Arrays, Pointers, and Pointer Arithmetic

1
Arrays, Pointers, and Pointer Arithmetic
2
Arrays and Pointers

• The name of an array is treated as a constant
  pointer that points to the first element of the
  array. Therefore the array name and pointers of
  the same type have some similarities
• int a 6 4,2,3,1,8, 11
• cout ltlt a // Using Pointer Notation
• cout ltlt a0 // Using Array Notation
• Both statements above print 4

3
Arrays and Pointers

• More Examples
• int a6 4,2,3,1,8, 11
• int b
• ba
• cout ltlt b //displays 4
• cout ltlta // displays 4
• cout ltlt b0 // displays 4
• cout ltlt b1 // displays 2

0 1 2
3 4 5
a
b
1000
1004
1008
1012
1016
1020
4
Arrays and Pointers

• In other words the array notations and pointer
  notations are interchangeable.
• What are the the following statements output?
• cout ltlt (b3)
• cout ltlt a 2
• cout ltlt (a1)
• cout ltlt a
• cout ltlt a 2

// Output is 1
// Output is the address of third element
// Output is 2
// Output is the address of first element
// Output is 6 (42)
0 1 2
3 4 5
a
b
1000
1004
1008
1012
1016
1020
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Passing Arrays and Strings to Functions

• Since arrays name and string name are pointers,
  therefore an array or string can appear as a
  pointer in a function parameter list. The
  following function prototypes are equivalent.
• void fun (int x)
• void fun (int x)
• void fun (char string)
• void fun (char string)

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Pointer Arithmetic

• Legal pointer arithmetic in C
• Pointer Integer
• Pointer Integer
• Pointer Pointer
• Pointer, or Pointer
• Pointer, or --Pointer
• Other arithmetic operations are illegal. An
  operation like Integer Pointer is not also
  allowed.

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Pointer Arithmetic

• pointer n refers to the address of nth
  element , from the current address. In other
  words
• pointer n current address n sizeof (type)
• E.g
• double d3 4.5, 6.9, 7.7
• double ptr d0

ptr
1000
1008
1016
d
ptr ptr 2 // moves ptr to the address1000
28
ptr
d
1000
1008
1016
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Pointer Arithmetic

• Pointer1 Pointer2, results in an integer
  value that represents the number of elements
  between the two pointers
• int a5 2, 6, 4, 7, 9
• int ptr
• ptr a2
• int diff
• diff ptr a
• In this example the value of diff will be 2.

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Example

• int my_strlen (char string)
• char p
• p string
• while (p ! \0)
• p
• return (p string)

int main () int length const char s
xyz length my_strlen ( s )
printf (The string length is\
d., length) return 0
stack
p
String constant area
AR my_strlen
string
x
y
s
s
z
AR main
length
\0
3
No args
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Initialization vs. Assignment

• In general, initialization of variables in C is
  only possible at the time of declaration of
  variables. Or, in other words, at time allocation
  of memory. For example, the following operations
  are called initialization
• int a 20
• int b 40
• int c3 3, 6,1
• Now, consider the following statement
• int x
• char y
• x 9
• y A

These two operation are assignment, NOT
initialization.
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Initialization of Arrays

• Consider the following initialization of array,
  some are valid, and some are invalid
• int myarray26
• int myarray16 2, 5, 7 //OK
• int myarray3 3,4, 6, 8 // OK
• int myarray4 3 4, 5, 6, 7 //ERROR, not
  enough space
• char string32 ABCD // ERROR, not
  enough space
• char string15 ABC // OK
• char string2 XYZ // OK
• char string3 KLH // OK, but
  bad style
• const char string4 CBC // better
  style

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Array Initialization

• Difference between pointer and array
  initialization
• char s Jack
• char ptr Joe

s
J
2000
String Constant Area
a
2001
J
1000
c
2002
o
1001
k
2003
e
1002
\0
2004
\0
1003
ptr
2005
No Args
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Pointer Initialization

• The following statement are all known as
  initialization operations in C/C.
• int a5, b a, cNULL
• int d a
• But the following statements are assignment
  operations
• a 9
• b a
• c a
• d a
• d 44