARY STANDARD LIBRARY
#include <stdlibm.h> // CCS LIBRARY (HEAP) MALLOC MODULE
int main()
{
long *h, t;
long pmr[2][5] = { {195, 989, 193, 111, 111 },
{194, 197, 112, 222, 333 } };
h = pmr;
t = *((long*)h);
t = *((long*)(h+1)); // inc by 1
t = *((long*)(h+2)); // inc by 1
return 0;
}
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In the above snippet, I am able to assign the two dimensional array's base address to a pointer called "h".
Then, I am able to use pointer arithmetic to access all the values in the two dimensional array *and* I do this by simply incrementing the pointer's address by 1 as shown above!
Now please take a look at this other snippet:
| Code: |
#include <Dv4685.h> // DEVICE INNITIALIZATIONS
#include <stdlib.h> // CCS LIBRARY STANDARD LIBRARY
#include <stdlibm.h> // CCS LIBRARY (HEAP) MALLOC MODULE
typedef struct tag_lb
{
long pmr;
}lb;
int main()
{
long t;
lb *pObj = NULL;
long pmr[2][5] = { {195, 700, 193, 111, 111 },
{194, 195, 112, 222, 111 } };
pObj = malloc (sizeof *pObj);
pObj->pmr = pmr;
t = *((long*) pObj->pmr);
t = *((long*)((pObj->pmr)+2)); // inc by 2 ?
t = *((long*)((pObj->pmr)+4)); // inc by 2 ?
return 0;
}
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In the example above the base address (pmr) is used indirtectly. Innitially, I assigned it to a long pointer member of the tag_lb structure called pmr. Then I simply fetch the pmr pointer whcih still contains the base address of the two dimensional array like this:
| Code: | | t = *((long*) pObj->pmr); |
As you can see, in the line above, I dereference the pointer get the contents of the two dimensional array. The problem is, that I have to do the base address +2 instead of +1 ?
I tried this piece of code in another C compiler and it works with +1. Why in PCWH, we have to do +2 ?
I really would like to know if this is specific to the PCWH compiler?
FvM, I thankyou for your help!
Finest regards
Robert! |
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FvM
Joined: 27 Aug 2008
Posts: 2337
Location: Germany
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| Posted: Thu Jul 30, 2009 3:21 pm |
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As a first comment, your pointer arithmetic variant is generating the same binary code as the one-dimensional array access I suggested previously. It can be simplified, because h is already a long pointer, thus these three lines are equivalent
| Code: | t = *((long*)(h+1)); // inc by 1
t = *(h+1); // inc by 1
t = h[1]; |
I don't exactly understand, why pObj is apparently treated as untypified pointer, so the respective pointer arithmetic uses a byte increment. But you can cast it to a long (int16) pointer.
| Code: | | t = *(((long*)pObj->pmr)+1); |
This variant is generating a considerable code size overhead anyway and shouldn't be used, I think. |
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Guest
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| Posted: Thu Jul 30, 2009 4:24 pm |
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Hello FvM,
| Quote: |
I don't exactly understand, why pObj is apparently treated as untypified pointer so the respective pointer arithmetic uses a byte increment. |
Isn't pObj a pointer typified as an lb type?
Not supposed to! I have tried it in other compilers and works fine lke this:
| Code: | | t = *(((long*)pObj->pmr)+1); |
as opposed to having to add 2 all the time.
| Quote: | | But you can cast it to a long (int16) pointer. |
type casting pObj, would not make the pointer an *lb type anymore. No?
I don't know, this is driving me up a wall. I don't understand this and I really feel like I need to grasp the fundamental understanding of this issue.
FvM I appreciate your help... should you see something I am not seeing, please do get back, I would be greatful.
Finest regrads
Robert |
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FvM
Joined: 27 Aug 2008
Posts: 2337
Location: Germany
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| Posted: Fri Jul 31, 2009 12:51 am |
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| Quote: | Isn't pObj a pointer typified as an lb type?
Not supposed to! I have tried it in other compilers and works fine lke this: |
I think, it's correct C syntax, but not correctly understood by CCS C.
| Quote: | | type casting pObj, would not make the pointer an *lb type anymore. |
Not exactly, but it works anyway in this case. |
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rougie
Joined: 14 Jun 2006
Posts: 31
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| Posted: Fri Jul 31, 2009 7:50 am |
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Hello FvM,
Well that sums that up!
Its been nice chatting with you, I wish you a nice day!
P.S. I thank you for your help!
Finest Regards
Robert |
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