pointer

• What it is

• How to declare it

• How to use it

• Relationship between arrays and pointers

• Relationship between strings and pointers

pointerA pointer is a variable that contains a memory address as its value

count

10

countPtr

pointer

count

countPtr

memory address 1000 10

1000

count

10

countPtr

1000

How to declare a pointer

int * countPtr;

countPtr is a variable of type “pointer to int”

example

count

countPtr

memory address 100010

1000

int count = 10;int *countPtr = 1000;

How to initialize a pointer to point to an address

count

countPtr

memory address 100010

1000

int count = 10; int * countPtr = &count;

& is an operator that returns the address of a variable.

Restrictions for using & operator• May be used for variables

int count;

int *countPtr = &count;

• May not be used with constants (&10)

How to initialize a pointer to point to nothing

int * aPtr = NULL;

OR

int * aPtr; aPtr = NULL;

Indirect and direct references

Direct reference – changing count memory location using count variable

Indirect reference – changing count memory location using countPtr variable

count

10

countPtr

Indirect and direct references

int count; int * countPtr = &count;

count = 12; // direct reference*countPtr = 12 // indirect reference

* is the indirection operator. Means change the memory location currently pointed to by countPtr.

count

10

countPtr

12

Exercises

1) Declare an integer variable called x. Declare a pointer to x called xPtr and initialize it so that it points to the variable x. Use xPtr to indirectly change the value of x to 20;

2) Declare a second integer variable called y. Declare a pointer to y called yPtr and initialize it so that it points to the variable y. Calculate the sum of x + y using xPtr and yPtr.

pointer arithmetic

int *aPtr;aPtr++; // is valid but what does it do?

integer (4 bytes) aPtr

aPtr is incremented by the size of whatever it is pointing to. In this example 4 bytes. So the value in aPtr is increased by 4 by the aPtr ++ statement

pointer arithmetic

char *aPtr;aPtr++;

char (1 byte) aPtr

aPtr is incremented by the size of whatever it is pointing to. In this example 1 byte. So the value in aPtr is increased by 1 by the aPtr ++ statement

using pointer arithmetic

grades[0]grades[1]grades[2]grades[3]grades[4]

10085506588

int gradesPtr = &grades[0];

gradePtr

gradesPtr++; gradePtr

using pointer arithmetic

letters[0]letters[1]letters[2]letters[3]letters[4]

‘a’‘b’‘c’‘d’‘e’

char letterPtr = &letters[0];

letterPtr

letterPtr ++; letterPtr

pointers and arrays

• array name is a pointer. This is why:can pass array name to a function without the

bracketsarrays are passed to functions as reference

parameters by default

arrays as function parametersPass the name of the array without the brackets

void PrintResults(int results[ ]); // function prototype

void PrintResults(int results[ ]) // function definition{

}

calling sequence: const int maxRange = 10; int frequency[maxRange ] = {0}; PrintResults (frequency);

pointers and arrays

int b[ ] = {1,2,3,4,5};

b[0]b[1]b[2]b[3]b[4]

12345

b

pointers and arrays

int *bPtr; int b[ ] = {1,2,3,4,5};

bPtr = &b[0]; or bPtr = b;

b[0]b[1]b[2]b[3]b[4]

12345

bPtr

b

pointers and strings

char digits [ ] = {“0123456789”};

‘0’‘1’

‘3’‘4’‘5’‘6’‘7’‘8’‘9’

digits[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

‘2’

NULL

pointers and strings

char digits [ ] = {“0123456789”};

‘0’‘1’

‘3’‘4’‘5’‘6’‘7’‘8’‘9’

digits[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

‘2’

NULL

char *digitsPtr = digits;

digitsPtr

simplify

Instead of char digits [ ] = {“0123456789”};

char *digitsPtr = digits;

do char *digitsPtr = “0123456789”;

simplify

char *digitsPtr = “0123456789”;

cout << digitsPtr << endl; // displays all the numbers in the string

Simplify Array of strings

const int Size = 5; const int WordSize = 10;

char article [Size] [WordSize] = {"the", "a", "one", "some", "any"}; char noun [Size] [WordSize] = {"boy", "girl", "dog", "town", "car"}; char verb [Size] [WordSize] = {"drove", "jumped", "ran", "walked", "skipped"}; char preposition [Size] [WordSize] = {"to", "from", "over", "under", "on"};

Array of strings

const int Size = 5; char *article [Size] = {"the", "a", "one", "some", "any"}; char *noun [Size] = {"boy", "girl", "dog", "town", "car"}; char *verb [Size] = {"drove", "jumped", "ran", "walked", "skipped"}; char *preposition [Size] = {"to", "from", "over", "under", "on"};

The End !!!!