More Pointers

Last time: Strings and Pointers

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Additional Credits

Announcements

• Don't forget project 3 is due Wednesday.
• Friday I will be away -- NO CLASS, but project 4 will be assigned by then (which builds on project 3).
• Exam #1 coming next Friday!

Pointer arithmetic

• In addition to single variables, pointers can be used to access parts of an array.

int *ip;
int a[20];

ip = &a[3];

• Given that ip points to element 3 of a, we can use pointer arithmetic to access elements before or after 3, as in

ip++;
*ip = 7;
*(ip+1) = 8;
*(ip-2) = 3;

which sets element 4 to 7, element 5 to 8 and element 2 to 3.

Connecting pointers to arrays

• It turns out that pointers and arrays have much in common.
• These are equivalent:

  ip = a;     ====     ip = &a;     ====     ip = &a[0];
  

• This is essentially how the compiler lets us pass arrays as parameters!
  • A function call: myfunc(a,10) is actually myfunc(&a[0],10)
  • void myfunc(int array[], int size) is treated as if it had been void myfunc(int *array, int size) since later uses of array[x] are still permitted when array is a pointer.

Connecting pointers to arrays

• Similarly, pointer arithmetic can be used interchangably with array subscripting, e.g.:

  ip[3] == *(ip+3) == a[3];
  

  is also valid and evaluates to true.

Character pointers and arrays for strings

• Often, arrays and pointers can be used interchangeably.
• It is common to refer to and manipulate strings via character pointers.
• This means:
  • Any function declared to take a string (declared as a char array), will also accept a character pointer, since even if an array is passed, the function actually receives a pointer to the first element of the array.
  • Any function declared to take a char pointer will also accept a character array.
  • printf()'s %s actually expects a character pointer.
  • Many programs extensively manipulate strings as character pointers and never explicitly declare any actual arrays.

String operations using pointers

• mystrcmp() using pointers

char *p1 = &str1[0], *p2 = &str2[0];

while(1) {
  if (*p1 != *p2)
    return *p1 - *p2;
  if (*p1 == '\0')
    return 0;
  p1++;
  p2++;
}

• mystrcpy() using pointers

String operations using pointers

• mystrcmp() using pointers

char *p1 = &str1[0], *p2 = &str2[0];

while(1) {
  if (*p1 != *p2)
    return *p1 - *p2;
  if (*p1 == '\0')
    return 0;
  p1++;
  p2++;
}

• mystrcpy() using pointers

char *dp = &dest[0], *sp = &src[0];

while (*sp != '\0')
  *dp++ = *sp++;
*dp = '\0';