DCT1063 Programming 2

CHAPTER 1 POINTERS

Mohd Nazri Bin IbrahimFaculty of Computer, Media & Technology

TATi University Collegenazri@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:-1. #include <iostream>

2. using namespace std;

3. int main()

4. {

5. int balance;

6. int *balptr;

7. int value;

8. balance = 3200;

9. balptr = &balance;

10.value = *balptr;

11.cout << "balance is: " << value << '\n';

12. return 0;

13. }

• Figure 6-1 depicts the actions of the two preceding statements.

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:

1. // This program will not work right.2. #include <iostream>3. using namespace std;4. int main()5. {6. double x, y;7. int *p;8. x = 123.23;9. p = (int *) &x; // use cast to assign double * to int *10.y = *p; // What will this do?11.cout << y; // What will this print?12.return 0;13.}

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.1. #include <iostream>2. using namespace std;3. int main()4. {5. int *p, num;6. p = &num;7. *p = 100;8. cout << num << ' ';9. (*p)++;10.cout << num << ' ';11. (*p)--;12.cout << num << '\n';13. return 0;14. }

• The output from the program is shown here.

100 101 100

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 expressionp1 = 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.

1. // Demonstrate pointer arithmetic.

2. #include <iostream>

3. using namespace std;

4. int main()

5. {

6. int *i, j[10];

7. double *f, g[10];

8. int x;

9. i = j;

10. f = g;

11. for(x=0; x<10; x++)

12.cout << i+x << ' ' << f+x << '\n';

13. return 0;

14. }

• 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.

1. // Tokenizing program: pointer version.2. #include <iostream>3. #include <cstdio>4. using namespace std;5. int main()6. {7. char str[80];8. char token[80];9. char *p, *q;10. cout << "Enter a sentence: ";11. gets(str);12. p = str;13. // Read a token at a time from the string.14. while(*p) {15. q = token; // set q pointing to start of token16. /* Read characters until either a space or

the17. null terminator is encountered. */18. while(*p!=' ' && *p) {19. *q = *p;20. q++; p++;21. }

22. if(*p) p++; // advance past the space

23. *q = '\0'; // null terminate the token

24. cout << token << '\n';25. }26. return 0;27. }28. Here

1. // Tokenizing program: array-indexing version.

2. #include <iostream>3. #include <cstdio>4. using namespace std;5. int main()6. {7. char str[80];8. char token[80];9. int i, j;10.cout << "Enter a sentence: ";11.gets(str);12. // Read a token at a time from the

string.13. for(i=0; ; i++) {14. /* Read characters until either a space

or the15.null terminator is encountered. */16. for(j=0; str[i]!=' ' && str[i]; j++, i++)

17. token[j] = str[i];18. token[j] = '\0'; // null terminate the token19. cout << token << '\n';20. if(!str[i]) break;21. }22. return 0;23. }

Indexing a Pointer• It is possible to index a pointer as if it were an array.

1. // Indexing a pointer like an array.2. #include <iostream>3. #include <cctype>4. using namespace std;5. int main()6. {7. char str[20] = "hello tom";8. char *p;9. int i;10.p = str; // put address of str into p11. // now, index p like an array12. for(i=0; p[i]; i++) p[i] = toupper(p[i]);13.cout << p; // display the string14. return 0;15. }

• The program displaysHELLO 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:-1. pass-by-value

2. 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;1213 cout << "The original value of number is " << number;1415 number = cubeByValue( number ); // pass number by value

to cubeByValue16 cout << "\nThe new value of number is " << number <<

endl;17 return 0; // indicates successful termination18 } // end main19

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

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

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;1213 cout << "The original value of number is " << number;1415 cubeByReference( &number ); // pass number address to

cubeByReference1617 cout << "\nThe new value of number is " << number <<

endl;18 return 0; // indicates successful termination19 } // 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 functionreturn 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.