cmpt 102 introduction to scientific computer programming

1 © Janice Regan, CMPT 102, Sept. 2006

CMPT 102Introduction to Scientific Computer Programming

Pointers

© Janice Regan, CMPT 102, Sept. 2006 2

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


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


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


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


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


Declaring pointer variables Pointer variables can point to only one type of

variable

int *v1p; // pointer to an integer double *v2p; // pointer to a double char *v3p; // pointer to a char myStruct *v5p; //pointer to a structure of // type myStruct


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;


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.!


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


Declared variables in memory

76.2

23.0

11.9

-23.7

-44.567

100011008

1032

1016

1024

address

Value of variable

Identifier of variable

v1

v2

v5

v3

v4


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;


& Operator & is the unary "address of" operator &myVariable

Used to determine the address of myVariable The value of the expression &myVariable is an address

Exampleint *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"


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;


After line 3

76.2

23.0

11.9

-23.7

-44.567

?

?

100011008

1032

1016

1024

address

Value of variable


v1

v2

v5

v3

v4

Pointer Identifier

v3p

v5p

Value of pointer variable


After line 4

76.2

23.0

11.9

-23.7

-44.567

1016

?

100011008

1032

1016

1024

address

Value of variable


v1

v2

v5

v3

v4

Pointer Identifier

v3p

v5p



After line 6

76.2

23.0

11.9

-23.7

-44.567

1016

1032

100011008

1032

1016

1024

address

Value of variable


v1

v2

v5

v3

v4

Pointer Identifier

v3p

v5p



& 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


* 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


"Pointing to" Example Consider:

v1 = 0; p1p = &v1;printf(“%d\n”, v1);*p1p = 42; printf(“%d, %d\n”, v1, *p1p);

Produces output:042 42

*p1p and v1 both refer to the value in the same location in memory


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


Pointer Assignments Graphic: Display 10.1 Uses of the Assignment Operator with Pointer Variables


Another example1. int *v1p, *v2p, v1, v2;2. v1p = &v1;3. *v1p = 47;4. v2p = v1p;5. *v2p = 23;6. v2p = &v2;7. *v2p = 111;


After line 1

?

??

?

100011004

address

Value of variable


v1

v2

Pointer Identifier

v1p

v2p



After line 2

?

?1000

?

100011004

address

Value of variable


v1

v2

Pointer Identifier

v1p

v2p



After line 3

47

?1000

?

100011004

address

Value of variable


v1

v2

Pointer Identifier

v1p

v2p



After line 4

47

?1000

1000

100011004

address

Value of variable


v1

v2

Pointer Identifier

v1p

v2p



After line 5

23

?1000

1000

100011004

address

Value of variable


v1

v2

Pointer Identifier

v1p

v2p


NOTE: this also changes the value of *v1p


After line 6

23

?1000

1000

100011004

address

Value of variable


v1

v2

Pointer Identifier

v1p

v2p



After line 7

23

1111000

1000

100011004

address

Value of variable


v1

v2

Pointer Identifier

v1p

v2p



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

cmpt 102 introduction to scientific computer programming

Documents

pointer variable reference

variable nameproduces

v2p pointer

v1p pointer

different pointer type

v3p pointer

v5p pointer

pointer identifiersto