chapter7 pointers

7/29/2019 Chapter7 Pointers

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Principles of Programming - NI2005 1

Chapter 7: Pointers

In this chapter you will learn about,

Basic concept of pointers

Pointer variables

Pointer declaration

Pointer definition and initializationPointer operator (& and *)

Parameter passing by reference


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What is a pointer

So far, we have seen that a variable is used to storea value.

Variables allow the programmer to directlymanipulate the data in memory.

A pointer variable, however, does not store a valuebut store the addressof thememory space whichcontain the value i.e. it directly points to a specificmemory address.

Why would we want to use pointers?

To call a function by reference so that the data passedto the function can be changed inside the function.

To create a dynamic data structure which can growlarger or smaller as necessary.


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Variable declaration

A variable declaration such as,int number = 20; causes the compiler to

allocate a memory location for the variable

numberand store in it the integer value 20.

This absolute address of the memory locationis readily available to our program during the

run time.

The computer uses this address to access its

content.number

20number directly references a

variable whose value is 7

11001100


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Pointer declaration

General Format:data_type *pointer_name;

A pointer declaration such as,

int *numberPtr;

declares numberptr as a variable that

points to an integer variable. Its

content is a memory address.

The * indicates that the variable beingdeclared is a pointer variable instead of anormal variable.


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Pointer declaration cont

Consider the following declaration

int *numberPtr, number = 20;

In this case, two memory address have beenreserved in the memory, namely the

numberPtr and number.

The value in variable number is of typeinteger, and the value in variable numberPtris an address for another memory.

20

1100110011111111

number*numberPtr


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Pointer Initialization

To prevent the pointer from pointing to arandom memory address, it is advisable thatthe pointer is initialized to 0, NULL oranaddress before being used.

A pointer with the value NULL, points tonothing.

Initializing a pointer to 0 is equivalent toinitializing a pointer to NULL, but NULL is

preferred.


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Pointer Operator (& and *)When a pointer is created, it is not pointing to any

valid memory address. Therefore, we need to assignit to a variables address by using the & operator.This operator is called a reference operator.

Look at this example:

int number = 20;int *numberPtr;

numberPtr = &number;

printf(number = %d, *numberPtr);

The statement numberPtr = &numberassigns theaddress of the variable number to a pointer variablenumberPtr. Variable numberPtr is then said as topoint to variable number.

Output:

number = 20


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Graphical representation

int *numberPtr, number = 20;

numberPtr = &number;

20

1100110011111111

number*numberPtr

2011001100

1100110011111111

number*numberPtr


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Pointer Operator (& and *) cont

After a pointer is assigned to a particularaddress, the value in the pointed address canbe accessed/modified using the * operator.

This operator is commonly called as theindirection operatorordereferencing

operator.The * operator returns the value of the objectto which its operand points. For example, thestatement

printf(number = %d, *numberPtr);prints the value of variable number, namely as20. Using * in this manner is calleddereferencing operator.


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Example: & and *#include

void main(void){

int var = 10;

int *ptrvar = &var;

printf(The address of the variable var is: %d\n, &var);

printf(The value of the pointer ptrvar is: %d\n, ptrvar);

printf(Both values are the same\n);

printf(The value of the variable var is: %d\n, var);

printf(The value of *ptrvar is: %d\n, *ptrvar);


printf(The address of the value pointed by ptrvar is: %d\n, &*ptrvar);

printf(The value inside the address of ptrvar is: %d\n, *&ptrvar);


}


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Example: & and */*Sample Output */

The address of the variable var is: 1245052

The value of the pointer ptrvar is: 1245052

Both values are the same

The value of the variable var is: 10

The value of *ptrvar is: 10


The address of the value pointed by ptrvar is: 1245052The value inside the address of ptrvar is: 1245052


Press any key to continue


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&* and *&

& and * are inverse operations. &* actsequivalent to *& and this leads back to theoriginal value.

Example: (Assume that the address of num is1245052)

#include

void main(void){

int num = 5;int *numPtr = &num;

printf("%d \n", numPtr);printf("%d \n", &*numPtr);printf("%d \n", *&numPtr);

}

Output:

1245052

1245052

1245052


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Parameter Passing by Reference/Pointer

A function may return multiple values bydeclaring their formal parameters (passingvalue) as pointers variables.

This way of passing the argument is knownas call by reference

When the value referenced by the pointer ischanged inside the function, the value in theactual variable will also change.

Therefore, we can pass the result of thefunction through the function argumentwithout having to use the return statement.


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When a pointer is passed to a function, weare actually passing the address of a variableto the function.

Since we have the address, we can directlymanipulate the data in the address.

In the case where a non-pointer variable ispassed, the function will create another spacein memory to hold the value locally while the

program is inside the function. Therefore, anychange to the variable inside the function willnot change the actual value of the variable.


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To pass the value of variable by pointersbetween two functions, we must do thefollowing:

Declare the variable that is meant to return a

value to the calling function as a pointer

variable in the formal parameter list of the

function.

void function_name(int *varPtr);

When to call the function, use a variabletogether with address operator (&)

function_name(&var);


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Note : the effect of the previous two actions isthe same as the effect of the declarations

int var;

int *varPtr=&var;

Declare the function with pointer parameterappropriately in a function prototype by using

symbol * as a prefix for pointer parameters.

void function_name(int *);


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Example

#include

void Func1(int, int);

void Func2(int *, int *);

void main( )

{ int a = 8, b = 9;

printf(Before Func1 is called, a = %d, b = %d\n, a, b);

Func1(a, b);

printf(After Func1 is called, a = %d, b = %d\n, a, b);

printf(\nBefore Func2 is called, a = %d, b = %d\n, a, b);

Func2(&a, &b);

printf(After Func2 is called, a = %d, b = %d\n, a, b);

}


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Example

void Func1(int a, int b)

{

a = 0;

b = 0;

printf(The value inside Func1, a = %d, b = %d\n, a, b);

}

void Func2(int *pa, int *pb)

{

*pa = 0;

*pb = 0;printf("The value inside Func2, *pa = %d, *pb = %d\n\n", *pa, *pb);

}


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Result

/* output */

Before Func1 is called, a = 8, b = 9

The value inside Func1, a = 0, b = 0

After Func1 is called, a = 8, b = 9

Before Func2 is called, a = 8, b = 9

The value inside Func2, *pa = 0, *pb = 0

After Func2 is called, a = 0, b = 0

Press any key to continue


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SummaryIn this chapter, you have learnt

The pointer operator: & and *

Pointer declaration and assignment

Parameter passing by reference

The 3 steps needed to pass the value of variable bypointers between two functions.

Declare the variable as a pointer variable in formal

parameter list of the function.

Use a variable together with address operator (&)

when to call a function.Using symbol * in function prototype to declare any

pointer parameter.

chapter7 pointers

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