Lecture 25:Pointers

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Lecture 25Pointers

• Introduction to Computer Science
• Spring 2006

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Contents

• Review searching algorithm on an ordered list
• Pointers

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Contents

• Review searching algorithm on an ordered list
• Pointers

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Sequential Search on an Ordered List

• General form of sequential search algorithm on a
  sorted list

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Sequential Search on an Ordered List
int seqOrderedSearch(const int list, int
listLength, int searchItem) int
loc //Line 1 bool found false //Line
2 for (loc 0 loc lt listLength
loc) //Line 3 if (listloc gt
searchItem) //Line 4 found
true //Line 5 break //Line 6
if (found) //Line 7 if
(listloc searchItem) //Line 8 return
loc //Line 9 else //Line 10
return -1 //Line 11 else //Line 12
return -1 //Line 13
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Binary Search

• Binary search can be applied to sorted lists
• Uses the divide and conquer technique
• Compare search item to middle element
• mid (firstlast)/2
• first is the starting index of the search list
• last is the ending index of the search list
• mid is the index of the middle element of the
  search list
• If search item is less than middle element,
  restrict the search to the lower half of the list
• Otherwise search the upper half of the list

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Search 75 in the following list
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Binary Search
int binarySearch(const int list, int
listLength, int searchItem) int first
0 int last listLength - 1 int mid bool
found false while(first lt last
!found) mid (first last) /
2 if(listmid searchItem) found
true else if(listmid gt searchItem)
last mid - 1 else first mid
1 if(found) return mid else
return -1
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Contents

• Review searching algorithm on an ordered list
• Pointers

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Three categories of data type

• Simple data type
• Structured data type
• Pointer

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Pointer Variables

• int count 5
• The value "5" is stored in memory and can be
  accessed by using the variable "count".
• Pointer variable content is a memory address
• Declaring Pointer Variables
• dataType identifier
• int p
• char ch

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Pointer Variables (continued)

• These statements are equivalent
• int p
• int p
• int p
• The character can appear anywhere between type
  name and variable name
• int p, q
• Only p is the pointer variable, not q. Here q is
  an int variable
• To avoid confusion, attach the character to the
  variable name
• int p, q
• The following statement declares both p and q to
  be pointer variables of the type int
• int p, q

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Address Of Operator ()

• Example
• int count 5
• int p
• The ampersand, , is called the address of
  operator
• p count
• The address of operator is a unary operator that
  returns the address of its operand

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Dereferencing Operator ()

• C uses as the binary multiplication operator
  and as a unary operator
• When used as a unary operator,
• Called dereferencing operator or indirection
  operator
• Refers to object to which its operand (that is, a
  pointer) points
• Example
• int count 5
• int p
• p count // Stores the address of count in
  p
•     // The unary operator
  returns the address of a variable
• int total
• total p     // The value in the address
  stored in p is assigned to total

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int p int num
num78
p num
p 24
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An application of pointer parameter passed by
address

• void swap(int x, int y)
• int tmpx
• xy
• ytmp
• int a1, b2
• swap(a, b)

a
b
1
2
2
1
x
y
tmp1
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Difference between pointers and references

• Reference is always the alias of some variable,
  but pointer does not have to be.
• int a int raa
• int pa //pa is not an alias of any variable, we
  call it dangling.
• Reference can not refer other variable after it
  is defined.
• int a, b int raa rab //ra is not the alias
  of b. Here, rab is equivalent to ab
• int pax pay //pa refers to y now
• pointer supports arithmetic operation, reference
  does not.
• Conclusion reference is more restricted, more
  safe than pointer. But Pointer is more powerful
  than reference.

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Pointer Arithmetic

• Operations of Pointers
• Add a number
• pp1 or p //assume p is a pointer
• Subtract a number
• pp-1 or p-- //assume p is a pointer
• Taken the difference of two pointers
• int dp1-p2 //assume p1, p2 are pointers

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The meaning of Pointer Arithmetic
q
p
p3
p-2
q-p5
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Another application of pointeraccess array by
using pointer
int A10
100
p
int p A0
(p4)100
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Other applications of Pointers

• Pointers of other data type
• Pointer of array, pointer of function, pointer of
  pointer.
• Dynamic memory allocation
• Heap based memory allocation.
• We skip these issues because time limit.

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End of lecture 23

• Thank you!