Associated Variables

1
Associated Variables

• Now we introduce structures, variable types which
  associate other variables.
• Consider the rectangle
• struct rect
• int top
• int bottom
• int left
• int right

top
left
right
bottom
2
Declaring rect Variables

• struct rect
• int top
• int bottom
• int left
• int right
• door, window
• door is a rectangle (rect) having four landmarks
  and window is also a rect.

3
Accessing The Landmarks

• door.top6
• door.bottom0
• door.left3
• door.right4
• window.top5
• window.bottom1
• window.left5
• window.right7

3
4
5
7
6
5
1
0
4
Calculate Areas

• da(door.top-door.bottom)
• (door.right-door.left)
• wa(window.top-window.bottom)
• (window.right-window.left)

5
Assigning all Values

• Copy values from door to window
• windowdoor
• Initialize while declaring
• struct rect
• int top
• int bottom
• int left
• int right
• door6, 0, 3, 4, window5, 1, 5, 7

6
Classlist

• struct StudentData
• char name
• long ID
• float final
• sd700
• sd0.name"Jones"
• sd0.ID1234567
• sd0.final76.5

7
Dynamic Memory Allocation

• Dynamic allocation of memory means that a program
  obtains memory elements while it is executing.
• Memory elements available for dynamic allocation
  are located in a region of memory called the
  heap dynamically allocated memory can only come
  from the heap.

8

• C standard functions are available to help with
  the management of dynamically allocated memory
  they perform the necessary calls to the operating
  system in order to
• fill the request for memory, or to
• release memory that has been allocated
  dynamically when it is no longer needed.

9

• The functions malloc and free, and the operator
  sizeof are essential to dynamic memory allocation
  (malloc and free are found in stdlib.h).

10
malloc

• The function malloc obtains memory from the heap
  and makes it available for use in a C program.
• The function malloc
• takes one argument which is the number of bytes
  to be allocated
• returns to the caller the address of the first
  byte of the memory area that has been allocated.
• The NULL address is returned if the requested
  memory could not be allocated.

11

• The number of bytes obtained from a call to
  malloc will always be located in contiguous
  memory locations.
• The memory area allocated through malloc is
  accessible via the address returned, which must
  be assigned into a pointer variable.
• Before the address is assigned to the pointer
  variable, it must be cast to the correct address
  type, defining in effect the type of the memory
  allocation and ensuring that any subsequent
  pointer arithmetic works properly.

12
Example of malloc

• int intPtr
• ?
• intPtr(int ) malloc(sizeof(int))
• intPtr9
• The call to malloc obtains a memory allocation
  large enough for storing an int.
• malloc returns the address of the first byte
  allocated which is cast to the address of an int
  and then assigned to intPtr, a pointer to an int.
• The memory allocation can now be used just like
  an int but it must be manipulated via its pointer.

13
free

• The function free de-allocates memory that has
  been previously allocated dynamically.
• The function free takes an argument which is a
  pointer to the memory area that is to be
  de-allocated.
• Based on the previous example
• free(intPtr)
• de-allocates the two bytes previously allocated
  by malloc to store an int.
• The de-allocated bytes are returned to the heap.

14
Dynamically Allocated Arrays

• Arrays can be created dynamically.
• A C standard function is invoked to obtained all
  of the required memory elements, and
• the array is managed using a pointer variable of
  the appropriate type.
• Not valid
• int n
• scanf("d", n)
• int arrayn

15
calloc

• The C standard function calloc, defined in
  stdlib.h, is especially well-suited to obtaining
  memory dynamically for the storage of an array.

16

• The function calloc takes 2 arguments
• the first is the size of the array to be created,
• the second is the number of bytes required to
  store a single element of the array this depends
  on the type of the array elements.
• The function calloc returns to the caller the
  address of the first byte of the allocated memory
  area.
• The NULL address is returned if the requested
  memory could not be allocated.
• Always be located in contiguous memory.

17
malloc vs calloc

• The function malloc can also be used to obtain
  the necessary memory but the argument sent must
  correspond to the total amount of memory needed.
• The argument must be of the form
  malloc(size_of_arraysizeof(type))
• Compare with
• calloc(size_of_array, sizeof(type))

18
Example
int arrayPtr arrayPtr(int )calloc(3,
sizeof(int))
?
(arrayPtr0)
7010 7011 7012 7013 7014 7015
arrayPtr0
(arrayPtr1)
arrayPtr1
(arrayPtr2)
arrayPtr2
?