What are convincing examples where pointer arithme

Question

I'm preparing some slides for an introductory C class, and I'm trying to present good examples (and motivation) for using pointer arithmetic over array subscripting.

A lot of the examples I see in books are fairly equivalent. For example, many books show how to reverse the case of all values in a string, but with the exception of replacing an a[i] with a *p the code is identical.

I am looking for a good (and short) example with single-dimensional arrays where pointer arithmetic can produce significantly more elegant code. Any ideas?

Answer 1

You're asking about C specifically, but C++ builds upon this as well:

Most pointer arithmetic naturally generalizes to the Forward Iterator concept. Walking through memory with *p++ can be used for any sequenced container (linked list, skip list, vector, binary tree, B tree, etc), thanks to operator overloading.

Answer 2

Pointer arithmetic may look fancy and "hackerish", but I have never encountered a case it was FASTER than the standard indexing. Just the opposite, I often encountered cases when it slowed the code down by a large factor.

For example, typical sequential looping through an array with a pointer may be less efficient than looping with a classic index on a modern processors, that support SSE extensions. Pointer arithmetic in a loop sufficiently blocks compilers from performing loop vectorization, which can yield typical 2x-4x performance boost. Additionally, using pointers instead of simple integer variables may result in needless memory store operations due to pointer aliasing.

So, generally pointer arithmetic instead of standard indexed access should NEVER be recommended.

Answer 3

#include ctype.h
void skip_spaces( const char **ppsz )
{
  const char *psz = *ppsz;
  while( isspace(*psz) )
    psz++;
  *ppsz = psz;
}

void fn(void)
{
  char a[]="  Hello World!";
  const char *psz = a;
  skip_spaces( &psz );
  printf("\n%s", psz);
}