problem passing pointer array

[pereges]

Hi, can some one please tell me why this program is not able to
function properly. I have a array a and i am trying to create a
pointer array b which points to elements less than 40 in a.

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

void create_ptr_list (int *a, int ***b, int n, int *size_ptr)
{
int i;
*size_ptr = 0;

for (i = 0; i < n; i++)
{
if (a[i] < 40)
(*size_ptr) ++;
}

*b = malloc(sizeof(i nt *) * (*size_ptr));
(*size_ptr) = 0;
for (i = 0; i < n; i++)
{
if (a[i] < 40)
(*b)[*size_ptr++]= &a[i];
}

}

int main(void)
{
int a[] = { 5, -6, 45, -100, 20, -150, 160, 40, 0, 0, 1};
int **b;
int size;
int i;

create_ptr_list (a, &b, 10, &size);
for(i = 0; i <size; i++)
printf("%d\n", *(b[i]));

return (0);
}

[vippstar]

On Jun 30, 6:33 pm, pereges <Brol...@gmail. comwrote:

Hi, can some one please tell me why this program is not able to
function properly. I have a array a and i am trying to create a
pointer array b which points to elements less than 40 in a.

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

void create_ptr_list (int *a, int ***b, int n, int *size_ptr)

Change int ***b to int **b.
Change the 'int **b' in your main() to int *b.

[pereges]

On Jun 30, 8:59 pm, vipps...@gmail. com wrote:

Change int ***b to int **b.
Change the 'int **b' in your main() to int *b.

Wouldn't int *b lead to an array instead of array of pointers ? I
needed array of pointers hence int **b.

[pete]

pereges wrote:

Hi, can some one please tell me why this program is not able to
function properly. I have a array a and i am trying to create a
pointer array b which points to elements less than 40 in a.

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

void create_ptr_list (int *a, int ***b, int n, int *size_ptr)
{
int i;
*size_ptr = 0;

for (i = 0; i < n; i++)
{
if (a[i] < 40)
(*size_ptr) ++;
}

*b = malloc(sizeof(i nt *) * (*size_ptr));
(*size_ptr) = 0;
for (i = 0; i < n; i++)
{
if (a[i] < 40)
(*b)[*size_ptr++]= &a[i];
}

}

int main(void)
{
int a[] = { 5, -6, 45, -100, 20, -150, 160, 40, 0, 0, 1};
int **b;
int size;
int i;

create_ptr_list (a, &b, 10, &size);
for(i = 0; i <size; i++)
printf("%d\n", *(b[i]));

return (0);
}

I would write that this way:

/* BEGIN new.c */

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

int *create_ptr_lis t(int *a, size_t n, size_t *size_ptr)
{
int *b;
size_t i;

*size_ptr = 0;
for (i = 0; n i; ++i) {
if (40 a[i]) {
++*size_ptr;
}
}
b = malloc(*size_pt r * sizeof *b);
*size_ptr = 0;
if (b != NULL) {
for (i = 0; n i; i++) {
if (40 a[i]) {
b[(*size_ptr)++]= a[i];
}
}
}
return b;
}

int main(void)
{
int a[] = {5, -6, 45, -100, 20, -150, 160, 40, 0, 0, 1};
int *b;
size_t size;
size_t i;

b = create_ptr_list (a, 10, &size);
if (b != NULL) {
for (i = 0; size i; ++i) {
printf("%d\n", b[i]);
}
} else {
puts("b == NULL");
}
free(b);
return 0;
}

/* END new.c */

The distinction between counters and sortable data
is more obvious with size_t counters.
I don't like to read sorting functions
where everything is int.

--
pete

[pereges]

On Jun 30, 9:33 pm, pete <pfil...@mindsp ring.comwrote:

I would write that this way:

/* BEGIN new.c */

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

int *create_ptr_lis t(int *a, size_t n, size_t *size_ptr)
{
int *b;
size_t i;

*size_ptr = 0;
for (i = 0; n i; ++i) {
if (40 a[i]) {
++*size_ptr;
}
}
b = malloc(*size_pt r * sizeof *b);
*size_ptr = 0;
if (b != NULL) {
for (i = 0; n i; i++) {
if (40 a[i]) {
b[(*size_ptr)++]= a[i];
}
}
}
return b;

}

int main(void)
{
int a[] = {5, -6, 45, -100, 20, -150, 160, 40, 0, 0, 1};
int *b;
size_t size;
size_t i;

b = create_ptr_list (a, 10, &size);
if (b != NULL) {
for (i = 0; size i; ++i) {
printf("%d\n", b[i]);
}
} else {
puts("b == NULL");
}
free(b);
return 0;

}

/* END new.c */

I actually need an array of pointers. The array b will contains
pointers to elements in a which are less than 40.

The distinction between counters and sortable data
is more obvious with size_t counters.
I don't like to read sorting functions
where everything is int.

Sometimes using size_t or any unsigned entity can cause trouble in
sorting algorithms depending on how we write the sorting algorithm.
eg. just check the post i made on a quick sort algo today.

[vippstar]

On Jun 30, 7:15 pm, pereges <Brol...@gmail. comwrote:

On Jun 30, 8:59 pm, vipps...@gmail. com wrote:

Change int ***b to int **b.
Change the 'int **b' in your main() to int *b.

Wouldn't int *b lead to an array instead of array of pointers ? I
needed array of pointers hence int **b.

Sorry, I just realized that.
The problem in your original code is in this line:

(*b)[*size_ptr++]= &a[i];

that increments 'size_ptr' as a pointer, not the value it points to.
Change it to (*b)[(*size_ptr)++] = &a[i];

[pereges]

On Jun 30, 9:51 pm, vipps...@gmail. com wrote:

Sorry, I just realized that.
The problem in your original code is in this line: (*b)[*size_ptr++]= &a[i];

that increments 'size_ptr' as a pointer, not the value it points to.
Change it to (*b)[(*size_ptr)++] = &a[i];

Thanks, that solved it. I realize it was a stupid mistake

[vippstar]

On Jun 30, 7:51 pm, vipps...@gmail. com wrote:

On Jun 30, 7:15 pm, pereges <Brol...@gmail. comwrote:

On Jun 30, 8:59 pm, vipps...@gmail. com wrote:

Change int ***b to int **b.
Change the 'int **b' in your main() to int *b.

Wouldn't int *b lead to an array instead of array of pointers ? I
needed array of pointers hence int **b.

Sorry, I just realized that.
The problem in your original code is in this line: (*b)[*size_ptr++]= &a[i];

that increments 'size_ptr' as a pointer, not the value it points to.
Change it to (*b)[(*size_ptr)++] = &a[i];

You also don't check the return value of malloc. It could be NULL, in
which case you just return; the caller can check for the
successfulness of create_ptr_list (a, &b, c, &d); with if(b !=
NULL) ...

Also the design is flawed, mainly because this works only for arrays
of ints, and because 40 is hardcoded, how about you change this to
struct vector { size_t nmemb, size; void *elements; };
struct vector *remove_if(cons t struct vector *v, int (*remove)(void
*));

though I don't like the 'remove_if' name, I chose it because there's a
similar function in common lisp named REMOVE-IF.

[pereges]

On Jun 30, 10:00 pm, vipps...@gmail. com wrote:

You also don't check the return value of malloc. It could be NULL, in
which case you just return; the caller can check for the
successfulness of create_ptr_list (a, &b, c, &d); with if(b !=
NULL) ...

Also the design is flawed, mainly because this works only for arrays
of ints, and because 40 is hardcoded, how about you change this to
struct vector { size_t nmemb, size; void *elements; };
struct vector *remove_if(cons t struct vector *v, int (*remove)(void
*));

though I don't like the 'remove_if' name, I chose it because there's a
similar function in common lisp named REMOVE-IF.

Well, I wrote the function just as a test function to understand about
array of pointers. Just chose int to make things simple.

[pete]

pereges wrote:

On Jun 30, 9:33 pm, pete <pfil...@mindsp ring.comwrote:

>I would write that this way:

I actually need an array of pointers.

Sorry about that.

--
pete