1. Data Structures and Algorithms
Functions as Data Types
PLSD210

2. Quicksort - library implementation
POSIX standard
void qsort( void *base, size_t n, size_t size, int (*compar)( const void *, const void * ) );
base address of array n number of elements size size of an element compar comparison function

3. Quicksort - library implementation
POSIX standard
Comparison function
C allows you to pass a function to another function!
void qsort( void *base, size_t n, size_t size, int (*compar)( const void *, const void * ) );
base address of array n number of elements size size of an element compar comparison function

4. Functions as data types - declaration
int (*compar)( const void *, const void * );
Parentheses around the * and the name
Distinguish this as a pointer to a function
Contrast with ...
int *compar( const void *, const void * );
This is a function returning an int *

5. Functions as data types
Declaration
So this declares a pointer to a function named compar that takes two void * arguments and returns an int
int (*compar)( const void *, const void * );
The function returns an int
and has two arguments, both void *

6. Functions as data types
Declaration
The compare function passed to qsort returns
*arg1 < *arg2
*arg1 == *arg2
*arg1 > *arg2
int (*compar)( const void *, const void * );
The function returns an int
and has two arguments, both void *
Interpret as defining order -
not magnitude

7. Functions as data types
Use
Library functions that need a special purpose function
Sorting needs a concept of order
compar function provides it
Ordering rule could be quite complex for a complex record
eg names in a telephone directory
Searching also needs an order
Notes on Web and in C texts - omitted from printed copy

8. Functions as data types
Generalising our collection class
We used pointers to objects
Enabling one collection class to store any type of object
All C pointers are the same!
but we had to assume external
itemkey or itemcmp functions
Restricting us to one collection per program!
Add the ordering function to the attributes
Ordering rule stored with each collection object
As many collections as we like!

9. Generalised Collection Class
Redefine the constructor
Add an element to the attributes
Collection ConsCollection( int max_items,
int (*compar)(const void *,const void * ) );
struct t_collection { int max_items;
int cnt; int (*compar)( const void *, const void * );
void *data; };
Assumes array - make appropriate
changes for list, tree, etc.

10. Generalised Collection Class
In the constructor code, treat the function just like any other data item:
Collection ConsCollection( int max_items,
int (*compar)(const void *,const void * ) ) { Collection c; c = malloc( sizeof( struct collection ) ); if ( c != NULL ) { c->max_items = max_items; c->compar = compar; …… }
return c;
compar is a pointer
to a function -
usually the address
of the beginning of the
function’s code
Copy the pointer (it’s an address) just like any other data value!

11. Using a pointer to a function
Use the attribute as the name of a function:
void *FindInCollection( void *key ) { int k; for(k=0;k<c->cnt;k++) { if( *c->compar( key, c->data[k] ) == 0 ) return c->data[k]; }
return NULL;
… and supply it with arguments, just like any other function!
c->compar is a pointer to a function -
de-reference to access the function

12. Using a pointer to a function
Use the attribute as the name of a function:
void *FindInCollection( void *key ) { int k; for(k=0;k<c->cnt;k++) { if( *c->compar( key, c->data[k] ) == 0 ) return c->data[k]; }
return NULL;
… but the compiler will be kind and understand if you forget this asterisk!
c->compar is a pointer to a function -
de-reference to access the function

13. Functions as Data - Other uses
Table-driven code
eg menus
Each entry
String
Action to be performed if the entry is selected
struct menu { char *desc; int (*action)( void ); } file_menu[ ] = {
{ “Save”, save_function }, { “Save as”, save_as_function }, … };
Pointer to a function
… which takes no arguments

14. Functions as Data - Other uses
Table-driven code
eg menus
Each entry
String
Action to be performed if the entry is selected
struct menu { char *desc; int (*action)( void ); } file_menu[ ] = {
{ “Save”, save_function }, { “Save as”, save_as_function }, … };
Functions which are called when the item is selected
Strings which appear in the menu

15. Functions as Data - Other uses
Using the table
struct menu { char *desc; int (*action)( void ); } file_menu[ ] = {
{ “Save”, save_function }, { “Save as”, save_as_function }, … };
void file_menu_callback() { int k = item_selected( file_menu ); file_menu[k].action(); }
Find which item was
selected
Call the action function