C Arrays and Pointers for Java programmers

C arrays

• C arrays must be declared in the "older" Java syntax.
  • double V[5] ;
• C arrays must be explicitly dimensioned except the last dimention is optional.
  • double W[5][6][7] ;
  • double W[5][6][] ;
• C arrays without a last dimension should only be used as fuction parameters where an additional argument gives the size of the last dimension.
  • double Sum(double A[], int N) ;
• It can be difficult to write generic multi-dimensional C functions.
  The following C prototype is illegal in C90.
  • double Identity(double I[][], int N) ;
• C arrays are not allocated with new. Generally they are statically allocated on the stack.
• C arrays can be dynamically be allocated with calloc.
  This is rarely done.
  • V = (double *)calloc(1000, sizeof(double)) ;
• Multidimensional C arrays are not implemented as arrays of pointer, as in Java. C arrays are stored in memory in row major order.
• C arrays and pointers are highly related.
  • double Sum(double *A, int N) ;
  • A[5] == *(A+5) ;

C pointers

• C pointers contain the addresses of C variables.
  • int *pI;
  • double *pD;
  • int *pAI[10]; /* Like a Java 2-D array */
• C pointers are typed, unlike PL/I pointers.
• The address-of operator, &, returns the address of an l-value.
  • int I = 5 ;
  • int *pI = I ;
• The dereference (or indirection) operator, *, returns the value stored in the l-value referenced by the pointer.
  • *pI + 10
• Arithmetic can be performed on pointers. Adding i to an integer pointer pI returns the i'th integer after pI.
• Pointers can be compared using C's relational operators.
• Technically, the C standard only defines the result of adding to pointers that contain the address of elements within an array.
  • int i = 5 ;
  • int A[3] = {1, 2, 3} ;
  • int *pI = &i ;
  • int *pA = &A[1] ;
  • i = *(pA+1) ; /* Sets i to 3 */
  • i = *(pI+1) ; /* Undefined */
• C increment and decrement operators can also be applied to integers.
  • ++pA ;
• & and * are unary prefix operators with a precedence just below that of unary prefix operators.
  • *++pX == *(++pX) ;
  • *pX++ == *(pX++) ;
• C arrays are considered constant pointers.
• All Java objects are implemented with pointers.

C array and pointer examples

Barely readable function to reverse an array

void reverse(int V[], int n) {
  int i, j ;
  for (i=0, j=n-1; i<j; ++i, ++j) {
    int t ;
    t = V[i] ;
    V[i] = V[j] ;
    V[j] = t ;
  }
}

Unreadable function to reverse an array

reverse(int *p, int *q) {
  if (p>q) {
    int *t ;
    t = p ;
    p = q ;
    q = t ;
  }
  for (; p<q; ++p, --q) {
    int t ;
    t  = *p ;
    *p = *q ;
    *q = t ;
  }
}

More unreadable function to reverse an array

reverse(int *p, int *q) {
  if (p>q) {
    int *t ;
    t = p ;
    p = q ;
    q = t ;
  }
  while (p<q) }
    int t ;
    t  = *p ;
    *p++ = *q ;
    *q++ = t ;
  }
}

Obscure question from Spring 2008 ECE 209 final

  int i, j ;
  int m[2] ;
  int *p, *q ;
  i = 5 ;
  j = 6 ;
  m[0] = 7 ;
  m[1] = 8 ;
  p = m ;
  q = &i ;
  *p = *q ;
  printf("%d %d %d %d\n", i, j, m[0], m[1]) ;
  p = q ;
  *q = j ;
  printf("%d %d %d %d\n", i, j, m[0], m[1]) ;