Dynamic Mutli dimensional Array question.

Hi guys, im trying to make a Dynamic Mutli dimensional Array,

There are lots of tutorials for making these (using a pointer array of poniters) but not for adding and retriving from them really.

and im having trouble retriving and adding iformation to the array,

Ill post my code for adding, creating and resizing the array, and getting information

Code:

---



#define oops(s) { perror((s)); exit(EXIT_FAILURE); }
#define MALLOC(s,t) if(((s) = malloc(t)) == NULL) { oops("error: malloc() "); }

double **dynarray;
int dynArraySize = 0;
int currentRow = -1;
const int COLUMS = 2;
const int INCREMENT = 10;

void createDynarray(double **arrayPtr)
{
        int i;

        MALLOC(arrayPtr, sizeof(double *) * (INCREMENT));
        for (i = 0; i < INCREMENT; i++)
        {
            MALLOC(arrayPtr[i], sizeof(double) * COLUMS);
        }
        dynArraySize = 10;
}
void resizeDynarray(double **arrayPtr)
{
        double **tmp;
        int i;
          if (currentRow >= dynArraySize)
          {
            if(tmp = realloc(arrayPtr, sizeof(double *) * (dynArraySize + (INCREMENT))) == NULL)
            {
              oops("realloc() error! ");
            }
            for(i = dynArraySize; i < (dynArraySize + INCREMENT); i++)
            {
              MALLOC(tmp[i], sizeof(double) * COLUMS);
            }
            dynArraySize += INCREMENT;
            arrayPtr = tmp;
          }
}

void addDoubleToArray(double **arrayPtr,double *data)
{
        int i;
        double **tmpPtr;

        currentRow++;
        if(currentRow < dynArraySize)
        {
                  arrayPtr += currentRow;
          arrayPtr = data;
        }
        else
        {
                resizeDynarray(arrayPtr);
                addDoubleToArray(arrayPtr,data);
        }
}
double *getDoubleFromArray(double **arrayPtr,int index)
{
        double *retVal;
        if(index <= dynArraySize)
        {
                arrayPtr += index;
                retVal = arrayPtr;
        }
        else
        {
        oops("Array out of bounds error! ");
        }
        return retVal;
}

---

I get a program crash when ever i try to retrive something from the array (when i call getDoubleFromArray)

its probly sometihing simple but im stumped? anyone see the problem? (i havent really dealt to much with pointers to pointers so i could be doing something very wrong i guess heh)

Well it could be the fact that you're not even trying to pay attention to your data types.

Code:

---

double *getDoubleFromArray(double **arrayPtr,int index)
{
        double *retVal;
        if(index <= dynArraySize)
        {
                arrayPtr += index;
                retVal = arrayPtr;

---

One of these things is not like the other...


Quzah.

Ahh good point,

i think is where i confused with pointers to pointers,

If i do retVal = *arrayPtr;

That means retval equals the pointer that arrayPtr was pointing to right?

so that means if ArrayPtr points to Pointer which points to 10,
retVal would = the Pointer which points to 10 (so the memory address of 10) right?

No. Don't mix up your operators your * would refer to the value of the variable your & is the address of

If you do

Code:

---

retVal = *arrayPtr;

---

We would assume retVal is plain data which is being assigned with the value of pointer arrayPtr in otherwords, if arrayPtr points to memory that stores a 10. Then retVal would equal 10.

Code:

---

#include <stdio.h>

int main() {
  int foo, baz = 10;
  int *bar;

  bar = &baz;  // Points to the address of baz

  foo = *bar;  // Equals the value that bar is pointing to

  printf("%d", foo);
}

---

hmmmm okay

So arrayPtr is a pointer that points to and array of pointers;
So if wanted to change one of the pointers in the array,
i though i could just do arrayPtr[index] = NewPointer;
but that crashed,

So to do it another way maybe (arrayPtr + index(do i need a sizeof here?)) = Newpointer

but that didnt work either?

Well you're almost right again. retVal is in this case a pointer. Thus, they're right when they say:

Quote:

---

Originally Posted by fatdunky

If i do retVal = *arrayPtr;

That means retval equals the pointer that arrayPtr was pointing to right?

so that means if ArrayPtr points to Pointer which points to 10,
retVal would = the Pointer which points to 10 (so the memory address of 10) right?

---

Yes. retval is a pointer. Thus, if arrayPtr points at a pointer, dereferencing that gives you a pointer. You then assign that pointer directly to retval. Just like doing this:

Code:

---

int *ptr1, *ptr2;
int x = 5;
ptr2 = &x;

ptr1 = ptr2;

---

Here you are assigning the value of ptr2 to ptr1. Thus, ptr1 effectively points to x.


Quzah.

hehe i think i figured out what my problem was *blush* i was correctly creating the array and it was null, if you look at the create array i was just creating a tempory array