malloc for multidimensional array !!please help

[shadab]

I am having big problem retrieving data assgined to dynamic 2-d array .

i am calculating and saving data in to dynamic 2-d array.
but when i retrieve them, it doesnt give correct values.

you can run following code in C compiler and see the difference

follwoing is my code, please have a look and please reply to
(e-mail address removed)


TIA



#include <stdio.h>
#include <memory.h>

main()

{


double *a;

int i, j, lx=5;

a= (double *)malloc(lx*2);
for (i=0; i<lx; i++)
{
for(j=0;j<lx; j++)
{
*(a+i+j)= i+j*0.3;
printf("a[%d][%d]=%f\n",i,j,*(a+i+j));

}

}


printf("copy data \n\n\n");

for (i=0; i<lx; i++)
{
for(j=0;j<lx; j++)
{
printf("a[%d][%d]=%f\n",i,j,*(a+i+j));

}

}

free(a);
}

 

[Richard Heathfield]

shadab said:

I am having big problem retrieving data assgined to dynamic 2-d array .

i am calculating and saving data in to dynamic 2-d array.
but when i retrieve them, it doesnt give correct values.

foo.c:6: warning: return-type defaults to `int'
foo.c:6: warning: function declaration isn't a prototype
foo.c: In function `main':
foo.c:13: warning: implicit declaration of function `malloc'
foo.c:13: warning: cast does not match function type
foo.c:38: warning: implicit declaration of function `free'
foo.c:39: warning: control reaches end of non-void function

Fix these problems first, then post again if you're still having trouble.

 

[santosh]

I am having big problem retrieving data assgined to dynamic 2-d array .

i am calculating and saving data in to dynamic 2-d array.
but when i retrieve them, it doesnt give correct values.

you can run following code in C compiler and see the difference

follwoing is my code, please have a look and please reply to
(e-mail address removed)

#include <stdio.h>
#include <memory.h>

Replace above header with stdlib.h

main()

Replace above with int main(void)

{
double *a;
int i, j, lx=5;

a= (double *)malloc(lx*2);

Do: a = malloc(lx * 2);
Check for success.

for (i=0; i<lx; i++)
{
for(j=0;j<lx; j++)
{
*(a+i+j)= i+j*0.3;

Completely wrong. Do:
a[j] = i + j * 0.3;

printf("a[%d][%d]=%f\n",i,j,*(a+i+j));

Again:
printf("a[%d][%d] = %f\n", i, j, a[j]);

}
}
printf("copy data \n\n\n");
for (i=0; i<lx; i++)
{
for(j=0;j<lx; j++)
{
printf("a[%d][%d]=%f\n",i,j,*(a+i+j));

}
}

What's the point of repeating the code?

 

[santosh]

Replace above header with stdlib.h


Replace above with int main(void)


Do: a = malloc(lx * 2);
Check for success.

Whoops. This should be:
a = malloc(lx * sizeof *a);

<snip>

 

[jaysome]

I am having big problem retrieving data assgined to dynamic 2-d array .

i am calculating and saving data in to dynamic 2-d array.
but when i retrieve them, it doesnt give correct values.

you can run following code in C compiler and see the difference

follwoing is my code, please have a look and please reply to
(e-mail address removed)

#include <stdio.h>
#include <memory.h>

Replace above header with stdlib.h

main()

Replace above with int main(void)

{
double *a;
int i, j, lx=5;

a= (double *)malloc(lx*2);

Do: a = malloc(lx * 2);
Check for success.

for (i=0; i<lx; i++)
{
for(j=0;j<lx; j++)
{
*(a+i+j)= i+j*0.3;

Completely wrong. Do:
a[j] = i + j * 0.3;

Completely wrong. See FAQ 6.16.

http://c-faq.com/aryptr/dynmuldimary.html

 

[santosh]

shadab wrote:

[...]
Completely wrong. See FAQ 6.16.

http://c-faq.com/aryptr/dynmuldimary.html

Thanks. Sorry everyone. Obviously, I shouldn't answer right after
waking up in future.

 

[Simon Biber]

I am having big problem retrieving data assgined to dynamic 2-d array .

i am calculating and saving data in to dynamic 2-d array.
but when i retrieve them, it doesnt give correct values.

#include <stdio.h>
#include <memory.h>

main()

{


double *a;

int i, j, lx=5;

a= (double *)malloc(lx*2);

You want lx rows and lx columns, that's lx * lx values. Not 2 * lx. You
also need to multiply by the size of the element, since malloc expects a
number in bytes.

a = malloc(lx * lx * sizeof (double));
or
a = malloc(lx * lx * sizeof *a);

for (i=0; i<lx; i++)
{
for(j=0;j<lx; j++)
{
*(a+i+j)= i+j*0.3;

You must multiply the row number by the width, so it will skip over that
many elements to keep each row separate.

Here the calculation is wrong again, should be
*(a + i*ix + j)
or
a[i*ix + j]
These two are equivalent.

printf("a[%d][%d]=%f\n",i,j,*(a+i+j));

Same here.

}

}


printf("copy data \n\n\n");

for (i=0; i<lx; i++)
{
for(j=0;j<lx; j++)
{
printf("a[%d][%d]=%f\n",i,j,*(a+i+j));

Same here.

}

}

free(a);

Add:
return 0;

 

[Skybuck Flying]

For 2D apply these rules:

Size = height * width

Offset = width * y + x

Bye,
Skybuck.

 

[CBFalconer]

shadab wrote:

[...]
Completely wrong. See FAQ 6.16.

http://c-faq.com/aryptr/dynmuldimary.html

Thanks. Sorry everyone. Obviously, I shouldn't answer right after
waking up in future.

The next time you do that please note stock prices, winners at the
horse races, etc. and report back here. :=)

 

[pete]

I am having big problem retrieving data assgined to dynamic 2-d array .

i am calculating and saving data in to dynamic 2-d array.
but when i retrieve them, it doesnt give correct values.

you can run following code in C compiler and see the difference

follwoing is my code, please have a look and please reply to
(e-mail address removed)

TIA

#include <stdio.h>
#include <memory.h>

main()

{

double *a;

int i, j, lx=5;

a= (double *)malloc(lx*2);
for (i=0; i<lx; i++)
{
for(j=0;j<lx; j++)
{
*(a+i+j)= i+j*0.3;
printf("a[%d][%d]=%f\n",i,j,*(a+i+j));

}

}

printf("copy data \n\n\n");

for (i=0; i<lx; i++)
{
for(j=0;j<lx; j++)
{
printf("a[%d][%d]=%f\n",i,j,*(a+i+j));

}

}

free(a);
}

/* BEGIN new.c */

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

int main(void)
{
double *a;
unsigned i, j;
const unsigned lx = 5;

a = malloc(lx * lx * sizeof *a);
if (a == NULL) {
puts("a == NULL");
exit(EXIT_FAILURE);
}
for (i = 0; i != lx; ++i) {
for(j = 0; j != lx; ++j) {
a[i * lx + j] = i + j * 0.3;
}
}
for (i = 0; i != lx; ++i) {
for(j = 0; j != lx; ++j) {
printf("a[%u][%u] = %f\n", i, j, a[i * lx + j]);
}
}
free(a);
return 0;
}

/* END new.c */

 

[Tom St Denis]

Here the calculation is wrong again, should be
*(a + i*ix + j)
or
a[i*ix + j]
These two are equivalent.

Better

(i*ix+j)[a]

/me ducks

hehehehe

Tom