RE: [Dev-C++] dynamic allocation and structs

[Ioannis V. <no...@ho...>]

> -----Original Message-----
> From: Per Westermark [mailto:pw...@ia...] 
> Sent: Friday, October 25, 2002 1:02 AM
> To: Ioannis Vranos
> Cc: dev...@li...
> Subject: RE: [Dev-C++] dynamic allocation and structs
> 
> 
> Well Ioannis, I did manage to get a star on the wrong side of 
> the struct 
> type name, see inserted correction below, so the example was slightly
> broken. However, the example was not a total goose chase try at a
> two-dimensional matrix of entries, but an example of creating a
> one-dimensional matrix (a vector) of pointers to single 
> entries, which was
> exactly what the heading suggested "array of pointers to 
> entries".


Lets avoid using the term "vector". Well an array of single pointers is
a 2 dimensional array of elements so it is the same.


> If the  
> code wasn't obvious, a hint to that fact should have been that I only
> used a single for loop to assign values to the entries.

Which i think doesn't work.



> 
> One reason to use alternative two, a vector of pointers to elements 
> instead of a vector of elements, is that sorting of the vector only
> require changing the place of the pointers, instead of 
> copying the full
> structures.


So you wanted to create a single-dimentional array of elements? Then a
single * would be enough.



> 
> Another reason is that it might be used to create a sparse 
> matrix, where
> the vector might be very long, but only some index values of 
> the vector
> will have an allocated entry.
> 
> A third reason is that a vector of pointers might be to store 
> different
> types of structs in the same vector, as in:
> 
> enum {
>     CIRCLE,
>     LINE,
>     BOX
> };
> 
> struct Circle {
>     int type;
>     int x,y;
>     int radius;    
> };
> 
> struct Line {
>     int type;
>     int x1,y1,x2,y2;
> }
> 
> struct Box {
>     int type;
>     int x1,y1,x2,y2;
> };
> 
> void fill_array() {
>     Circle *pCircle;
>     Line *pLine;
>     Box *pBox;
>     int item_count,i;
> 
>     item_count = 0;
>     pCircle = (Circle*)malloc(sizeof(Circle));
>     pCircle->type = CIRCLE;
>     pCircle->x = 10;
>     pCircle->y = 20;
>     pCircle->radius = 5;
>     myarray[item_count++] = pCircle;
> 
>     pBox = (Box*)malloc(sizeof(Box));
>     pBox->type = BOX;
>     pBox->x1 = 15;
>     pBox->y1 = 2;
>     pBox->x2 = 18;
>     pBox->y2 = 5;
>     myarray[item_count++] = pBox;
> 
>     ...
> 
>     // Draw items.
>     for (i = 0; i < item_count; i++) {
>         if (myarray[i]) {
>             switch (myarray[i]->type) {
>                 case CIRCLE:
>                     pCircle = (Circle*)myarray[i];
>                     // Draw circle
>                     break;
>                 case BOX:
>                     pBox = (Box*)myarray[i];
>                     // Draw box
>                     break;
>                 case LINE:
>                     pLine = (Line*)myarray[i];
>                     // Draw line
>                     break;
>                 default:
>                     // Complain about unsupported object type.
>             }
>         }
>     }


I did not get your meaning. Where is myarray  definition. I assume
myarray is a single arrray of void *. And?

Ioannis

* Ioannis Vranos
* Programming pages: http://www.noicys.freeurl.com
* Alternative URL: http://run.to/noicys