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

[Per W. <pw...@ia...>]

The term vector is valid, since the allocated information is only supposed
to be used with one single index, i.e. one-dimensional.

It must always be observed - yes this is outside scope of Dev-C++ but
standard C/C++ requirements - that a while a pointer might point to the
forst element of a vector, i.e.
mystruct *p;
p[0].x = 1;
p[1].x = 2;
...
it is not required or guaranteed that there are more than one element the
pointer is pointing at. Never assume a p[1] just because p[0] is fine.
The code below uses vector[i]->id, not vector[i][0].id, to signify that
this is a one-dimensional allocation, and that while the compiler would
accept vector[i][0], vector[i][1], vector[i][1] would point outside
allocated memory.

Full source compilable in c:
----test.c----
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Obviously this struct should contain more than just an integer.
typedef struct {
    int id;
    char *str;
} mystruct;

int cmp(const void* a,const void* b) {
    const mystruct *p1 = *(const mystruct**)a;
    const mystruct *p2 = *(const mystruct**)b;
    return p1->id - p2->id;
}

int main() {
    const int count = 10;
    mystruct **vector;
    char buf[100];
    int i;

    // Allocate vector of pointers TO SINGLE ELEMENTS!
    // Yes, it does look like allocating vector of pointers to vectors,
    // but that is the charm of C/C++. It is up to the end user to
    // know if a pointer points to first element of a vector, or just
    // to a single element.
    vector = (mystruct**)malloc(count*sizeof(mystruct*));
    for (i = 0; i < count; i++) {
        // Allocate count number of single elements.
        vector[i] = (mystruct*)malloc(sizeof(mystruct));
    }
    // Assign values.
    for (i = 0; i < count; i++) {
        vector[i]->id = rand()%100;
        sprintf(buf,"This was element vector[%d]",i);
        vector[i]->str = strdup(buf);
    }
    // Sort values.
    qsort(vector,count,sizeof(*vector),cmp);
    // Print values.
    for (i = 0; i < count; i++) {
        printf("Vector element %d has value %d and str '%s'\n",
            i,vector[i]->id,vector[i]->str);
    }
    // Clean up.
    for (i = 0; i < count; i++) {
        free(vector[i]->str);
        free(vector[i]);
    }
    free(vector);
    return 0;
}
-------------------

/Per W

On Fri, 25 Oct 2002, Ioannis Vranos wrote:

> > -----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
> 
> 
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