Engineering Problem Solving with C Fundamental Concepts

1
Engineering Problem Solving with C Fundamental
Concepts

• Chapter 6
• Pointers

2
Addresses and Pointers
3
Address Operator

• A variable can be referenced using the address
  operator
• example
• scanf(f, x)
• This statement specifies that the value read is
  to be stored at the address of x

4
Pointer Assignment

• A pointer is a variable that holds the address of
  a memory location
• If a variable p holds the address of another
  variable q, then p is said to point to q
• If q is a variable at location 100 in memory,
  then p would have the value 100 (qs address)

5
How to declare a pointer variable

• pointer variables are declared using an asterisk
  ( ) The asterisk is called the indirection
  operator or the de-referencing operator).
• example
• int a, b, ptr
• ptr is a pointer to an integer
• when a pointer is defined, the type of variable
  to which it will point must be specified. (i.e.
  a pointer defined to point to an integer cannot
  also point to a floating point variable.)

6
Example

• int iPtr
• double dPtr
• the variable iPtr is declared to point to an int
• the variable dPtr is declared to point to a
  double
• neither variable has not been initialized in the
  above example
• declaring a pointer creates a variable capable of
  holding an address

7
Example
int a, iPtrchar sdouble dPtr
a
?
iPtr s dPtr
?
-
?
?
8
More about declaring pointers

• When using the form
• int p, q
• the operator does not distribute.
• In the above example p is declared to be a
  pointer to int. p is declared to be an int.

9
Assigning values to a pointer

• the assignment operator () is defined for
  pointers
• the right operand can be any expression that
  evaluates to the same type as the left
• the operator in front of an ordinary variable
  produces the address of that variable. The
  operator is called to address of operator

10
Example

• example -
• int I6, j
• int iPtr
• iPtr I
• j iPtr
• I iPtr
• j

6
6
11
Practice!
Give a memory snapshot after each set of
assignment statements int a1, b2, ptr ptr
b int a1, b2, ptrb a ptr
12
NULL pointer

• A pointer can be assigned or compared to the
  integer zero, or, equivalently, to the symbolic
  constant NULL, which is defined in ltstdio.hgt.
• A pointer variable whose value is NULL is not
  pointing to anything that can be accessed

13
Example-
int iPtr0char s0double dPtrNULL
iPtr s dPtr
14
Pointer Assignments

• A pointer can point to only one location at a
  time, but several pointers can point to the same
  location.
• Example
• / Declare and initialize variables. /
• int x-5, y 8, ptr1, ptr2
• / Assign both pointers to point to x. /
• ptr1 x
• ptr2 ptr1
• The memory snapshot after these statements are
  executed is
• ptr1 ptr2
• x y

-5
8
15
Pointer Arithmetic

• Four arithmetic operations are supported
• , -, , --
• only integers may be used in these operations
• Arithmetic is performed relative to the variable
  type being pointed to
• Example p
• if p is defined as int p, p will be incremented
  by 4 (system dependent)
• if p is defined as double p, p will be
  incremented by 8(system dependent
• when applied to pointers, means increment
  pointer to point to next value in memory

16
Comparing Pointers

• You may compare pointers using relational
  operators
• Common comparisons are
• check for null pointer (p NULL)
• check if two pointers are pointing to the same
  object
• (p q) Is this equivalent to
• (p q)
• compare two pointers that are pointing to a
  common object such as an array.

17
Pointers and Arrays

• The name of an array is the address of the first
  elements (i.e. a pointer to the first element)
• The array name is a constant that always points
  to the first element of the array and its value
  can not be changed.
• Array names and pointers may often be used
  interchangeably.
• Example
• int num4 1,2,3,4, p
• p num
• / above assignment is the same as p num0
  /
• printf(i, p)
• p
• printf(i, p)

18
More Pointers and Arrays

• You can also index a pointer using array notation
• Example
• char string This is a string
• char str
• int i
• str string
• for(i 0 stri i) //look for null
• printf(c, stri)

19
Two-Dimensional Arrays

• A two-dimensional array is stored in sequential
  memory locations, in row order.

Array definition int s23 2,4,6,
1,5,3, sptrs
Memory allocation s00 2
s01 4 s02 6 s10 1 s11 5 s1
2 3 A pointer reference to s01 would be
(sptr1) A pointer reference to s11 would be
(sptr4) row offset number of columns
column offset
20
Pointers in Function References

• In C, function references are call-by-value
  except when an array name is is used as an
  argument.
• An array name is the address of the first element
• Values in an array can be modified by statements
  within a function
• To modify a function argument, a pointer to the
  argument must be passed
• The actual parameter that corresponds to a
  pointer argument must be an address or pointer.

21
switch Example
void switch2(int a, int b) / Declare
Variables. / int temp / Switch values
pointed to by a and b. / temp
a ab btemp / Void return.
/ return
22
Dynamic Memory Allocation

• Dynamically allocated memory is determined at
  runtime
• A program may create as many or as few variables
  as required, offering greater flexibility
• Dynamic allocation is often used to support data
  structures such as stacks, queues, linked lists
  and binary trees.
• Dynamic memory is finite
• Dynamically allocated memory may be freed during
  execution

23
Dynamic Memory Allocation

• Memory is allocated using the
• malloc function (memory allocation)
• calloc function (cleared memory allocation)
• Memory is released using the
• free function
• The size of memory requested by malloc or calloc
  can be changed using the
• realloc function

24
malloc and calloc

• Both functions return a pointer to the newly
  allocated memory
• If memory can not be allocated, the value
  returned will be a NULL value
• The pointer returned by these functions is
  declared to be a void pointer
• A cast operator should be used with the returned
  pointer value to coerce it to the proper pointer
  type

25
Example of malloc and calloc
int npts 500 double x int p / Allocate
memory for 500 doubles. / x (double
)malloc(nptssizeof(double)) / Allocate
memory for 500 integers. / p (int
)calloc(nptssizeof(int))
26
Common Pointer Problems

• Using un-initialized pointers
• int iPtr
• iPtr 100
• iPtr has not been initialized. The value 100
  will be assigned to some memory location,
  resulting in various types of errors.
• Failing to reset a pointer after changing its
  value
• Incorrect/unintended syntax