Monash University > CSSE > CSE1303 > Part A > Lectures > Lecture A02 notes

CSE1303 Computer Science
Semester 3 (summer), 2003
Part A
Lecture A02 notes: Pointers (revision)

In this lecture

• Kruse - Appendix C.6
• Deitel & Deitel - Chapter 7
• King - Chapter 11, 12
• Langsam - Chapter 1.1
• Standish - Chapter 2.3
• Kernighan and Ritchie - Chapter 5.1 to 5.10, 6.2

• A variable that contains the memory address of another variable.
• Every pointer "points to" a specific data type.
• All pointers must be initialized.

• pointer = &object assigns the address of object to the variable pointer.
• Can only be applied to objects in memory: variables and array elements.
• Cannot be applied to expressions, constants, or register variables.

• *pointer accesses the object that the variable pointer "points to".
• objectType* pointer defines pointer as a variable which points to objects of type objectType.

typedef struct SomeRec Something;
Something* pointer;

• pointer->membername is equivalent to (*pointer).membername.

• To enable a function to access and change an object, a pointer to that object needs to be passed to the function.
• If a large structure is to be passed to a function, it is generally more efficient to pass a pointer than to copy the whole structure.
• Use the const specifier whenever a constant is intended.

• If aPtr points to some element of an array, then aPtr++ and ++aPtr increments aPtr to point to the next element.
• If aPtr points to some element of an array, then aPtr-- and --aPtr decrements aPtr to point to the previous element.
• If aPtr points to some element of an array, then aPtr += n increments aPtr to point to n elements beyond where it currently points to.
• If aPtr points to some element of an array, then aPtr -= n decrements aPtr to point to n elements before where it currently points to.
• If aPtr points to the i^th element of an array and bPtr points to the j^th element of the same array, then aPtr-bPtr equals i-j.

• The name of an array is a synonym for the location of the first element of the array, i.e. array and &array[0] are equivalent.
• The expression array[n] is equivalent to *(array + n).
• The expression &array[n] is equivalent to array + n.
• The definition float a[2][3]; defines a to be an array of 2 elements, each of which is an array of 3 floats.
• The expression a[n][m] is equivalent to *(a[n] + m) and *(*(a+n)+m).
• Multidimensional arrays are stored in row-major order. For example in array above the elements are stored (in increasing address) as:
  a[0][0], a[0][1], a[0][2], a[1][0], a[1][1], a[1][2]

Code in this lecture

#include <stdio.h>

void fakeSwap(int a, int b)
{
   int tmp;

   tmp = a;
   a = b;
   b = tmp;
}

int main()
{
   int x = 1;
   int y = 2;

   fakeSwap(x, y);

   printf("%d %d\n", x, y);
}

#include <stdio.h>

void trueSwap(int* a, int* b)
{
   int tmp;

   tmp = *a;
   *a = *b;
   *b = tmp;
}

int main()
{
   int x = 1;
   int y = 2;

   trueSwap(&x, &y);
   printf("%d %d\n", x, y);
}

/* strcpy: copy t to s; array subscript version */
char* strcpy(char* s, char* t)
{
   int i;

   i = 0;
   while (t[i] != 0)
   {
     s[i] = t[i];
     i++;
   }
   s[i]=0;
   return s;
}

/* strcpy: copy t to s; pointer version */
char* strcpy(char* s, char* t)
{
   char* p = s;

   while (*p++ = *t++)
   {
     ;
   }
   return s;
}

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Last Updated: Tuesday 02 December 2003 22:29:28