How to complete function to use pointer arithmetic instead of array subscripting in C?

Question

I am struggling with specific parts in my code that I seem to have formatted wrong. This code was taken from my programming book and the parts that were blank have a '$' around them. However, there are two blanks I can't seem to figure out. My current code is:

int sum_two_dimensional(const int a[][LEN], int n)
{
  int i,j, sum = 0;
  for(i=0, i<n; i++)
     for(j = 0; j< LEN; j++)
       sum+=a[i][j];
  return sum;
}

int sum_two_dimensional_array(const in a[][LEN], int n)
{
  int *p, sum = 0;

  for(p= a[0]; p < a[0] ______; p++)
      sum += ________;                     //my guess is a[p][sum];
  return sum;
}

I tried several things in these blanks an it seems that I keep getting errors. I do not fully understand the array/pointer situation. The blanks that I filled in, (encased in $$$), I feel are right but feel free to double check me. I appreciate any help.

The title says you want to rewrite a function, but the question suggests you want to know what to put in the blanks. Which is it? — M.M
– M.M, Commented Oct 27, 2016 at 3:43
@CodeFreak you first need to understand using a pointer to iterate over a 1-D array. I'd suggest reading material on that first. Once you understand that then this will be a small step on top of that. (*p is the same in both cases, it is just specifying the end condition that is slightly different for the 2-D array) — M.M
– M.M, Commented Oct 27, 2016 at 4:07

xvan · Accepted Answer · 2016-10-27 05:15:13Z

1

This exploits the fact that an array a[N][M] uses the same memory as a single dimension array a[N*M]

So you can "safely" iterate a[0] "out of bound" without triggering memory exception up to the index a[0][N*M-1]

int sum_two_dimensional_array( int a[][LEN], int n)
{
  int *p, sum = 0;

  for(p= a[0]; p < a[0]+n*LEN; p++)
      sum += *p;
  return sum;
}

edited Oct 27, 2016 at 5:15

answered Oct 27, 2016 at 5:02

xvan

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