CMPT 102
Introduction to Scientific
Computer Programming
Pointers
© Janice Regan, CMPT 102, Sept. 2006
0
Pointer Introduction

Pointer
 A special type of variable that holds the memory address of
another variable

Think about the memory in your computer
 Conceptually, consider memory as a sequence of bytes (eight bit
chunks, that can hold 8 zeros or ones)
 All bytes are numbered, the first byte would be byte 0, the second
byte 1 and so on
 Variables of different types can have different sizes (use different
numbers of bytes of memory)
 The address of the variable is byte number in memory where the
stored variable starts

You’ve seen pointers already
 Call-by-reference parameters: Address of actual argument was
passed
© Janice Regan, CMPT 102, Sept. 2006
1
What is a pointer?
 Each variable is stored at some memory address,

Each location in memory has an address, that address can be
represented as an integer
 A pointer is a special type of variable that holds a
memory address



A pointer is printed with a %p
A printed memory address looks like 0x22ccdc
The integer includes characters because it is shown in
hexadecimal (base 16)
 A pointer containing the address of a variable 'points to'
that variable
© Janice Regan, CMPT 102, Sept. 2006
2
Addresses and Numbers
 Pointers have their own types, these types
 Hold addresses (represented by integers)
 Point to variables of a particular type
There is a different pointer type to point to
each type of variable
 Even though an address is represented by an
integer it is not of type integer


Therefore print with special conversion %p
© Janice Regan, CMPT 102, Sept. 2006
3
Data Types, pointers and integers

int is a data type
 a set of objects, the integers

… -10, -9,-8, … ,123, 124, …
 A set of operations that can be done on those objects

+, -, *, /, % …

A pointer to an integer is a data type
 A set of objects, integers that represent the addresses of each
location in the computers memory
 A set of operations that can be done on those objects

+, - ,++, -- …

Does not make sense to %, * or / an address, %,*, / are not
valid operations on addresses
© Janice Regan, CMPT 102, Sept. 2006
4
Declaring Pointer Variables
 Pointers declared like other types

Add "*" before variable name
 Produces "pointer to" that type
 "*" must precede each variable that is to have a pointer
type
 int *v1p, *v2p, v1, v2;

v1p, v2p hold pointers to int variables
 v1, v2 are ordinary int variables
 Note that each pointer variable (reference) must
have the * as the first character, the * is associated
with the variable identifier not the type
© Janice Regan, CMPT 102, Sept. 2006
5
Declaring pointer variables
 Pointer variables can point to only one type of
variable
int *v1p;
double *v2p;
char *v3p;
myStruct *v5p;
© Janice Regan, CMPT 102, Sept. 2006
// pointer to an integer
// pointer to a double
// pointer to a char
//pointer to a structure of
// type myStruct
6
Pointers and Style
 It is useful to easily be able to see which variables in
your function are pointers
 C does not enforce any particular structure on your
variable and pointer identifiers
 To distinguish pointers from other variables a number of
conventions are used in different coding standards
 In this course the coding standard suggested is to
assure that all pointer identifiers end in p to indicate they
are pointers.
 double myVariable, *myVariablep;
© Janice Regan, CMPT 102, Sept. 2006
7
Pointer Variables
 Similarity of pointer variables and other types
of variables

Can store value of the variable in a memory
location
 For pointer variables the address of the
variable pointed to is contained in the
associated memory location
 For non pointer types the value of the variable
is contained in the associated memory location
 Not int, double, etc.
 Instead: A POINTER to int, double, etc.!
© Janice Regan, CMPT 102, Sept. 2006
8
Pointer Variables
 Example:
1: double *v3p, *v5p, v1, v2, v3, v4, v5;
2: v1 = 76.2; v2 = 23.0; v3 = 11.9;
3: v4 = -23.7; v5 = -44.567;
 v3p, v5p are declared as "pointer to double"
variables
 v3p can hold pointers to variables of type double


Cannot hold pointers to other types!
v1, v2, v3, v4, v5 are double variables

each double takes 8 bytes
© Janice Regan, CMPT 102, Sept. 2006
9
Declared variables in memory
Value of
variable
76.2
address
Identifier of
variable
v1
23.0
1000
1
1008
11.9
1016
v3
-23.7
1024
v4
-44.567
1032
v5
© Janice Regan, CMPT 102, Sept. 2006
v2
10
Where do declared pointers point
 When you declare a pointer
int *v1p, v1;
 Pointer variable v1p points to a random location in memory.
 Why? v1p has not been initialized, it contains whatever
was in the memory location associated with v1p before it
was associated with v1p
 This can cause serious problems, accessing and changing
values that are not even associated with your program.
 It is good programming practice to initialize all pointer to
NULL. This means initialize the pointer to point nowhere.
v1p = NULL;
© Janice Regan, CMPT 102, Sept. 2006
11
& Operator

& is the unary "address of" operator

&myVariable


Used to determine the address of myVariable

The value of the expression &myVariable is an address
Example
int *v3p, *v4p, v1, v2;
v1 = 23;
v3p = &v1;

Sets pointer variable v3p to "point to" int variable v1

Read like:


"v3p is assigned the address of v1"
Or "v3p points to v1"
© Janice Regan, CMPT 102, Sept. 2006
12
Pointer Variables
 Example:
1: double *v3p, *v5p, v1, v2, v3, v4, v5;
2: v1 = 76.2; v2 = 23.0; v3 = 11.9;
3: v4 = -23.7; v5 = -44.567;
4: v3p = &v3;
5: v5p = &v5;
© Janice Regan, CMPT 102, Sept. 2006
13
After line 3
Pointer
Identifier
v3p
v5p
Value of
pointer
variable
?
?
© Janice Regan, CMPT 102, Sept. 2006
Value of
variable
76.2
address
Identifier of
variable
v1
23.0
1000
1
1008
11.9
1016
v3
-23.7
1024
v4
-44.567
1032
v5
v2
14
After line 4
Pointer
Identifier
v3p
v5p
Value of
pointer
variable
1016
?
© Janice Regan, CMPT 102, Sept. 2006
Value of
variable
76.2
address
Identifier of
variable
v1
23.0
1000
1
1008
11.9
1016
v3
-23.7
1024
v4
-44.567
1032
v5
v2
15
After line 6
Pointer
Identifier
v3p
v5p
Value of
pointer
variable
1016
1032
© Janice Regan, CMPT 102, Sept. 2006
Value of
variable
76.2
address
Identifier of
variable
v1
23.0
1000
1
1008
11.9
1016
v3
-23.7
1024
v4
-44.567
1032
v5
v2
16
& Operator

& is the unary "address of" operator

Also used to specify call-by-reference parameter
 For example the function declared below has one parameter.
v1. that is called by reference.

int funSample(int *v1);
 In the code calling this function

variables v1 and v2 are defined

int v1; int v2, int solution;

The function is called twice

solution = funSample(&v1) + funSample(&v2);
 The arguments of the function calls (&v1, &v2) are references
to or “pointers to” the variables used as arguments
© Janice Regan, CMPT 102, Sept. 2006
17
* Operator: dereferencing



* The dereferencing operator
This operator can only be applied to a pointer variable
Consider the pointer variable f2p that points to variable
f2 (f2 = &f2p)
 f2p hold the address of the memory location in which
f2 is stored
 There are two ways to refer to the value of variable f2 in
an expression
 f2
 *f2p
© Janice Regan, CMPT 102, Sept. 2006
18
"Pointing to" Example
 Consider:
v1 = 0;
p1p = &v1;
printf(“%d\n”, v1);
*p1p = 42;
printf(“%d, %d\n”, v1, *p1p);
 Produces output:
0
42 42
 *p1p and v1 both refer to the value in the same location
in memory
© Janice Regan, CMPT 102, Sept. 2006
19
Pointer Assignments
 Pointer variables can be "assigned":
int *p1, *p2;
p2 = p1;
 Assigns one pointer to another
 "Make p2 point to where p1 points"
 Do not confuse with:
*p1 = *p2;
 Assigns "value pointed to" by p1, to "value
pointed to" by p2
© Janice Regan, CMPT 102, Sept. 2006
20
Pointer Assignments Graphic:
Display 10.1 Uses of the Assignment
Operator with Pointer Variables
© Janice Regan, CMPT 102, Sept. 2006
21
Another example
1. int *v1p, *v2p, v1, v2;
2. v1p = &v1;
3. *v1p = 47;
4. v2p = v1p;
5. *v2p = 23;
6. v2p = &v2;
7. *v2p = 111;
© Janice Regan, CMPT 102, Sept. 2006
22
After line 1
Pointer
Identifier
v1p
v2p
Value of
pointer
variable
?
Value of
variable
?
?
address
1000
1
1004
Identifier of
variable
v1
v2
?
© Janice Regan, CMPT 102, Sept. 2006
23
After line 2
Pointer
Identifier
v1p
v2p
Value of
pointer
variable
1000
Value of
variable
?
?
address
1000
1
1004
Identifier of
variable
v1
v2
?
© Janice Regan, CMPT 102, Sept. 2006
24
After line 3
Pointer
Identifier
v1p
v2p
Value of
pointer
variable
1000
Value of
variable
47
?
address
1000
1
1004
Identifier of
variable
v1
v2
?
© Janice Regan, CMPT 102, Sept. 2006
25
After line 4
Pointer
Identifier
v1p
v2p
Value of
pointer
variable
1000
Value of
variable
47
?
address
1000
1
1004
Identifier of
variable
v1
v2
1000
© Janice Regan, CMPT 102, Sept. 2006
26
After line 5
Pointer
Identifier
v1p
v2p
Value of
pointer
variable
1000
Value of
variable
23
?
address
1000
1
1004
Identifier of
variable
v1
v2
1000
NOTE: this also changes the value of *v1p
© Janice Regan, CMPT 102, Sept. 2006
27
After line 6
Pointer
Identifier
v1p
v2p
Value of
pointer
variable
1000
Value of
variable
23
?
address
1000
1
1004
Identifier of
variable
v1
v2
1000
© Janice Regan, CMPT 102, Sept. 2006
28
After line 7
Pointer
Identifier
v1p
v2p
Value of
pointer
variable
1000
Value of
variable
23
111
address
1000
1
1004
Identifier of
variable
v1
v2
1000
© Janice Regan, CMPT 102, Sept. 2006
29
Define Pointer Types
 Can "name" pointer types
 To be able to declare pointers like other
variables

Eliminate need for "*" in pointer declaration
 typedef int* IntPtr;
 Defines a "new type" alias
 Consider these declarations:
IntPtr p;
int *p;

The two are equivalent
© Janice Regan, CMPT 102, Sept. 2006
30