Dynamic Memory Allocation

[houksyndrome]

I am writing a program that reads (x,y,z) coordinates from a text file and then manipulates them. I need to store the (x,y,z) coordinates in an array. The problem is that there is a different number of points in each file, so I need to use dynamic memory allocation. I have tried using a command like:
float ptarray=new datatype[arraysize], with arraysize being a variable. However, this doesn't work. I would be grateful for any help.

 

[rpet]

Try this syntax to create a dynamic array:
datatype * name = new datatype[size]; // create array
delete[] name; // destroy array

You can also use a struct to represent your coordinate type.


struct _coord {
float x, y, z;
} coord;

coord * pArray = new coord[arraysize];

/* ... */

pArray[n].x = 1.0f; /* access any coord */


Do not forget to free the memory if no longer need
the array (delete[] pArray).

hth

-- ralf

 

[netcomander]

what about a linked list?

you can compile and try this:

#include <stdio.h>
#include <stdlib.h>

typedef struct p{
int x,y,z;
struct coord *prev;
struct coord *next;
}coord;

// param current is NULL for the first element
coord *add_coord(coord *current){
coord *newcoord;
if((newcoord = (coord *)malloc(sizeof(coord))) == NULL)
return NULL;
// init the new element and link it
newcoord->prev = current;
newcoord->next = NULL;
newcoord->x = newcoord->y = newcoord->z = 0;
// nextcoorder of the previous element to this element
if(current != NULL)
current->next = newcoord;
return newcoord;
}


int main(){
coord* firstElement;
coord* currentElement;

firstElement = add_coord(NULL);
currentElement = firstElement;

currentElement->x = 3;
currentElement->y = 9;
currentElement->z = 15;

currentElement = add_coord(currentElement);
// now currentElement is the second Element...
currentElement->x = 38;
currentElement->y = 99;
currentElement->z = 35;
// the third and so on...
currentElement = add_coord(currentElement);
currentElement->x = 987;
currentElement->y = 87;
currentElement->z = 25;

currentElement = add_coord(currentElement);
currentElement->x = 4562;
currentElement->y = 1235;
currentElement->z = 1;

currentElement = firstElement;
while(currentElement->next){
printf(&quot;x: %d\ny: %d\nz: %d\n\n&quot;, currentElement->x,currentElement->y,currentElement->z);
currentElement = currentElement->next;
}
// not to forget the last...
printf(&quot;x: %d\ny: %d\nz: %d\n\n&quot;, currentElement->x,currentElement->y,currentElement->z);
}

if you insist on using array, you could try
realloc for dynamic memoryallocation


netcomander

 

[Lim]

Linked list can be overkill to store XYZ coordinates (it can have a sense when in one node you will have more then one point)
I would sugest use realloc function with some reallocation number more around 50. (I has performed some test with reallocation and found that reallocation by 50 is the optimal solution (reallocation by 1 takes in 100 times more time, by 100 almost the same as by 50).
Note: it is a C forum, so you can not use new, delete in C.
You can try this:

double (*padPoints)[3] = NULL; /*ptr to array of XYZ */
int nPnt = 0, nAlloc = 0, nRealloc = 50;

int AddPoint(double adPoint[3])
{
double (*padPntTmp)[3] = NULL;

if (nPnt >= nAlloc)
{
/* allocate/reallocate memory */
padPntTmp = realloc(padPoints,
nAlloc+nRealloc * sizeof (double[3]));
if (padPntTmp != NULL)
{
padPoints = padPntTmp;
nAlloc += nRealloc;
}
else
return 1; /* not enough mem */
}
/* add point */
memcpy(padPoints, adPoint, sizeof (double[3]));
nPnt++;
return 0;
}
int main()
{
double adPoint[3];
/* read X,Y,Z */
if (AddPoint(adPoint))
{
return 1; /* not enough memory */
}
for(i=0; i<nPoint; i++)
printf(&quot;XYZ: %12.5e, %12.5e, %12.5e\n&quot;,
padPoint[0],padPoint[1],padPoint[2]);
.....
if (padPoints != NULL)
free (padPoints);
return 0;
}

I did not test it, but should work.

 

[houksyndrome]

Thanks a lot everybody, I got it to work.

-Andrew

 

[Lim]

sorry in memcpy should be:
memcpy(padPoints+nPnt, adPoint, sizeof (double[3]));