DCT1063 Programming 2 CHAPTER 1 POINTERS

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DCT1063 Programming 2 CHAPTER 1 POINTERS. Mohd Nazri Bin Ibrahim Faculty of Computer, Media &amp; Technology TATi University College nazri@tatiuc.edu.my. Pointer. After completing this chapter student should be able to:-. What Are Pointers?.

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### DCT1063 Programming 2 CHAPTER 1 POINTERS

Mohd Nazri Bin Ibrahim

Faculty of Computer, Media & Technology

TATi University College

nazri@tatiuc.edu.my

Pointer
• After completing this chapter student should be able to:-
What Are Pointers?
• A pointer is a variable that contains a memory address.
• For example, if x contains the address of y, then x is said to "point to" y.
• Pointer declaration
• type *var-name;
• For example, to declare p to be a pointer to an integer, use this declaration:
• int *p;
• For a float pointer, use
• float *p;
The Pointer Operators
• There are two special operators that are used with pointers: * and &.
• The & is a unary operator that returns the memory address of its operand.
• balptr = &balance;

puts into balptr the memory address of the variable balance.

• The * is a unary operator that returns the value of the variable located at the address
• value = *balptr;

will place the value of balance into value.

Consider the following program:-
• #include <iostream>
• using namespace std;
• int main()
• {
• int balance;
• int *balptr;
• int value;
• balance = 3200;
• balptr = &balance;
• value = *balptr;
• cout << "balance is: " << value << '\n';
• return 0;
• }
The Pointer Operators(2)
• What is the output of the program above?

Balance is : 3200

The Base Type Is Important
• Your pointer variables must always point to the correct type of data.
• When you declare a pointer to be of type int, the compiler assumes that anything it points to will be an integer value.
• For example, the following fragment is incorrect:

int *p;

double f;

// ...

p = &f; // ERROR

The Base Type Is Important(2)
• You can override this restriction (at your own risk) by using a cast. For example, the following fragment is now technically correct:

int *p ;

double f;

// ...

p = (int *) &f; // Now technically OK

• To better understand why using a cast to assign one type of pointer to another is not usually a good idea, consider the following short program:
// This program will not work right.
• #include <iostream>
• using namespace std;
• int main()
• {
• double x, y;
• int *p;
• x = 123.23;
• p = (int *) &x; // use cast to assign double * to int *
• y = *p; // What will this do?
• cout << y; // What will this print?
• return 0;
• }
Assigning Values Through a Pointer
• You can use a pointer to assign a value to the location pointed to by the pointer.
• Example. Assuming that p is an integer pointer, this assigns the value 101 to the location pointed to by p:

*p = 101;

("at the location pointed to by p, assign the value 101“)

• To increment or decrement the value at the location pointed to by a pointer, you can use a statement like this:

(*p)++;

Assigning Values Through a Pointer(2)
• The following program demonstrates assignment using a pointer.
• #include <iostream>
• using namespace std;
• int main()
• {
• int *p, num;
• p = &num;
• *p = 100;
• cout << num << ' ';
• (*p)++;
• cout << num << ' ';
• (*p)--;
• cout << num << '\n';
• return 0;
• }
Pointer Expressions
• Pointers can be used in most valid C++ expressions.
• Remember also that you may need to surround some parts of a pointer expression with parentheses
Pointer Arithmetic
• There are only four arithmetic operators that can be used on pointers: ++, – –, +, and –.
• Let p1 be an integer pointer with a current value of 2,000.
• After the expression

p1++;

the contents of p1 will be 2,004, not 2,001!

• Each time that a pointer is incremented, it will point to the memory location of the next element of its base type.
Pointer Arithmetic(2)
• You can also add or subtract integers to or from pointers. The expression

p1 = p1 + 9;

makes p1 point to the ninth element of p1’s base type, beyond the one to which it is currently pointing.

• To see the effects of pointer arithmetic, execute the next short program.
// Demonstrate pointer arithmetic.
• #include <iostream>
• using namespace std;
• int main()
• {
• int *i, j[10];
• double *f, g[10];
• int x;
• i = j;
• f = g;
• for(x=0; x<10; x++)
• cout << i+x << ' ' << f+x << '\n';
• return 0;
• }
Here is sample output. 0012FE5C 0012FE84

0012FE60 0012FE8C

0012FE64 0012FE94

0012FE68 0012FE9C

0012FE6C 0012FEA4

0012FE70 0012FEAC

0012FE74 0012FEB4

0012FE78 0012FEBC

0012FE7C 0012FEC4

0012FE80 0012FECC

Pointer Comparisons
• Pointers may be compared by using relational operators, such as ==, <, and >.
• If p1 and p2 are pointers that point to two separate and unrelated variables, then any comparison between p1 and p2 is generally meaningless.
• If p1 and p2 point to variables that are
• related to each other, such as elements of the same array, then p1 and p2 can be meaningfully compared.
Pointers and Arrays
• In C++, there is a close relationship between pointers and arrays.
• In fact, frequently a pointer and an array are interchangeable.
• consider this fragment:

char str[80];

char *p1;

p1 = str;

• if you want to access the fifth element in str, you could use:-

str[4]

or

*(p1+4)

Pointers and Arrays(2)
• C++ allows two methods of accessing array elements: pointer arithmetic and array indexing.
• Pointer arithmetic can sometimes be faster than array indexing.
• Example: two versions of the same program will be shown next.
// Tokenizing program: pointer version.
• #include <iostream>
• #include <cstdio>
• using namespace std;
• int main()
• {
• char str[80];
• char token[80];
• char *p, *q;
• cout << "Enter a sentence: ";
• gets(str);
• p = str;
• // Read a token at a time from the string.
• while(*p) {
• q = token; // set q pointing to start of token
• /* Read characters until either a space or the
• null terminator is encountered. */
• while(*p!=' ' && *p) {
• *q = *p;
• q++; p++;
• }
• if(*p) p++; // advance past the space
• *q = '\0'; // null terminate the token
• cout << token << '\n';
• }
• return 0;
• }
• Here
// Tokenizing program: array-indexing version.
• #include <iostream>
• #include <cstdio>
• using namespace std;
• int main()
• {
• char str[80];
• char token[80];
• int i, j;
• cout << "Enter a sentence: ";
• gets(str);
• // Read a token at a time from the string.
• for(i=0; ; i++) {
• /* Read characters until either a space or the
• null terminator is encountered. */
• for(j=0; str[i]!=' ' && str[i]; j++, i++)
• token[j] = str[i];
• token[j] = '\0'; // null terminate the token
• cout << token << '\n';
• if(!str[i]) break;
• }
• return 0;
• }
Indexing a Pointer
• It is possible to index a pointer as if it were an array.
• // Indexing a pointer like an array.
• #include <iostream>
• #include <cctype>
• using namespace std;
• int main()
• {
• char str[20] = "hello tom";
• char *p;
• int i;
• p = str; // put address of str into p
• // now, index p like an array
• for(i=0; p[i]; i++) p[i] = toupper(p[i]);
• cout << p; // display the string
• return 0;
• }
The program displays

HELLO TOM

• Remember, the expression p[i] is functionally identical to *(p+i).
Are Pointers and Arrays Interchangeable?
• Pointers and arrays are interchangeable in many cases, but not completely. Consider this fragment:

int num[10];

int i;

for(i=0; i<10; i++) {

*num = i; // this is OK

num++; // ERROR -- cannot modify num

}

Pointers and String Literals
• The following program demonstrate how pointer is used with string literals.

#include <iostream>

using namespace std;

int main()

{

char *s;

s = "Pointers are fun to use.\n";

cout << s;

return 0;

}

A Comparison Example
• It is legal to compare the value of one pointer to another (which is point to elements of the same array).
• The following program demonstrates a pointer comparison.

// A pointer comparison example.

#include <iostream>

using namespace std;

int main()

{

int num[10];

int *start, *end;

start = num;

end = &num[9];

// enter the values

while(start <= end) {

cout << "Enter a number: ";

cin >> *start;

start++;

}

start = num; // reset the starting pointer

// display the values

while(start <= end) {

cout << *start << ' ';

start++;

}

return 0;

}

Arrays of Pointers
• Pointers can be arrayed like any other data type. For example,

int *ipa[10]; //each element in ipa holds a pointer to an int value.

• To assign the address of an int variable called var to the third element of ipa

ipa[2] = &var;

• Using the ipa array to assign the value of var to an int variable called x

x = *ipa[2];

Arrays of Pointers(2)
• Like other arrays, arrays of pointers can be initialized.

char *fortunes[] = {

"Soon, you will come into some money.\n",

"A new love will enter your life.\n",

"You will live long and prosper.\n",

"Now is a good time to invest for the future.\n",

"A close friend will ask for a favor.\n"

};

• Thus, to print the second message, use a statement like this:

cout << fortunes[1];

The Null Pointer Convention
• Any type of pointer should be initialized to null when it is declared. For example, the following initializes p to null:

float *p = 0; // p is now a null pointer

• If you follow the null pointer convention, you will avoid many problems when using pointers.
Problems with Pointers
• When a pointer accidentally contains the wrong value, it can be the most difficult bug to track down.
• The classic example of a pointer error is the uninitialized pointer. Consider this example:

// This program is wrong.

int main(){

int x, *p;

x = 10;

*p = x; // where does p point?

return 0;

}

//You will have no way of knowing where the value of x has been written.

Passing Arguments to Functions by Reference with Pointers
• There are three ways in C++ to pass arguments to a function:-
• pass-by-value
• pass-by-reference (with pointer)
• Figure 8.6 and Figure 8.7 present two versions of a function that cubes an integercubeByValue and cubeByReference
Cube a variable using pass-by-value.

2 // Cube a variable using pass-by-value.

3 #include <iostream>

4 using std::cout;

5 using std::endl;

6

7 int cubeByValue( int ); // prototype

8

9 int main()

10 {

11 int number = 5;

12

13 cout << "The original value of number is " << number;

14

15 number = cubeByValue( number ); // pass number by value to cubeByValue

16 cout << "\nThe new value of number is " << number << endl;

17 return 0; // indicates successful termination

18 } // end main

19

Cube a variable using pass-by-value.(cont)

20 // calculate and return cube of integer argument

21 int cubeByValue( int n )

22 {

23 return n * n * n; // cube local variable n and return result

24 } // end function cubeByValue

Output

The original value of number is 5

The new value of number is 125

2 // Cube a variable using pass-by-reference with a pointer argument.

3 #include <iostream>

4 using std::cout;

5 using std::endl;

6

7 void cubeByReference( int * ); // prototype

8

9 int main()

10 {

11 int number = 5;

12

13 cout << "The original value of number is " << number;

14

15 cubeByReference( &number ); // pass number address to cubeByReference

16

17 cout << "\nThe new value of number is " << number << endl;

18 return 0; // indicates successful termination

19 } // end main

Cube a variable using pass-by-reference with a pointer argument(cont)

21 // calculate cube of *nPtr; modifies variable number in main

22 void cubeByReference( int *nPtr )

23 {

24 *nPtr = *nPtr * *nPtr * *nPtr; // cube *nPtr

25 } // end function cubeByReference

Output

The original value of number is 5

The new value of number is 125

Calling Functions with Arrays
• When an array is an argument to a function, only the address of the first element of the array is passed.
• Remember- in C++, an array name without an index is a pointer to the first element in the array.)
• Examine the following example. To call display( ), pass the address of the array as the first argument, and the size of the array as the second.
#include <iostream>

using namespace std;

//void display(int *, int );

void display(int *num,int n)

{

int i;

for(i=0; i<n; i++) cout << num[i] << ' ';

}

int main()

{

int t[10],i;

for(i=0; i<10; ++i) t[i]=i;

display(t,10); // pass array t and it size to a function

return 0;

}

Summary
• Pointers are variables that contain as their values memory addresses of other variables.
• The declarationint *ptr; declares ptr to be a pointer to a variable of type int and is read, "ptr is a pointer to int." The * as used here in a declaration indicates that the variable is a pointer.
• There are three values that can be used to initialize a pointer: 0, NULL or an address of an object of the same type. Initializing a pointer to 0 and initializing that same pointer to NULL are identical0 is the convention in C++.
Summary(2)
• The & (address) operator returns the memory address of its operand.
• A function receiving an address as an argument must have a pointer as its corresponding parameter.
• There are four ways to pass a pointer to a functiona nonconstant pointer to nonconstant data, a nonconstant pointer to constant data, a constant pointer to nonconstant data and a constant pointer to constant data.
• The value of the array name is the address of (a pointer to) the array's first element.
Summary(3)
• To pass a single element of an array by reference using pointers, pass the address of the specific array element.
• The arithmetic operations that may be performed on pointers are incrementing (++) a pointer, decrementing (--) a pointer, adding (+ or +=) an integer to a pointer, subtracting (- or -=) an integer from a pointer and subtracting one pointer from another.
• When an integer is added or subtracted from a pointer, the pointer is incremented or decremented by that integer times the size of the object to which the pointer refers.
Summary(4)
• Pointers can be compared using the equality and relational operators. Comparisons using relational operators are meaningful only if the pointers point to members of the same array.
• Pointers that point to arrays can be subscripted exactly as array names can.