HSEARCH(3C-SVR4)    RISC/os Reference Manual     HSEARCH(3C-SVR4)



NAME
     hsearch, hcreate, hdestroy - manage hash search tables

SYNOPSIS
     #include <search.h>

     ENTRY *hsearch (ENTRY item, ACTION action);

     int hcreate (sizet nel);

     void hdestroy (void);

DESCRIPTION
     hsearch is a hash-table search routine generalized from
     Knuth (6.4) Algorithm D.  It returns a pointer into a hash
     table indicating the location at which an entry can be
     found.  The comparison function used by hsearch is strcmp
     [see string(3C)].  item is a structure of type ENTRY
     (defined in the search.h header file) containing two
     pointers:  item.key points to the comparison key, and
     item.data points to any other data to be associated with
     that key.  (Pointers to types other than void should be cast
     to pointer-to-void.)  action is a member of an enumeration
     type ACTION (defined in search.h) indicating the disposition
     of the entry if it cannot be found in the table.  ENTER
     indicates that the item should be inserted in the table at
     an appropriate point.  Given a duplicate of an existing
     item, the new item is not entered and hsearch returns a
     pointer to the existing item.  FIND indicates that no entry
     should be made.  Unsuccessful resolution is indicated by the
     return of a null pointer.

     hcreate allocates sufficient space for the table, and must
     be called before hsearch is used.  nel is an estimate of the
     maximum number of entries that the table will contain.  This
     number may be adjusted upward by the algorithm in order to
     obtain certain mathematically favorable circumstances.

     hdestroy destroys the search table, and may be followed by
     another call to hcreate.

EXAMPLE
     The following example will read in strings followed by two
     numbers and store them in a hash table, discarding dupli-
     cates.  It will then read in strings and find the matching
     entry in the hash table and print it out.

          #include <stdio.h>
          #include <search.h>
          #include <string.h>
          #include <stdlib.h>




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HSEARCH(3C-SVR4)    RISC/os Reference Manual     HSEARCH(3C-SVR4)



          struct info {       /* this is the info stored in table */
               int age, room; /* other than the key */
          };

          #define NUM_EMPL    5000    /* # of elements in search table */

          main( )
          {
               /* space to store strings */
               char string_space[NUM_EMPL*20];
               /* space to store employee info */
               struct info info_space[NUM_EMPL];
               /* next avail space in string_space */
               char *str_ptr = string_space;
               /* next avail space in info_space */
               struct info *info_ptr = info_space;
               ENTRY item, *found_item;
               /* name to look for in table */
               char name_to_find[30];
               int i = 0;

               /* create table */
               (void) hcreate(NUM_EMPL);
               while (scanf("%s%d%d", str_ptr, &info_ptr->age,
                      &info_ptr->room) != EOF && i++ < NUM_EMPL) {
                    /* put info in structure, and structure in item */
                    item.key = str_ptr;
                    item.data = (void *)info_ptr;
                    str_ptr += strlen(str_ptr) + 1;
                    info_ptr++;
                    /* put item into table */
                    (void) hsearch(item, ENTER);
               }

               /* access table */
               item.key = name_to_find;
               while (scanf("%s", item.key) != EOF) {
                   if ((found_item = hsearch(item, FIND)) != NULL) {
                    /* if item is in the table */
                    (void)printf("found %s, age = %d, room = %d\n",
                         found_item->key,
                         ((struct info *)found_item->data)->age,
                         ((struct info *)found_item->data)->room);
                   } else {
                    (void)printf("no such employee %s\n",
                         name_to_find)
                   }
               }
               return 0;
          }





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HSEARCH(3C-SVR4)    RISC/os Reference Manual     HSEARCH(3C-SVR4)



SEE ALSO
     bsearch(3C), lsearch(3C), malloc(3C), malloc(3X),
     string(3C), tsearch(3C).

DIAGNOSTICS
     hsearch returns a null pointer if either the action is FIND
     and the item could not be found or the action is ENTER and
     the table is full.

     hcreate returns zero if it cannot allocate sufficient space
     for the table.

NOTES
     hsearch and hcreate use malloc(3C) to allocate space.

     Only one hash search table may be active at any given time.







































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