Table of Contents

## Name

im_list_add, im_list_len, im_list_pos, im_list_member, im_list_append, im_list_remove, im_list_eq, im_list_map, im_list_map_rev, im_list_fold, im_list_fix, im_list_free, im_list_insert - linked list functions

## Synopsis

#include <vips/vips.h>
#include <vips/list.h>

typedef struct list_type {
`` `` struct list_type *next;
`` `` void *this;
} List;

#define hd(L) ((L)->this)
#define tl(L) ((L)->next)

typedef void *(*im_list_map_fn)( void *, void *, void * );
typedef void (*im_list_free_fn)( void *, void *, void * );
typedef void *(*im_list_fold_fn)( void *, void *,
`` `` void *, void * );

int im_list_len( List *l );
int im_list_pos( List *l, void *t );
int im_list_member( List *l, void *t );
void *im_list_index( List *l, int n );
int im_list_add( List **base, void *new );
int im_list_insert( List **base, void *new, void *old );
int im_list_append( List **base, void *new );
int im_list_remove( List **base, void *t );

void *im_list_eq( void *a, void *b );
void *im_list_map( List *l,
`` `` im_list_map_fn fn, void *a, void *b );
void *im_list_map_rev( List *l,
`` `` im_list_map_fn fn, void *a, void *b );
void *im_list_fold( List *l,
`` `` void *start, im_list_fold_fn fn, void *a, void *b );
void im_list_fix( List **base,
`` `` im_list_map_fn fn, void *a, void *b );
void im_list_free( List **base,
`` `` im_list_free_fn fn, void *a, void *b );

## Description

Manipulate linked lists in various ways. These functions are heavily used by the VIPS IO system; use them yourself if you like. VIPS lists store lists of void * pointers - use casts if you want to store some other type. Note that if sizeof( your object ) != sizeof( void * ), you will be in trouble!

All are based on the List type (see above). An empty list is a NULL pointer, a one element list is a pointer to a List struct, whose this field contains a pointer to the object in the list and whose next field is NULL. Macros hd(3) and tl(3) (head and tail) return this and next respectively.

im_list_len(3) returns the number of elements in list l. im_list_pos(3) searches list l for stored object t, returning an index. The first list element has index zero. im_list_pos(3) returns -1 for not present. im_list_index(3) returns the item at position n in the list, or NULL for index out of range. im_list_member(3) returns non-zero if the list contains the element.

im_list_map(3) applies a void * valued function to every element in a list, running from beginning to end. If the function returns NULL, im_list_map continues with the next element. If the function returns non-NULL, im_list_map(3) abandons the map and returns immediately, returning the value the user function returned. If the list is empty, im_list_map(3) returns NULL.

The two extra arguments a and b are carried around for you by VIPS and fed into each call of the function. They are useful for communicating context information.

You can use im_list_map to implement many kinds of list search/apply operation. VIPS supplies the function im_list_eq(3) which tests two void * pointers for equality, returning the pointer if they match, and returning NULL otherwise.

Example: search a list for an object

im_list_map( list,
(im_list_map_fn) im_list_eq, object, NULL );

This could also be written as

List *p;

for( p = list; p; p = tl( p ) )
if( object == hd( p ) )
break;

I prefer the first.

im_list_map_rev(3) behaves exactly as im_list_map(3) , but applies the function running from te end to the beginning of the list. It is much slower than im_list_map(3) and should be used only in emergencies.

im_list_fold(3) folds up a list with a dyadic function. If a list contains [1,2], return fn( 2, fn( 1, start, a, b ), a, b ). If the list is empty, return start.

The two extra arguments a and b are carried around for you by VIPS and fed into each call of the function. They are useful for communicating context information.

Example: find a pointer to the largest element in a list of ints (assume sizeof(int) <= sizeof(void *))

max_pair( int *new, int *old )
{
if( !old || *new > *old )
return( new );
else
return( old );
}

largest = im_list_fold( list,
NULL, (im_list_map_fn) max_pair, NULL, NULL );

im_list_add(3) adds a new element to the head of a list. Since the head of the list will move, you must pass in a *pointer* to your pointer to your old head.

Example: make a list of the numbers 9-0 (assume sizeof(int) <= sizeof(void *))

int i;
List *nlist = NULL;

for( i = 0; i < 10; i++ )
im_list_add( &nlist, (void *) i );

im_list_insert(3) adds a new element to a list, placing it just before the indicated old element. If the old element is not found, im_list_insert(3) returns an error.

im_list_append(3) appends a new element to the end of a list. This is much slower than im_list_add(3) , and should be avoided if possible.

im_list_remove(3) removes the specified element from the list. Since the head of the list may move, you must pass in a *pointer* to your pointer to your old head.

im_list_fix(3) finds the fixed-point of a list-altering function. It repeatedly maps a function over the list until the function returns NULL. Note that, since the list may be changing, you must pass in a *pointer* to the pointer you store the list in.

The two extra arguments a and b are carried around for you by VIPS and fed into each call of the function. They are useful for communicating context information.

Example: remove all elements less than x from a list of numbers (assume sizeof(int) <= sizeof(void *))

int *
test_ele( int *n, List **base, int x )
{
if( *n < x ) {
im_list_remove( base, n );
return( base );
}
else
return( NULL );
}

im_list_fix( &nlist,
(im_list_map_fn) test_ele, &nlist, x );

im_list_free(3) frees the list, applying a user free function to every element as it is freed. You may pass NULL instead of a pointer to a function, in which case im_list_free(3) will just free the memory used by the list nodes.

The two extra arguments a and b are carried around for you by VIPS and fed into each call of the function. They are useful for communicating context information.

## Return Value

The functions returns a 0 or a pointer on sucess, and non-zero or NULL on failure.

## See Also

im_rect_intersectrect(3) , etc.

## Copyright

National Gallery, 1992

## Author

J. Cupitt

Table of Contents

- Name
- Synopsis
- Description
- Return Value
- SeeAlso
- Copyright
- Author