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Pointers

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Getting the Address of a Variable

} Each variable in program is stored at a unique

address

}

Use address operator& to get address of avariable:int num = -23;

cout

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Pointer Variables

}Pointer variable pointer): variable that holds an

address 

} Can perform some tasks more easily with anaddress than by accessing memory via a symbolic

name:}  Accessing unnamed memory locations

}  Array manipulation

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Pointer Variables

} Definition:int *intptr;

} Read as:

“intptr can hold the address of an int”

} Spacing in definition does not matter:int * intptr;

int* intptr;

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Pointer Variables

}  Assignment:int num = 25;int *intptr;intptr = #

}

Memory layout:

Can access num  using intptr and indirection

operator *: cout

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Indirection Operator (dereferencing)

}

Value of a variable pointed to by ptr }  Achieved by * operator 

int num = 25;

int *intptr;intptr = #

cout

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Pointers & Arrays

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}  Array name is starting address of array

int a[] = {4, 7, 11};

cout

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Example

void main (){

int a[ ] = {4, 7, 11};

  int *ptr = a;

cout

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Pointers in Expressions

Given:

int a[]={4,7,11};

int *ptr = a;

cout

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 Array Access

}  Array notation

a[i] 

is equivalent to the pointer notation

  *(a + i)

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 Pointer Arithmetic

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 Pointer Arithmetic

} Some arithmetic operators can be used with pointers:

  - Increment and decrement operators ++, --

  - Integers can be added to or subtracted from

pointers using the operators +, -, +=, and -=

  - One pointer can be subtracted from another 

  by using the subtraction operator -

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Pointer arithmetic

} Integer math operations can be used with pointers.

} If you increment a pointer, it will be increased by the

size of whatever it points to.

a[0] a[1] a[2] a[3] a[4]

int *ptr = a;

*ptr

*(ptr+2)*(ptr+4)

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Pointer Arithmetic (++/--)

}  Assume the variable definitions

  int a[]={4,7,11};

int *ptr = a;

}  Examples of use of ++ and –- ptr++; // points at 7

 ptr--; // now points at 4

 

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  Pointer Arithmetic (+)

• Assume the variable definitions:

  int a[]={4,7,11};

int *ptr = a;

•  Example of use of + to add an int to a pointer:

  cout

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 Pointer Arithmetic (+=)

• Assume the variable definitions:

  int a[]={4,7,11};

int *ptr = a;

•  Example of use of +=:

 ptr = a; // points at 4

 ptr += 2; // points at 11 

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 Pointer Arithmetic

}  Assume the variable definitions

  int a[] = {4,7,11};

int *ptr = a;

}  Example of pointer subtraction ptr += 2;

cout

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Comparing Pointers

}

Relational operators can be used to compareaddresses in pointers

} Comparing addresses in pointers is not the same as

comparing contents pointed at by pointers: if (ptr1 == ptr2) // compares  // addresses

 if (*ptr1 == *ptr2) // compares  // contents

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Pointers & Functions

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 Pointers as Function Parameters

}  A pointer can be a parameter 

} Works like a reference parameter to allow change toargument from within function

}  A pointer parameter must be explicitly dereferencedto access the contents at that address

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Pointers as Function Parameters

} Requires:

  1) asterisk * on parameter in prototype and

  heading  void getNum(int *ptr); 

2) asterisk * in body to dereference the pointer 

  cin >> *ptr; 

3) address as argument to the function  getNum(&num); 

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Pointers as Function Parameters

  void swap(int *x, int *y)

  {

  int temp;

  temp = *x;  *x = *y;

  *y = temp;

  }

  int num1 = 2, num2 = -3;  swap(&num1, &num2);

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Example (returning search key index)

int find(int *p, int key)

{

for(int i=0;i

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Contd…

void main ()

{

int a[]={1,12,45,63,18,14,6,9,10,7};

int num;

coutnum;

int *ptr=a;

int k=find ptr,num);

if (k!=-1)

cout