Question

i am trying to create a 2d array using double pointer…
my code is…

---

int **p1;
p1=(int **) malloc(2*sizeof(int *));
for(int i=0;i<2;i++)
{
    p1[i]=(int *) malloc(3*sizeof(int));
    for(int j=0;j<3;j++)
    {
       scanf("%d",(p1+i)+j);
    }

}
for(int i=0;i<2;i++)
{

    for(int j=0;j<3;j++)
    {
       printf("%d\n",*(*(p1+i)+j));
    }

}

---

as i have declared a double pointer (**p1) for it and i am able to put data at all those places using my scan scanf("%d",(p1+i)+j); statement. And to dereference i can use print statement as i have done printf("%d\n",*(*(p1+i)+j));

but why it is breaking during print statement but accepting my scan statement.

but why this is giving me correct response…

int mybox[][4]={{1,2,3,4},{5,6,7,8}};//we have to provide subscript;
for(int i=0;i<2;i++){
     for(int j=0;j<4;j++){
        printf("%d",mybox[i][j]);//will print all elements
        }
}
        printf("%d",*(*(mybox)+1));//give me 2
        printf("%d",*(*(mybox+1)+1));//give me 6
        printf("%d",*(*(mybox+1)+3));//give me 8

Answer 1

Your scanf portion is implemented incorrectly. It destroys your data. Ironically, the problem does not reveal itself during scanf phase, but instead causes a crash at printf stage.

The correct scanf code might look as follows

for(int i=0;i<2;i++)
{
    p1[i]=(int *) malloc(3*sizeof(int));
    for(int j=0;j<3;j++)
    {
       scanf("%d",*(p1+i)+j);
    }
}

Note the extra *. What you really need here is

scanf("%d", &(*(*(p1 + i)) + j));

You can write it this way if you want. But you can notice that the outer & “annihilates” with the next nested *, so the above is equivalent to

scanf("%d", *(p1 + i) + j);

However, my advice to you is to stop using the barely-readable *-and-+ combination for array element access and start using the [] instead. This is how your code should have looked from the very beginning

...
scanf("%d", &p1[i][j]);
...
printf("%d\n", p1[i][j]);

On top of that, since this is taggged C, avoid using sizeof with types as much as possible and stop casting the result of malloc. A better variant would be

int **p1;

p1 = malloc(2 * sizeof *p1);
for(int i = 0; i<2; i++)
{
  p1[i] = malloc(3 * sizeof *p1[i]);
  ...