1 / 9

The Hash Table Data Structure

The Hash Table Data Structure. Mugurel Ionu ț Andreica Spring 2012. Operations. put(key, value) Inserts the pair (key, value) in the hash table If a pair (key, value’) (with the same key) already exists, then value’ is replaced by value

zazu
Download Presentation

The Hash Table Data Structure

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. The Hash Table Data Structure Mugurel Ionuț Andreica Spring 2012

  2. Operations • put(key, value) • Inserts the pair (key, value) in the hash table • If a pair (key, value’) (with the same key) already exists, then value’ is replaced by value • We say that the value value is associated to the key key • get(key) • Returns the value associated to the key key • If no value is associated to key, then an error occurs • hasKey(key) • Returns 1 if the key key exists in the hash table, and 0 otherwise

  3. Example • put(3, 7.9) • put(2, 8.3) • get(3) -> returns 7.9 • put(3, 10.2) • get(3) -> returns 10.2 • get(2) -> returns 8.3 • hasKey(5) -> returns 0 • hasKey(2) -> returns 1 • get(5) -> generates an error

  4. Possible implementation • Maintain an array H[HMAX] of linked lists • The info field of each element of a list consists of a struct containing a key and a value • Each key is mapped to a value hkey=hash(key), such that 0≤hkey≤HMAX-1 • hash(key) is called the hash function • put(k, v) • Searches for the key k in the list H[hkey=hash(k)] • If the key is found, then we replace the value by v • If the key is not found, then we insert the pair (k,v) in H[hkey] • get(k) • Search for the key k in H[hkey=hash(k)] • If it finds the key, then it returns its associated value; otherwise, an error occurs • hasKey(k) • Search for the key k in H[hkey=hash(k)] • If it finds the key, then it returns 1; otherwise, it returns 0

  5. Possible implementation (cont.) • Class Hashtable • HMAX and hash => arguments in the constructor • The function hash will be passed as an argument (actually, a pointer to the function will be passed in fact) • Obviously, hash must be defined differently according to the data type of the keys (see later some examples for int and char*) • The array H: allocated dynamically in the constructor & deallocated in the destructor

  6. Hash Table – Implementation (hash_table.h) void put(Tkey key, Tvalue value) { struct list_elem<struct elem_info<Tkey, Tvalue> > *p; struct elem_info<Tkey, Tvalue> info; int hkey = hash(key); p = H[hkey].pfirst; while (p != NULL) { /* the == operator must be meaningful when comparing values of the type Tkey ; otherwise, an equality testing function should be passed as an argument to the constructor */ if (p->info.key == key) break; p = p->next; } if (p != NULL) p->info.value = value; else { info.key = key; info.value = value; H[hkey].addLast(info); } } #include "linked_list.h" template<typename Tkey, typename Tvalue> struct elem_info { Tkey key; Tvalue value; }; template<typename Tkey, typename Tvalue> class Hashtable { private: LinkedList<struct elem_info<Tkey, Tvalue> > *H; int HMAX; int (*hash) (Tkey); public: Hashtable(int hmax, int (*h) (Tkey)) { HMAX = hmax; hash = h; H = new LinkedList<struct elem_info<Tkey, Tvalue> > [HMAX]; } ~Hashtable() { for (int i = 0; i < HMAX; i++) { while (!H[i].isEmpty()) H[i].removeFirst(); } delete H; }

  7. Hash Table – Implementation (hash_table.h) (cont.) Tvalue get(Tkey key) { struct list_elem<struct elem_info<Tkey, Tvalue> > *p; int hkey = hash(key); p = H[hkey].pfirst; while (p != NULL) { if (p->info.key == key) break; p = p->next; } if (p != NULL) return p->info.value; else { fprintf(stderr, "Error 101 - The key does not exist in the hashtable\n"); Tvalue x; return x; } } int hasKey(Tkey key) { struct list_elem<struct elem_info<Tkey, Tvalue> > *p; int hkey = hash(key); p = H[hkey].pfirst; while (p != NULL) { if (p->info.key == key) break; p = p->next; } if (p != NULL) return 1; else return 0; } };

  8. Using the Hash Table - example char *k3 = "Abc"; char *k4 = "abcD"; int main() { hid.put(3, 7.9); hid.put(2, 8.3); printf("%.3lf\n", hid.get(3)); hid.put(3, 10.2); printf("%.3lf\n", hid.get(3)); printf("%.3lf\n", hid.get(2)); printf("%d\n", hid.hasKey(5)); printf("%d\n", hid.hasKey(2)); printf("%.3lf\n", hid.get(5)); hci.put(k1, 10); hci.put(k2, 20); printf("%d\n", hci.get(k1)); hci.put(k1, 30); printf("%d\n", hci.get(k1)); printf("%d\n", hci.get(k2)); printf("%d\n", hci.hasKey(k3)); printf("%d\n", hci.hasKey(k2)); printf("%d\n", hci.get(k4)); char *k5 = new char[4]; k5[0] = ‘a’; k5[1] = ‘b’; k5[2] = ‘c’; k5[3] = 0; printf("%d\n", hci.get(k5)); // what happens ? return 0; } #include <stdio.h> #include <string.h> #include “hash_table.h” #define VMAX 17 #define P 13 int hfunc(int key) { return (P * key) % VMAX; } Hashtable<int, double> hid(VMAX, hfunc); int hfunc2(char* key) { int hkey = 0; for (int i = 0; i < strlen(key); i++) hkey = (hkey * P + key[i]) % VMAX; return hkey; } Hashtable<char*, int> hci(VMAX, hfunc2); char *k1 = "abc"; char *k2 = "xyze";

  9. Final remarks • The Hash table is a fundamental data structure in many situations • Packet routing in the Internet • Session management in web servers • Web search (e.g. Google) • etc. • Many other implementations exist, besides using an array of linked lists • For example: Linear probing, Cuckoo hashing, etc. • Many of them are more efficient, but more difficult to understand

More Related