CS 192

CS 192

Lecture 12Winter 2003

December 31, 2003 - January 1, 2004Dr. Shafay Shamail

Pointer• Points to an item• Holds the memory address of the item

• Has its own memory storage• Occupies space in memory• Hence can itself be accessed too

• Allows C/C++ to support dynamic memory allocation

• If x contains the address of y, x is said to “point to” y• Pointer variables are declared as

type *var-name;e.g. int *p; pointer p contains the memory address of an int variablefloat *fl; points to a float variable

Assumptions

• characters are one byte in length• integers are four bytes long• floats are four bytes long• doubles are eight bytes long

& address of

* value at address

&a address of variable a

*p contents of location pointed to by variable p

Pointer Operators

• & (address operator)– a unary operator that returns the memory address of its

operand.

int balance = 350;int *balptr;balptr = &balance;

• This address is the location of the variable in memory, it has nothing to do with the value of balance.

Pointer Operators

• * (indirection operator)– Is the complement of &.– It is a unary operator that returns the value of

variable located at address specified by its operand.

int balance = 350;int *balptr;balptr = &balance;int value;value = *balptr;// what does value contain?

Pointer Operators

Pointers• int a=1, b=2, *p;• Picture in memory at this point:

• p=&a; //p is assigned the address of a

• b=*p; //b is assigned the value pointed to by pAs p points to a, the statement b=*p is equivalent to b=a

a b p1 2

a b p1 2 ?

a b p1 1

int main() {

int balance; int *balptr; int value;

balance = 3200; balptr = &balance; value = *balptr;

cout << "balance is: " << value << '\n'; cout << "Memory address where balance is stored is: ” << balptr << endl;

return 0;}

Pointer Operators

Output

balance is: 3200Memory address where balance is stored is: 0012FF7C

(Note memory address may be different when you run it on your machine)

value = *balptr;

• The compiler transfers the proper number of bytes according to base type.

int *p;double f;// ...p = &f; // ERROR

Base Type

int *p;int x = 12;p=&x;cout << x << endl;//Assign a value to the location pointed to by p*p = 101;cout << x << endl; //what is value of x?cout << *p << endl;cout << p << endl;cout << &x << endl;

Assigning Values Through a Pointer

Assigning values through a pointer

121011010012FF780012FF78

(*p)++; //increment value to the location pointed to by p

int main(){

int *p, num; p = &num; *p = 454;

cout << num << ' '; (*p)++; /* parentheses are necessary because * operator

has lower precedence than ++operator */ cout << num << ' '; (*p) - -; cout << num << '\n';

return 0; } //Output?

Assigning Values Through a Pointer

//Output is 454 455 454

Assigning Values Through a Pointer

• There are only four arithmetic operators that can be used on pointers: ++, --, +, -

• Let p1 be an integer pointer which contains the address 5000

• p1++; // now p1 will be 5004• Each time p1 is incremented, it shall point to

the next integer.• p1--; will cause p1 to be 4996 if initially it was

5000.

Pointer Arithmetic

• Let p1 be a char pointer which contains the address 4000

• p1++; // now p1 will be 4001• Each time p1 is incremented, it shall point to the

next character.• p1--; will cause p1 to be 3999 if initially it was

4000.• Pointer of type other than char shall increase or

decrease by length of base type.

Pointer Arithmetic

• You cannot add two pointers• You can subtract two pointers (if they are of same

base type).• Other than

– addition or subtraction of a pointer and an integer, OR– csubtraction of two pointers,

no other arithmetic operations can be performed on pointers.

• You cannot add or subtract float or double values.

Pointer Arithmetic

int main(){

int *iptr, a; //which is pointer to int? which is int? double *fptr, b; int x;

a = 10; b = 20; iptr = &a; fptr = &b;

for ( x = 0; x < 10; x++) cout << iptr+x << " " << fptr+x << '\n';

return 0;}

Pointer Arithmetic

• In C++, there is a close relationship between pointers and arrays.

• Two examples

Pointers and Arrays

int main(){

int *iptr; int iarray[4] = { 5, 6, 7, 8};

iptr = iarray;

cout << iptr << " " << iarray << “\n”; cout << iptr[0] << " " << iarray[0] << “\n”; cout << *(iptr+1) << " " << iarray[1] <<“\n”;

return 0;} //Output?

Pointers and Arrays

Output is:

0012FF6C 0012FF6C5 56 6

Pointers and Arrays

int b[] = {10, 20, 30, 40, 50, 60, 70, 80, 90, 100}; int *bptr = b; // set bPtr to point to array b int *b2ptr = b+5; /* set b2Ptr to point to sixth

element of array b */

cout << "b[" << 5 << "] = " << b[5] << endl; cout << "*b2ptr = " << *b2ptr << endl; cout << "(b2ptr - bptr) = " << (b2ptr - bptr) << endl;

Pointers and Arrays

int b[] = {10, 20, 30, 40, 50, 60, 70, 80, 90, 100}; int *bptr = b; // set bPtr to point to array b int *b2ptr = b+5; /* set b2Ptr to point to sixth

element of array b */

cout << "b[" << 5 << "] = " << b[5] << endl; cout << "*b2ptr = " << *b2ptr << endl; cout << "(b2ptr - bptr) = " << (b2ptr - bptr) << endl;

Output is:b[5] = 60*b2ptr= 60(b2ptr - bptr) = 5

Pointers and Arrays

int i1[4]={5,6,7,8};int i2[4]={1,2,3,4};i2=i1; //NOT ALLOWED

Why? /*name of array is a constant that points to beginning of array*/

char str1[]=“i am a string”;char str2[]=“i am a string”;str1==str2; //WRONG way of string comaprison

Pointers may be compared in C++ using relational operators like >, >=, <, <=, == etc, but they must have some relationship to be meaningful

Pointers and Arrays

int i1[4]={5,6,7,8};int i2[4]={1,2,3,4};

int*ptr1=i1;int *ptr2;ptr2=ptr1; //this is ok

Pointers and Arrays

int i1[4]={5,6,7,8};What is wrong with the following statement?i1++;

//The following is okint *iptr=i1;iptr++;cout<<*iptr;

//The following is ok *(i1+3) = 100; // This is OK because i1 has not changed

Pointers and Arrays

int b[] = {10, 20, 30, 40}; int *bptr = b; // set bptr to point to array b

cout << "Array b printed with:" << endl << "Array subscript notation" << endl;

for (int i = 0; i <= 3; i++) cout <<b[i] << endl;

cout << "Pointer subscript notation" << endl;

for (i = 0; i <= 3; i++) cout << bptr[i] << endl;

Using Subscripting and Pointer Notations with Arrays

int offset; cout << "Pointer/offset notation where" << endl << "the pointer is the array name" << endl;

for (offset = 0; offset <= 3; offset++) cout << *(b + offset) << endl;

cout <<"Pointer/offset notation" << endl;

for (offset = 0; offset <= 3; offset++) cout << *(bptr + offset) << endl;

Using Subscripting and Pointer Notations with Arrays

int *iptr; int iarray[4] = {5,6,7,8}; iptr = iarray;

cout << iptr << " " << iarray <<endl;

cout << iptr[0] << " " << iarray[0]<< endl ; cout << &iptr[0] << " " << &iarray[0]<< endl;

Using Subscripting and Pointer Notations with Arrays

int main(){

int a[3] = {10, 20, 30}; double b[3] = {10.5, 20.5, 30.5}; int *iptr; double *fptr; int x, size=3;

iptr = a; //name of array is a constant that points to beginning of array fptr = b;

for(x=0; x<size; x++) { cout << iptr+x << " " << fptr+x << '\n'; cout <<*( iptr+x) << " " <<*( fptr+x) << '\n'; cout << *iptr+x << " " << *fptr+x << '\n'; }

return 0; } //pointer-add example

Pointer Arithmetic

Output:0x0012FF74 0x0012FF5C10 10.510 10.50x0012FF78 0x0012FF6420 20.511 11.50x0012FF7C 0x0012FF6C30 30.512 12.5

Pointer Arithmetic

int a[3] = {1, 2, 3};int *iptr; iptr = a;

//what is the difference between *iptr++ and (*iptr)++ and *(iptr++)?

cout<<*iptr++; cout<<*++iptr; cout<<(*iptr)++; cout<<++(*iptr); cout<<*(iptr++); cout<<*(++iptr);

Pointer Arithmetic

int a[3] = {1, 5, 9};int *iptr; iptr = a;

//what is the difference between *iptr++ and (*iptr)++ and *(iptr++)?

Output: (looking at each statement independently)

cout<<*iptr++; // 1 cout<<*++iptr; // 5 cout<<(*iptr)++; // 1 cout<<++(*iptr); // 2 cout<<*(iptr++); // 1 cout<<*(++iptr); // 5

Pointer Arithmetic

int a[3]={1, 5, 9};int *iptr; iptr = a;

Self Test: what is output if the statements are executed one after the other? //Be-careful is there array overrun? garbage values?

cout<<*iptr++; cout<<*++iptr; cout<<(*iptr)++; cout<<++(*iptr); cout<<*(iptr++); cout<<*(++iptr);

Self Test: Pointer Arithmetic

What is wrong here?

int* myptr;*myptr=32;

Self Test-2

What is output here?int x=235;int a[3]={1, 5, 9};cout<<*(&x);cout<<*(&a[1]);

Self Test

Pointers & Arrays• Is anything wrong with the following:

int num[3]; int i; for(i=0; i<10; i++) { *num = i; // is this OK? num++; // is this OK? }

• Can’t modify num. Can’t modify a pointer constant (an array’s name)

• But this is ok:

int num[3]= {0, 1, 2}; int *p = num; *(p++);

Pointers & Strings• A string constant, like an array name, is treated by the

compiler as a pointerchar *p = “abc”;cout<<p<<endl<<p+1<<endl<<*p<<endl<<*(p+1); //output?

• abc bc a b

• p is assigned the base address of the character array “abc”. String printed till null character, as usual

• Can we use such expressions: “abc”[1] and *(“abc” + 2)• Of course, output is b and c respectively

a b c \0

p

Pointer Comparisons• Pointers may be compared using relational operators (e.g. ==, <, >). • To be meaningful, pointers should be of same type

int num[10]; int *start, *end; start = num; end = &num[9]; while(start!=end) { cout << "Enter a number: "; cin >> *start; start++; } start = num; /* reset the starting pointer */ while(start!=end) { cout << *start << ' '; start++; }

…

start

end

num

Arrays of Pointers

• Can sure do that like other data typese.g. int *zztop[10]; //array of 10 int pointers

• Now, say int var; zztop[3]=&var; *zztop[3];• Arrays of pointers to strings commonly used

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" };

cout << fortunes[1] << endl << fortunes<<endl << *(fortunes) << endl << *(fortunes[2]) << endl;

• Output: A new love will enter your life

0012FFC6Soon, you will come into some moneyY

Arrays of Pointers• Two dimensional string pointers e.g. C++ dictionary

char *keyword[][2] = { "for", "for(initialization; condition; increment)", "if", "if(condition) ... else ...", "switch", "switch(value) { case-list }", "while", "while(condition) ...", // add the rest of the C++ keywords here "", "" // terminate the list with nulls };

int main(){ char str[80]; int i; cout << "Enter keyword: "; cin >> str;

for(i=0; *keyword[i][0]; i++) if(!strcmp(keyword[i][0], str)) cout << keyword[i][1]; return 0; }

Array of Strings vs. Array of Pointers• Spot the difference:

char movies[5][20] ={“Godfather”, “Maula Jatt”, “A Fish Called Wanda”, “Blade Runner”, “Spiderman”};

char *movies[5] = {“Godfather”, “Maula Jatt”, “A Fish Called Wanda”, “Blade Runner”, “Spiderman”};

• In the first case, each column is 20 characters wide due to the longest movie name; wasted space

• In the second case, the strings are placed contiguously in memory without wasting space; pointers in the array movie point to them

The new operator

int *x_ptr = new int;

OR

int *xptr;xptr = new int;//heap

Dynamic Allocation

int *xptr=new int;*xptr = 73;

int *x2ptr = new int;*x2ptr = 65;

Dynamic Allocation

What is wrong here?

int *xptr = new int;*xptr = 73;

int *x2ptr;*x2ptr=65;

Dynamic Allocation

int *xptr = new int;*xptr = 73;

int *x2ptr;x2ptr = xptr;*x2ptr = 65;

Dynamic Allocation

//What is wrong here?int *xptr = new int;*xptr = 73;

int *x2ptr = new int;x2ptr = xptr;*x2ptr = 65;

//memory leak

Dynamic Allocation

int *myptr = new int(73);cout << *myptr;

delete myptr;

Dynamic Allocation

const int SIZE = 10;double *ptr = new double[SIZE]; /*10 element array*/int i;

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

ptr[i] = 2.0*i;}

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

cout << ptr[i] << endl; }

delete []ptr;

Dynamic Allocation

int *intPtr; int i[10]={10,11,12,13,14,15,16,17,18,19}; char *charPtr; char c[]="We are testing char pointers";

intPtr = i; charPtr = c;

cout << i <<" "<< c << endl; cout << intPtr << " " << charPtr << '\n'; cout << *intPtr <<" "<< i[0] << '\n';

Char Pointers

0x0012FF54 We are testing char pointers0x0012FF54 We are testing char pointers10 10

char pointers

When the compiler encounters a string constant, it stores it in the program string table and generates a pointer to the string.

char *s; s = "Pointers are fun to use.\n"; cout << s;

char s2[]="Pointers are fun to use.\n"; cout << s2;

Char Pointers

int size;cin>>size;int* array=new int[size];int i;

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

array[i]=2*i;}

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

cout<<array[i]<<endl; }

delete []array;

Pointers - Dynamic Allocation

• This program scans the input string, copying characters from the string into another array, called token, until a space is encountered. It prints the token and repeats the process until null at end of string is encountered.

• e.g “This is a test:Thisisa test

Tokenizing Example

int main(){

char str[80], 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; }

SELF_TEST:Tokenizing Example Using Arrays

char str[80], token[80]; int i=0, j=0; cout << "Enter a sentence: "; cout.flush(); gets(str); // Read a token at a time from the string.

Tokenizing Example Using Arrays

while(str[i]) {

j=0; while(str[i]!=' ' && str[i])

{ token[j]=str[i]; i++; j++; } token[j]='\0'; if(str[i]) i++; cout << token << '\n';

}

Tokenizing Example Using Arrays

int main(){

char str[80], token[80];char *p, *q;

cout << "Enter a sentence: ";

gets(str); p = str;

Tokenizing Example Using Pointers

// 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;}

Tokenizing Example Using Pointers