Pointers

1
Pointers

• Pointers are variables that contain memory
  addresses as their values.
• A variable directly contains a specific value.
• A pointer contains an address of a variable that
  contains a specific value.
• A variable name directly references a value.
• A pointer indirectly references a value.
• Referencing a value through a pointer is called
  indirection.

2
Directly and Indirectly referencing a variable

• sum
• sumPtr sum

sum directly references a variable whose value
is 5
5
sumPtr indirectly references a variable
whose value is 5
5
3
Initialization

• A pointer may be initialized to 0 , NULL or an
  address.
• NULL is a symbolic constant defined in ltstdio.hgt
• A pointer with the value NULL points to nothing.
• The only integer that can be assigned to a
  pointer is 0.
• Dereferencing
• The operator indirection or dereferencing
  operator, returns the value of the object that
  its operand points to in memory. This is called
  dereferencing the pointer.

4
/ Program to display the address and value of a
pointer /

• include ltstdio.hgt
• int main()
• int a
• int aPtr
• a 200
• aPtr a
• printf(The address of a is p \n, a)
• printf(The value of aPtr is p \n, aPtr)
• printf(The value of a is d \n, a)
• printf(The value of aPtr is d \n,
  aPtr)

5
Calling functions by reference

• include ltstdio.hgt
• void cubebyref( int )
• int main()
• int num 5
• printf(The number is d \n,num)
• cubeByRef(num)
• printf(The new value of number is d
  \n,num)
• return 0
• void cubeByRef(int numPtr)
• numPtr nPtr nPtr nPtr

6
Relationship between Pointers and Arrays

• An array name is the address of the first element
  of an array.
• int b5
• The address of the first element
• using p to print - b, b0, b.
• Assume that int b5 and pointer variable bPtr
  have been declared. Since the array name is a
  pointer to the first element of the array
• bPtr b
• This is equivalent to bPtr b0
• Array element b1 can be referenced by
• ( bPtr 1)
• 1 is the offset to the pointer.
• The preceding notation is called pointer / offset
  notation.

7

• In pointer/offset notation, the offset is the
  same as the array subscript.
• Pointers can be subscripted exactly as arrays
  can.
• bPtr1 refers to the array element b1.
• This is called pointer / subscript notation.

8

• include ltstdio.hgt
• int main()
• int b 10 , 20 , 30 , 40
• int bPtr b
• int i, offset
• printf(Pointer/offset notation where the
  pointer is )
• printf(the array name \n)
• for(offset 0 offset lt 4 offset)
• printf((b d) d \n, offset, ( b
  offset) )
• printf(Pointer/subscript notation \n)
• for( i 0 i lt 4 i)
• printf(bPtr d d \n, i, bPtr i )
• printf(pointer/offset notation \n)

9

• include ltstdio.hgt
• void copy( char , const char )
• int main()
• char string110, string2 CS230
• char string310, string4 Lab
• copy(string1, string2)
• printf(string1 s \n,string1)
• copy(string3,string4)
• printf(string3 s \n,string3)
• return 0

10

• void copy(char s1, const char s2)
• int i 0
• do
• s1i s2i
• i
• while (s2i ! \0)
• s1i \0