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 0012FE8C0012FE64 0012FE940012FE68 0012FE9C0012FE6C 0012FEA40012FE70 0012FEAC0012FE74 0012FEB40012FE78 0012FEBC0012FE7C 0012FEC40012FE80 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 OKnum++; // 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 valueswhile(start <= end) {cout << "Enter a number: ";cin >> *start;start++;}start = num; // reset the starting pointer// display the valueswhile(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 main22 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.