1. Pointers Introduction CSCI 230
Department of Computer and Information Science, School of Science, IUPUI
CSCI 230
Pointers
Introduction
Dale Roberts, Lecturer
Computer Science, IUPUI

2. What is Pointer A pointer is an address. Hostage CS Dept.
Location A: Highway Intersection
Location C:
Crabapple
Tree
Bring your
money to
IUPUI SL-280
to exchange
hostage
Phone rings and says
“Deliver ransom to
I-65 and West St. intersection
“Find next
message
on the top of
traffic light
on Michigan
St. and
West St.
Location D:
IUPUI SL-280
“Find
Instruction
Under
The crabapple
tree next to old
law school ”
Location B: Traffic Light
Hostage CS A B C D
Address of next location
A pointer is an address.

3. What is Pointer
Example: Find out who is with Woodstock among peanut gang?
Pig Pen
“ Lucy knows ”
Lucy
“Sally knows”
Sally
“ Linus knows ”
Linus
“ Charlie knows ”
Charlie
“ Snoopy knows ”
Woodstock
Woodstock is with Snoopy
Snoopy
Pig Pen points to Lucy; Lucy points to Sally; Sally points to Linus;
Linus points to Charlie; Charlie points to Snoopy;
Snoopy has Woodstock
Snoopy
Charlie
Linus …
An instance uses a pointer to link to a next instance making a chain.

4. Pointer declarations:
datatype snoopy, *charlie, **linus;
snoopy = ; /* snoopy’s content is Woodstock */
charlie = &snoopy;
/* charlie’s content is the info (pointer) to locate snoopy, which is snoopy’s address*/
linus = &charlie;
/* linus’s content is the info (pointer) to locate charlie, which is charlie’s address*/
In general, we can rewrite charlie using variable of snoopyPtr (Snoopy’s pointer) and rewrite linus using variable of snoopyPtrPtr (pointer to Snoopy’s pointer);
Note that this is only a naming convention. The only requirement is that the variable be a valid identifier: begin with a letter followed by letters, digits or _.

5. Pointer Variable Declarations and Initialization
Pointers
Definition:
A pointer is a variable that contains address of another variable.
A powerful feature of C, but difficult to master
Pointers enable programs to simulate call-by-reference and create/manipulate dynamic data structures
Close relationship with arrays and strings

6. Pointer variables Contain memory addresses as their values
Normal variables contain a specific value (direct reference)
Pointers contain address of a variable that has a specific value (indirect reference)
Indirection – referencing a pointer value
count 7
countPtr
count 7

7. Pointer Variable Declarations
Pointer Declarations
type *variable_name
* used with pointer variables
Example:
int *myPtr;
Declares a pointer to an int (pointer of type int *)
Multiple pointers require using a * before each variable declaration
Can declare pointers to any data type
int *myPtr1, *myPtr2;
float *pq;
char *pc;

8. Pointer Variable Initialization
Initialize pointers to 0, NULL, or an address
0 or NULL – points to nothing (NULL preferred)
Example:
int *ptr, x;
x=10;
ptr = &x;
5000
ptr
FFFF x
FFFF
5000
ptr x 10
FFFF
5000
ptr x 10

9. &: Address Operator Pointer Operators Returns address of operand
int y = 5;
int *yPtr;
yPtr = &y;
/* yPtr “points to” y */
/* yPtr gets address of y */
yPtr y 5
yptr y
500000
600000
600000 5
address of y is the value of yPtr

10. * : Indirection / De-referencing Operator
Pointer Operators
* : Indirection / De-referencing Operator
Returns a synonym/alias of what its operand points to
*yptr returns y (because yptr points to y)
* can be used for assignment that returns alias to an object
*yptr = 7; // changes y to 7
Dereferenced pointer (operand of *) must be a variable (no constants)
* and & are inverses
They cancel each other out

11. Pointer Operators i pi Example: 874 902 5 pi i int i = 5; int *pi;
pi = &i; /* place the address of i into pi */
Assume Symbol Table
i.e. &i = 874, i = 5; &pi = 902, pi = 874;
*pi = ?
*pi = 5; // same as i = 5;
*pi = *pi * 2; // same as i = i * 2;
*pi *= 2; // same as i *= 2;
variable
address
value at the address i
874 5 pi
902
874
902 5 pi i

12. Example: int i,*pi,**ppi; i = 5; pi = &i; ppi = π i 100 5 pi 104
Variable
Address
Value at address i
100 5 pi
104
ppi
108
100
104
108 5
ppi pi i
Usage
Meaning
Value pi
address of int
100
*pi
int value 5
&pi
address of pointer
104
ppi
*ppi
**ppi