Dale Roberts, Lecturer

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Department of Computer and Information
Science,School of Science, IUPUI
CSCI 230
Pointers Pointer Arithmetic

• Dale Roberts, Lecturer
• Computer Science, IUPUI
• E-mail droberts_at_cs.iupui.edu

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Pointer Expressions and Pointer Arithmetic

• pvn
• ? pv nsizeof(variable type that pointer point
  to)
• Arithmetic operations can be performed on
  pointers
• Increment/decrement pointer ( or --)
• Example
• vPtr, vPtr,
• --vPtr, vPtr--
• Add an integer to a pointer( or , - or -)
• Pointers may be subtracted from each other
• Operations meaningless unless performed on an
  array

 
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• Example
• Five element int array on machine with 4 byte
  ints
• vPtr points to first element v 0 whose address
  location is 3000 (vPtr 3000)
• vPtr 2
• // sets vPtr to 3008
• vPtr points to v 2 (incremented by 2), but
  the machine has 4 byte integers, so it points to
  address 3008

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Pointer Expressions and Pointer Arithmetic

• Example (double pointer)
• Assume long (long integer) is 4 bytes, and
  pointer variable is 2 bytes.
• long a105, 10, 15,
• long pa, ppa
• int i5
• pa a
• ppa pa

Questions
Expression Value Note
pa1 644 64014
pa3 652 64034
pai 660 640i4
ppa1 702 70012
ppai 710 700i2
pa1 6 51
(pa1) 10 a1pa1(a1)
pa2 15 648
ppa 640 value of pa
ppa1 644 pa1
(ppa1) invalid (702)
ppa1 6 a01 51
(ppa1) 10 (pa1)(64014)
Variable Address Value
a 640 5
644 10
648 15

pa 700 640
ppa 800 700
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Pointer Expressions and Pointer Arithmetic

• Subtracting pointers
• Returns number of elements from one to the other.
  If
• vPtr2 is a pointer pointing to v 2
• vPtr is a pointer pointing to v 0
• vPtr2 - vPtr would produce 2
• Pointer comparison ( lt, , gt )
• See which pointer points to the higher numbered
  array element
• Also, see if a pointer points to 0
• Pointers of the same type can be assigned to each
  other
• If not the same type, a cast operator must be
  used
• Exception pointer to void (type void )
• Generic pointer, represents any type
• No casting needed to convert a pointer to void
  pointer
• void pointers cannot be dereferenced

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The Relationship Between Pointers and Arrays

• Arrays and pointers are closely related
• Array name like a constant pointer
• Pointers can do array subscripting operations
• Example Declare an array b 5 and a pointer
  bPtr
• bPtr b
• // To set them equal to one another
• // The array name (b) is actually the address of
  first element of the array
• bPtr b 0
• // Explicitly assigns bPtr to address of first
  element of b
• To access element b 3
• x( bPtr 3 ) // Where n is the offset. Called
  pointer/offset notation
• xbptr 3 // Called pointer/subscript notation
• // bPtr 3 same as b 3
• x( b 3 ) // Performing pointer arithmetic on
  the array itself

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Pointers and Arrays

• Strong relation between pointers and arrays
• Pointers and arrays can be used interchangeably.
• The array name is equivalent to the address of
  the first element in the array
• Example
• int a10
• int pa
• pa a0 / is equivalent to pa a /
• So, a1 ? (pa1) ? pa1 ? (a1)
• a1 ? pa1 ? a1
• ai ? (pai) ? pai ? (ai)
• ai ? pai ? ai
• ai5 ? (pai)5 ? pai5
• Example
• f(int s)

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Arrays of Pointers

• Arrays can contain pointers
• For example an array of strings
• char suit4 "Hearts", "Diamonds",
• "Clubs", "Spades"
• Strings are pointers to the first character
• char each element of suit is a pointer to a
  char
• The strings are not actually stored in the array
  suit, only pointers to the strings are stored
• suit array has a fixed size, but strings can be
  of any size

 
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Pointers to Functions

• Pointer to function
• Contains address of function
• Similar to how array name is address of first
  element
• Function name is starting address of code that
  defines function
• Function pointers can be
• Passed to functions
• Stored in arrays
• Assigned to other function pointers

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Example Bubble Sort

• define SIZE 10
• void bubble(int ,const int,
• int ()(int,int))
• int ascending( int, int )
• int descending( int, int )
• int main()
• int aSIZE 2, 6, 4, 8, 10,
• 12, 89, 68, 45, 37
• bubble( a, SIZE, ascending )
• bubble( a, SIZE, descending )
• void bubble( int work, const int size,
• int (compare)(int,int))

void swap( int element1Ptr, int
element2Ptr ) int temp temp
element1Ptr element1Ptr element2Ptr elemen
t2Ptr temp int ascending( int a, int b )
return b lt a /swap if b is less than
a/ int descending( int a, int b ) return b
gt a /swap if b is greater than a/
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• Function bubble takes a function pointer
• bubble calls this helper function
• this determines ascending or descending sorting
• The argument in bubblesort for the function
  pointer
• bool ( compare )( int, int )
• tells bubblesort to expect a pointer to a
  function that takes two ints and returns a bool
• If the parentheses were left out
• bool compare( int, int )
• Declares a function that receives two integers
  and returns a pointer to a bool