Pointer

1
Pointer

• To hold the location of another variable
• Declaration
• a type and a name with an asterisk
• E.g. int ptr
• Assigning a value
• ptr k

2
Pointers and Structures

• Declaration
• struct date datePtr
• Initialization
• datePtr today
• Access members
• (datePtr).year
• datePtr-gtyear

struct date int day char month10 int
year struct date today
3
Structures contains pointers

• Pointers can be members in a structure
• struct date
• int day
• char month
• int year
• Special case
• struct date
• int day
• char month
• int year
• struct date next

4
Linked List

• What is a list?
• Node
• Link
• An alternative to Array
• Pros/Cons
• More flexible
• Can grow and shrink as necessary
• Insert and delete node with particular index
• - Cannot be randomly accessed
• sorted (ordered list)

5
Linked List

• Operations
• Start a list
• Create a node
• Insert
• Delete
• Search
• Print

6
Memory Allocation

• malloc is by far most frequently used
• Defined in ltstdlib.hgt
• void malloc(size_t size)
• Allocate memory space with size bytes
• Why does it return a void pointer?
• Because it doesn't matter to malloc to what type
  this memory will be used for
• If no memory of the size available, will return
  null
• Be sure to check whether it is null

7
Function sizeof

• Helpful when dynamically allocating memory
• sizeof(data type) returns a size_t of the data
  type in byte(s)
• For a typical 32-bit machine
• sizeof(int) returns 4

8
Free Allocated Space

• Very important
• System wont automatically take back memory space
  allocated through malloc
• If not free, a memory leak
• How?
• Use function
• void free(void ptr)
• It release the memory space referenced by ptr
• Note that free can take in NULL, as specified by
  ANSI

9
Build a Linked List

• Build a structure representing a node
• struct studentRecord
• int idNum
• struct studentRecord next
• Initialize the node
• struct studentRecord first NULL

NULL
first
10
Create a node

• Follow these steps
• Allocate memory for the node
• Set data into the node
• struct studentRecord new_student
• new_student malloc(sizeof(struct studentRecord
  ))
• ( new_student).idNum 1
• new_student ?next NULL

new_student
11
Insert a node insert in the front(empty)

• Follow these steps
• Create a node
• Set the node pointing to the front of the list
• Set it as the starting node of this list
• new_student malloc(sizeof(struct studentRecord
  ))
• new_student ? idNum 2
• new_student ?next first
• first new_student

NULL
new_student
first
12
Insert a node insert in the front(not empty)

• Follow these steps
• Create a node
• Set the node pointing to the front of the list
• Set it as the starting node of this list
• new_student malloc(sizeof(struct studentRecord
  ))
• new_student ? idNum 2
• new_student ?next first
• first new_student

NULL
first
new_student
13
Insert a node insert in middle

• To insert a new node after node called pt, follow
  these steps
• Create a node
• Set the node pointing to the next node after pt
  in the list
• Set it as the next node of this list after pt
• new_student malloc(sizeof(struct studentRecord
  ))
• new_student ? idNum 2
• new_student ?next pt ? next
• pt ? next new_student

first
pt
NULL
new_student
14
Traversing along the List

• for (p first p ! NULL p p?next)
• .
• int Length(struct studentRecord first)
• int count 0
• struct studentRecord current first
• while (current ! NULL)
• count
• currentcurrent-gtnext
• return(count)

15
Get nth Element in List

• int GetNth(struct studentRecord first, int
  index)
• struct studentRecord current first
• int count 0 // the index of the node
• while (current ! NULL)
• if (count index)
• return(current ? idNum )
• count
• current current ? next
• return(-1) // if we get to this line, the caller
  was asking
• // for a non-existent element

16
Delete

• Delete (almost reverse of insertion)
• locating the node to be deleted (see search a
  linked list)
• altering the previous node to bypass the deleted
  node
• calling free to reclaim the space occupied by the
  deleted node

17
Example for deleting

• To delete a node p from a linked list, you should
  also know the node which is in front of p in the
  list (q)
• if (p ! NULL)
• q ? next p ? next
• free(p)

q
p
NULL
first
18
Example for deleting

• If deleting the first node in the list
• if( p first )
• first p-gtnext
• free(p)

p
NULL
first
19
Demo

• http//www.cosc.canterbury.ac.nz/mukundan/dsal/Lin
  kListAppl.html