CS154 Data Structure in C

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CS154 Data Structure in C

• Chapter 2
• Dynamic variables Pointers
• Tutor Angie Hui

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Objective

• Dynamic variables
• Pointers
• Allocating and freeing dynamic variables
• Static variables
• Static variables Vs Dynamic variables

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Static variable

• Definition
• Size is fixed during programming
• running
• E.g. array

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Static variable - Array

• Disadvantage
• int num1000
• - May overestimate the length of
• the array, which cause memory
• waste
• - May underestimate the length of
• the array, which result in
• insufficient memory to hold all the
• required data

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Dynamic variable

• Definition
• Size can vary during programming
• running
• E.g. pointer, linked list

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Dynamic variable - pointer

• Declare a pointer
• e.g1. char c
• e.g2. int no
• Define a pointer type using typedef
• e.g. typedef int intPtr
• intPtr ptr

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Dynamic variable - pointer

• Result
• b11

• E.g1
• int a
• int b 5
• a b //a points to b
• a a 6
• printf(bd, b)

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Dynamic variable - pointer
Explanation

• Effect of the below instructions
• int a
• int b 5,

Effect of the below instructions a b
//a points to b a a 6
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Dynamic variable - pointer

• E.g2 int ptr, b5, c10
• //assign the address of b to ptr
• ptr b
• / change the content in the memory
• location pointed by ptr, to 8 /
• ptr 8

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Dynamic variable - pointer

• E.g3
• int x10, ptrx
• printf(xd\n, x)
• printf(ptrd\n, ptr)
• printf(ptrp\n, ptr)

Result x10 ptr10 ptrFF02 ( Assume the
address of x is FF02 )
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Dynamic variable - pointer

• printf(ptrd\n, ptr)
• - ptr means the content of the
• memory location pointed by ptr
• - So this statement will print out the
• content of x since ptr is pointing
• to x.

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Dynamic variable - pointer

• printf(ptrp\n, ptr)
• - ptr means the address of the
• location pointed by ptr
• - So this statement will print out the
• address of x since ptr is pointing to
• x!
• - We use p if we want to print out
• an address

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Dynamic variable - pointer

• printf(ptrp\n, x)
• - This statement will print out the
• address of x
• - The above statement should print out
• the same value as the following one if
• ptr is pointing to x
• printf(ptrp\n, ptr)

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Direct Access Vs Indirect Access

• int x int b x
• Direct access
• e.g. x 12 //directly assign 12 to x
• Indirect access
• e.g. b 3 //Indirectly assign 3 to x

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Dynamic variable - pointer

• Exercise 1
• (Assume the addr. of a is FE01)
• Line 1 int a15, b6
• Line 2 int ptr
• Line 3 ptr a
• Line 4 ptr ptr 3
• Line 5 b ptr 10

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Dynamic variable - pointer
a b ptr ptr
Line 1 (i) (ii)
Line 3 (iii)
Line 4 (iv) (v) (vi) (vii)
Line 5 (viii) (ix)
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Answer

• (i) 15
• (ii) 6
• (iii) FE01
• (iv) 18
• (v) 6
• (vi) FE01
• (vii) 18
• (viii) 28
• (ix) 18

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Dynamic variable - pointer

• Exercise 2
• long x10, ptrx
• ptrptr7
• printf(xd\n, x)
• printf(ptrd, ptr)

Result x70 ptr70
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Dynamic variable - pointer

• Exercise 3
• long x10, ptrx
• ptr
• printf(xd\n, x)
• printf(ptrd, ptr)
• printf(xd, x)

Result x11 ptr11 x12
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Using pointer notation to refer to the item in an
array

• int no 11, 12, 13, 14, 15
• no address of no0 (i.e. no0)
• no1 address of no1(i.e. no1)
• no3 address of no3(i.e. no3)
• (no2) 13
• (no1) 12
• (no4) 15 and so on

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Array - Exercise

• int no 11, 12, 13, 14, 15
• int i
• for(i0 ilt5 i)
• printf(d , (noi))

Result 11 12 13 14 15
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Array Exercise (Cont)

• for(i0 ilt5 i)
• printf(d , (noi))
• is equivalent
  to
• for(i0 ilt5 i)
• printf(d , noi)

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Dynamic memory allocation

• Programmer can vary the size of dynamic variables
  during program running
• In C, programmer can use the following 2
  functions to allocate memory space to any dynamic
  variable
• (i) malloc or
• (ii) calloc (Out of syllabus)

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Dynamic memory allocation

• E.g. To allocate 3 char spaces to ch
• char ch
• malloc (memory allocation)
• e.g. ch (char)malloc( 3sizeof(char) )
• calloc (contiguous allocation)
• e.g. ch (char)calloc( 3, sizeof(char) )

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Dynamic memory allocation

• If the computer system is unable to allocate the
  necessary memory, NULL is returned
• Otherwise, the starting address of the memory
  block will be returned

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malloc Vs calloc

• calloc
• - memory allocated is automatically
• initialized to 0
• malloc
• - memory allocated is not initialized.
• It starts with garbage values

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malloc Exercise 1

• Ex1 How to allocate 5 long int spaces
• to lptr, using malloc
• long lptr

Answer lptr (long)malloc(5sizeof(long))
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malloc Exercise 2

• struct student
• char name20
• char studID6
• struct student ptr

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malloc Exercise 2 (Cont)

• Q1 How to allocate 10 student struct to ptr?
• Q2 How to assign Mary Wong as the
• name of the 1st student?
• Q3 How to assign FT1111 as the student
• ID of the 1st student

Answer Q1. ptr(struct student)malloc(10sizeof(s
truct student)) Q2. strcpy(ptr0?name, Mary
Wong) Q3. strcpy(ptr0?studID, FT1111)
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Release of unused memory

• Its better to free all the unused memory so as
  to use the memory effectively!!
• In C, programmer can use the function, free to
  free all the unnecessary memory!

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Release of memory

• free
• - char ch
• ch (char)malloc( 3sizeof(char) )
• .
• free(ch)

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Heap

• It is a part of the memory which is reserved for
  dynamic variables.

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NULL pointer

• If a pointer equals to NULL, that means it points
  to nothing
• Initially, we can set the pointer to NULL to
  indicate the pointer is pointing to nothing at
  the beginning
• e.g. int no NULL
• To check whether the dynamic memory allocation is
  successful or not ?
• if((ch(char) malloc(3sizeof(char)))
  NULL)
• printf(Not enough memory allocated to
  ch!!)

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Compare two pointers

• Case 1. Compare the reference
• - i.e. check whether the 2
• pointers point to the
  same
• location
• Case 2. Compare the contents

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Case 1 Compare the reference

• e.g1.
• int first, second, a10
• first a second a
• if(first second)
• printf(they point to the same location)
• else
• printf(they point to different
  location)

Result they point to the same
location.
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Case 1 Compare the reference

• e.g2.
• int first, second, a10
• first a second first
• if(first second)
• printf(they point to the same location)
• else
• printf(they point to different
  location)

Result they point to the same
location.
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Case 1 Compare the reference

• e.g3.
• int first, second, a10, b10
• first a second b
• if(first second)
• printf(they point to the same location)
• else
• printf(they point to different
  location)

Result they point to the different
location.
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Case 2 Compare the contents

• e.g1.
• int first, second, a10, b10
• first a second b
• if( first second )
• printf(they have the same content)
• else
• printf(they have the different content)

Result they have the same content.
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Changing address of a pointer

• Method 1
• float f(float)malloc(sizeof(float))
• Method 2
• int no NULL
• Method 3
• char a, b
• a b

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Changing address of a pointer

• Method 4
• char a, b, ptr1, ptr2
• ptr1 a
• ptr2 ptr1

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Static Vs Dynamic variable
Static variable Dynamic variable
Size is fixed during program running Size can vary during program running
Cannot be created or destroyed during the program execution Can be created or destroyed during the program execution
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• END