2D Int Array print using pointers

Question

While executing the below code:

int main()
{
  int abc[3][3]={0};
  for(int *ip=&abc[0][0];ip<=&abc[3][3];ip++)
  {
    printf("%d   \n",*ip);
  }
}

Expected result is 9 zeros but it displays 12 data. What might be reason?

R Sahu · Accepted Answer · 2018-07-21 02:59:38Z

2

If you look at the memory layout of a 3x3 array, it looks like:

[0][0]          [1][0]        [2][0]         
+----+----+----+----+----+----+----+----+----+
|    |    |    |    |    |    |    |    |    |
+----+----+----+----+----+----+----+----+----+

Where is the element [3][3]?

[0][0]          [1][0]        [2][0]         [3][0]         [3][3] 
+----+----+----+----+----+----+----+----+----+----+----+----+----+
|    |    |    |    |    |    |    |    |    |    |    |    |    |
+----+----+----+----+----+----+----+----+----+----+----+----+----+

That explains why you end up accessing 12 elements.

Your code is subject to undefined behavior for accessing the beyond valid indices but that's another issue.

You could use:

for (int *ip = &abc[0][0]; ip <= &abc[2][2]; ip++)
{
  printf("%d   \n",*ip);
}

However, it is better to access a 2D array as a 2D array.

for (size_t i = 0; i < 3; ++i )
{
  for (size_t j = 0; j < 3; ++j )
  {
    printf("%d   \n", abc[i][j]);
  }
}

edited Jul 21, 2018 at 2:59

answered Jul 21, 2018 at 2:54

R Sahu

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2 Comments

Rajeesh Rarankurissi Over a year ago

Yes Its right. While printing last 3 values shows junk values instead zero.

R Sahu Over a year ago

@alk, <= is right. abc[2][2] is a valid element.

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