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);
}

[pete]

pereges wrote:

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.

Then, I would write that this way:

/* BEGIN new.c */

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

int **create_ptr_li st(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 */
--
pete

[Chris Torek]

[warning: all my code here is untested]

In article <OM************ *************** ***@earthlink.c om>
pete <pf*****@mindsp ring.comwrote:

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

int **create_ptr_li st(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;
}

I never really understand what people have against local variables :-)

I would begin by rewriting the above function as:

/*
* Create an array of pointers to elements that exceed the
* specified limit. The input array is a[i], where 0 <= i < n,
* and the limit is the given value. The size of the pointer
* array is placed in *size_ptr.
*/
int **create_ptr_li st(int *a, size_t n, int limit, size_t *size_ptr) {
int **b;
size_t i, over_limit;

for (i = over_limit = 0; i < n; i++)
if (a[i] limit)
over_limit++;
b = malloc(over_lim it * sizeof *b);
if (b != NULL) {
for (i = over_limit = 0; i < n; i++)
if (a[i] limit)
b[over_limit++] = &a[i];
*size_ptr = over_limit;
} else {
/* optionally: handle error here if over_limit 0 */
*size_ptr = 0;
}
return b;
}

More generally, one might pass the function a pointer to another
function that selects elements that pass some criterion or
criteria. This does make things substantially more complicated,
and possibly significantly slower (depending on how long it takes
to call functions):

int **create_ptr_li st(int *a,
size_t n,
int (*match)(int, void *),
void *context,
size_t *size_ptr) {
int **b;
size_t i, nmatch;

for (i = nmatch = 0; i < n; i++)
if (match(a[i], context))
nmatch++;
b = malloc(nmatch * sizeof *b);
... the rest is obvious ...
}

We can get also rid of the "int" restriction by changing "int *a"
to "void *base" and adding a "size" parameter. Now, however, we
must build an array of "void *"s, so this may be going too far
(and indeed the match-with-context may already be too far). This
time I will verify that the match function does not change its
mind though:

void **create_ptr_li st(void *base0, size_t n, size_t size,
int (*match)(void *, void *),
void *context,
size_t *size_ptr) {
unsigned char *base = base0;
void **b;
size_t i, nmatch1, nmatch2;

for (base = base0, i = nmatch1 = 0; i < n; base += size, i++)
if (match(base, context))
nmatch1++;
b = malloc(nmatch1 * sizeof *b);
if (b != NULL) {
for (base = base0, i = nmatch2 = 0; i < n; base += size, i++) {
if (match(base, context) {
if (nmatch2 >= nmatch1)
... handle error ...
b[nmatch2++] = base;
}
}
... optionally, make sure nmatch2 == nmatch1 ...
*size_ptr = nmatch2;
} else {
/* optionally: handle error here if over_limit 0 */
*size_ptr = 0;
}
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);

Seems like this should be:

b = create_ptr_list (a, sizeof a / sizeof a[0], &size);

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

--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) [phone]
email: gmail (figure it out) http://web.torek.net/torek/index.html

[Nick Keighley]

On 30 Jun, 16:33, pereges <Brol...@gmail. comwrote:

<snip>

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

<snip>

there are 11 elements in this array

* * create_ptr_list (a, &b, 10, &size);

you pass 10 to this function. Is this correct?

I use a macro like this

#define ARRAY_SIZE(A) (sizeof(A)/sizeof(*A))

(oops! another use for #define!)

<snip>

--
Nick Keighley

"If builders built houses the way programmers write programs,
the first woodpecker coming along would destroy civilization"

[pete]

Chris Torek wrote:

[warning: all my code here is untested]

In article <OM************ *************** ***@earthlink.c om>
pete <pf*****@mindsp ring.comwrote:

>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);

Seems like this should be:

b = create_ptr_list (a, sizeof a / sizeof a[0], &size);

.... especially with eleven elements in the array.

--
pete