Section 7 Pointers and Arrays

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• Section 7 Pointers and Arrays
• Pointer is one of the most important features of
  C
• 7-1 Pointers and Addresses
• (with more than 10 examples)
• A typical computer has an array of consecutively
  numbered hardware cells
• This array is called the memory, and the number
  of a cell is called the address
• Just like a set of rooms, each having a
  room-number, and somebody inside it
• A simplified picture of memory

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00100000 00100001 00100010 00100011 00100100
00100101 00100110 00100111 00101000 00101001
00101010 00101011
10110111
00100010
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• A pointer is a C variable used to hold the
  MEMORY ADDRESS
• of another item
• Example 1
• x10110111 px00100010
• then px is a pointer that points to x
• An item can be
• A basic variable whose type is int, float,
  double, char.
• An array element
• Another pointer
• A structure (user defined data type)
• A function

4

• Pointer Declaration
• To declare a pointer variable, put a in front
  of the name of the variable
• Example 2
• char x, px
• float y, py
• int i, pi
• px is a pointer that holds the memory address of
  a char variable
• py is a pointer that holds the memory address of
  a float variable
• pi is a pointer that holds the memory address of
  an int variable

5

• Pointer Basics
• To assign a variables MEMORY ADDRESS to a
  pointer, use operator
• x means address of x
• Example 3
• char x,px
• short i,pi
• px x
• pi i
• px now holds the MEMORY ADDRESS of x (x is
  pointed to by px)
• pi now holds the MEMORY ADDRESS of i(i is
  pointed to by pi)

6

• To store/retrieve a value in/from a variable
  pointed to by a pointer, use the Indirection
  operator
• z means the content of the cell whose address
  is z
• Example 4
• double x,y,px
• pxx (x is pointed to by px)
• px3.0 (a value of 3.0 is now stored in x)
• ypx (this is equivalent to yx)

7

• Example 5 (pointer pitfall)
• Dont put a value into a location with the
  unknown memory address
• int q, p
• (At this moment, p itself has not been
  initialized yet!)
• (You should not put anybody into a room with an
  unknown room-number)
• p5
• No compiler error or run-time error
• Value 5 is stored in an unknown memory location
  (Your program may crash!)