CS 2130

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CS 2130

• Presentation 13
• Void Pointers
• Function Pointers

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ReminderACM is providing free tutoring

• http//tutor.gtacm.org

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Void Pointers

• We talked about void pointers in an earlier
  lecture
• We stressed some things not to do
• We also mentioned that they had useful purposes
• For example
• malloc returns a void
• It is just returning a pointer to some newly
  allocated space
• The space has no associated type
• The programmer must make that association by
  assigning the returned value into the appropriate
  type pointer
• Since this type isn't known to the author of
  malloc it returns the generic void

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Another Example

• One of the problems with the standard swap( )
  function is that it is "hard-wired" to a
  particular type (i.e. we must know ahead of time
  that we are swapping two ints, or two floats,
  etc).
• Write a swap( ) function that will swap two items
  of arbitrary size. Your function will need to
  accept a third parameter, which is the sizeof the
  two items to be swapped. Do not use any library
  calls in your solution.

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• / Is this okay???
• 1 Yes
• 2 No /
• void swap(void a, void b, size_t size)
• int i
• for(i0 iltsize i)
• char t ai
• ai bi
• bi t

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• void swap(void a, void b, size_t size)
• char ap a
• char bp b
• int i
• for(i0 iltsize i)
• char t api
• api bpi
• bpi t

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• int a 42
• int b 78
• double d1 56.90
• double d2 392.4567
• swap(a, b, sizeof(int))
• swap(d1, d2, sizeof(double))
• Node n1
• Node n2
• / assume we initialize the nodes /
• swap(n1, n2, sizeof(Node))

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Questions?
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Function Pointers
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What is...

• A variable?
• A symbol representing an address where we store
  something
• An array variable?
• A symbol representing an address where a block of
  memory has been reserved
• A function name?
• A symbol representing the address of a piece of
  code which we can jump to (and we normally expect
  it will contain code that will return control to
  the caller.)
• Assembly language for int fact(int n)
• fact save sp, -112, sp Beginning of
• fact function

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If we...

• Want to store the address of a variable we use a
  pointer (e.g. int ip)
• Same is true for holding the address of a
  function
• int fi(void) / Function that returns an int
  /
• int fpi(void) / Function that returns a
  pointer
• to an int
  /
• (int )fpi(void) / What compiler sees (sort
  of!) /
• int (pfi)(void) / Declaring pfi to be a
  pointer to
• a function!
  /

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Using it...

• int fi(void) / Function that returns an int
  /
• int fpi(void) / Function that returns a
  /
• / pointer to an int
  /
• int (pfi)(void) / Declaring pfi to be a
  /
• pointer to a function!
  /
• pfi fi / Legal assignment
  /
• pfi fi() / NO NO NO NO NO NO NO NO NO
  NO /
• Notice similarity to
• int ia10
• int ip
• ip ia

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Using it (Part Deux)...

• i fi()
• i pfi() ALL THE SAME!!!
• i pfi()

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But what good is a function pointer?

• Say you are writing a general purpose sorting
  function.
• You want it to be able to sort anything
• Numbers
• Strings
• ???
• Obviously comparing numbers and strings calls for
  two different techniques
• What if we write functions that do the comparison
  we need
• A function to compare numbers
• A function to compare strings
• A function to compare ???

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But what good is a function pointer?

• Now when we call the function to do the sorting
  we pass in a pointer to the appropriate function
  for the type of data we have!
• "But wait," I hear you say, "It would be easier
  to write my own sorting function!"

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• qsort(3)
  qsort(3)
• NAME
• qsort - sort an array
• ANSI_SYNOPSIS
• include ltstdlib.hgt
• void qsort(void base, size_t nmemb,
  size_t size,
• int ( compar)(const void , const void )
  )
• TRAD_SYNOPSIS
• include ltstdlib.hgt
• qsort( base, nmemb, size, compar )
• char base
• size_t nmemb
• size_t size
• int ( compar)()

nmemb
base
size
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• qsort(3)
  qsort(3)
• DESCRIPTION
• qsort sorts an array (beginning at base)
  of nmemb objects.
• size describes the size of each element of
  the array.
• You must supply a pointer to a comparison
  function, using
• the argument shown as compar. (This
  permits sorting
• objects of unknown properties.) Define
  the comparison
• function to accept two arguments, each
  a pointer to an
• element of the array starting at base.
  The result of
• (ltcompar)gtgt must be negative if the
  first argument is
• less than the second, zero if the two
  arguments match, and
• positive if the first argument is greater
  than the second
• (where less than'' and greater than''
  refer to what-
• ever arbitrary ordering is appropriate).
• The array is sorted in place that is,
  when qsort returns,
• the array elements beginning at base have
  been reordered.

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• qsort(3)
  qsort(3)
• RETURNS
• qsort does not return a result.
• PORTABILITY
• qsort is required by ANSI (without
  specifying the sorting
• algorithm).
• SOURCE
• src/newlib/libc/stdlib/qsort.c

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QSort Demo

• include ltstdlib.hgt
• void qsort (
• void base,
• size_t nmemb,
• size_t size,
• int ( compar)(const void , const void )
  
• )
• The result of "compar" must be negative if the
  first argument is less than the second, zero if
  the two arguments match, and positive if the
  first argument is greater than the second (where
  "less than" and "greater than" refer to whatever
  arbitrary ordering is appropriate).

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• include ltstdio.hgt
• include ltstdlib.hgt
• define MAX 100
• int compar_ints(const void pa, const void pb)
• return ((int )pa) - ((int )pb)
• int compar_strings(const void ppa, const void
  ppb)
• return strcmp( ((char )ppa) , ((char )ppb)
  )

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• define MAX 100
• int main(int argc, char argv)
• char strings "dec", "sun", "ibm", "apple",
  "hp", "ti", "univac"
• int i, s
• int aMAX
• if(argc 2 (argv1) 'a')
• s sizeof(strings)/sizeof(strings0)
• qsort(strings, s, sizeof(strings0),
• compar_strings)
• for(i 0 i lt s i)
• printf(" s", stringsi)
• printf("\n")
• else...

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• else
• for(i 0 i lt MAX i)
• ai rand() 100
• printf(" d", ai)
• printf("\n\n")
• qsort(a, MAX, sizeof(int),
• compar_ints)
• for(i 0 i lt MAX i)
• printf(" d", ai)
• return 0

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Questions?
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But what good is a function pointer?

• Okay, so sorting is one application. How about
  another???
• Imagine we are writing a routine to traverse a
  data structure
• What exactly do we do at each node?
• print it?
• modify it?
• test it?

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But what good is a function pointer?

• What if we passed into our traversal routine a
  pointer to a function which did what needed to be
  done???
• Advantage No need to continually modify our
  traversal routine. It's now generic!

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Basic Treenode with Root Pointer

• struct treenode
• char itemName
• int itemCount
• struct treenode left, right
• struct treenode root NULL
• int totalItems

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Post Order Traversal

• void
• postOrder( struct treenode theNode,
• void ( doThis )( struct treenode ) )
• if( theNode NULL )
• return
• postOrder( theNode-gtleft, doThis )
• postOrder( theNode-gtright, doThis )
• doThis( theNode )

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Closer Look

• void
• postOrder( struct treenode theNode,
• void ( doThis )( struct treenode ) )
• Note This function "postOrder" take two
  parameters
• A pointer to a treenode
• A pointer to a function

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Closer Look

• void
• postOrder( struct treenode theNode,
• void ( doThis )( struct treenode ) )

Function Pointer
Parameter list for function
Return value of function
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Counting Function

• void
• countItems( struct treenode theNode )
• totalItems theNode-gtitemCount

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Printing Function

• void
• printItems( struct treenode theNode )
• printf( "s d\n", theNode-gtitemName,
  theNode-gtitemCount )

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Less than 5 items Function

• void
• checkItems( struct treenode theNode )
• if( theNode-gtitemCount lt 5 )
• printf( "s\n", theNode-gtitemName )

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Clear Items Function

• void
• clearItems( struct treenode theNode )
• theNode-gtitemCount 0

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Free Nodes Function

• void
• freeNodes( struct treenode theNode )
• free( theNode-gtleft )
• free( theNode-gtright )
• free( theNode-gtitemName )

void postOrder( struct treenode theNode, void (
doThis )( struct treenode ) ) if(
theNode NULL ) return postOrder(
theNode-gtleft, doThis ) postOrder(
theNode-gtright, doThis ) doThis( theNode
)
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Main Function

• int
• main( void )
• root buildTree( )
• totalItems 0 / Global /
• postOrder( root, countItems )
• printf( "There are d items.\n",
• totalItems )

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Main Function

• printf( "List of all the items and"
• " how many there are\n" )
• postOrder( root, printItems )
• printf( "The following items are in"
• " short supply\n" )
• postOrder( root, checkItems )
• / clear itemCount for all items /
• postOrder( root, clearItems )

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Main Function

• / free all nodes /
• postOrder( root, freeNodes )
• free(root)
• return EXIT_SUCCESS

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NOTE

• Function Header
• void
• postOrder( struct treenode theNode,
• void ( doThis )( struct treenode ) )
• Function Call
• doThis( theNode )
• Function Header
• void
• countItems( struct treenode )
• Function Call
• postOrder( root, countItems )

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What's the difference?

• LLNode (llnpfp) (LLNode , int )
• typedef LLNode (llnfp_t) (LLNode , int )

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Function that returns a function pointer

• float (GetPtr1(const char opCode))(float, float)
• The function name and parameters
• float (GetPtr1(const char opCode))(float, float)
• The return value for the function
• float (GetPtr1(const char opCode))(float, float)
• float (GetPtr1(const char opCode))(float, float)
• if(opCode '') return add
• if(opCode '-') return sub
• else return mult

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Additional Information

• http//www.function-pointer.org/
• Don't let the C stuff confuse you!

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Questions?
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