Pointers

1
Chapter 5

• Pointers

2
Addresses and Pointers
3
Pointers

• Fundamental part of C.
• Secret to C is in its use of pointers.
• C uses pointers a lot. Why?
• Only way to express some computations.
• Produces compact and efficient code.
• Provides a very powerful tool.
• C uses pointers explicitly with
• Arrays,
• Structures,
• Functions.
• Pointers are perhaps the most difficult part of C
  to understand.

4
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

5
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)

6
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.)

7
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 been initialized in the
  above example
• declaring a pointer creates a variable capable of
  holding an address

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. q 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
Practice

• int x 1, y 2
• int ip
• ip x
• y ip  
• x ip  
• ip 3
• What is the final value of x, y, ip ?

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
Practice

• Write a function to swap variables around.

16
First Attempt

• include ltstdio.hgt
• void main (void)
• float a 3, b 6
• void swap(float a, float b)
• swap(a,b)
• printf(f f\n, a, b)
• return
• void swap(float a, float b)
• float temp
• temp a
• a b
• b temp
• return

What is the output?
17
Practice

• The usual function call
•     swap(a, b)   WON'T WORK.
• Pointers provide the solution Pass the address
  of the variables to the functions and access
  address of function.
• Thus our function call in our program would look
  like this
•     swap(a, b)

18
Swap function

• The Code to swap is fairly straightforward
• void swap(int px, int py)
• int temp
• temp px / contents of pointer /
• px py
• py temp

19
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

20
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.

21
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, sptrs00

//correction 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
22
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.

23
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
24
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