Last Updated : 23 Jul, 2025
Introduction:Hashing is a technique that maps a large set of data to a small set of data. It uses a hash function for doing this mapping. It is an irreversible process and we cannot find the original value of the key from its hashed value because we are trying to map a large set of data into a small set of data, which may cause collisions. It is not uncommon to encounter collisions when mapping a large dataset into a smaller one. Suppose, We have three buckets and each bucket can store 1L of water in it and we have 5L of water also. We have to put all the water in these three buckets and this kind of situation is known as a collision. URL shorteners are an example of hashing as it maps large size URL to small size
Some Examples of Hash Functions:The value returned by the Hash function is the bucket index for a key in a separate chaining method. Each index in the array is called a bucket as it is a bucket of a linked list.
Rehashing:Rehashing is a concept that reduces collision when the elements are increased in the current hash table. It will make a new array of doubled size and copy the previous array elements to it and it is like the internal working of vector in C++. Obviously, the Hash function should be dynamic as it should reflect some changes when the capacity is increased. The hash function includes the capacity of the hash table in it, therefore, While copying key values from the previous array hash function gives different bucket indexes as it is dependent on the capacity (buckets) of the hash table. Generally, When the value of the load factor is greater than 0.5 rehashings are done.
Collision is the situation when the bucket index is not empty. It means that a linked list head is present at that bucket index. We have two or more values that map to the same bucket index.
Major Functions in our ProgramImplementation without Rehashing:
C
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// Linked List node
struct node {
// key is string
char* key;
// value is also string
char* value;
struct node* next;
};
// like constructor
void setNode(struct node* node, char* key, char* value)
{
node->key = key;
node->value = value;
node->next = NULL;
return;
};
struct hashMap {
// Current number of elements in hashMap
// and capacity of hashMap
int numOfElements, capacity;
// hold base address array of linked list
struct node** arr;
};
// like constructor
void initializeHashMap(struct hashMap* mp)
{
// Default capacity in this case
mp->capacity = 100;
mp->numOfElements = 0;
// array of size = 1
mp->arr = (struct node**)malloc(sizeof(struct node*)
* mp->capacity);
return;
}
int hashFunction(struct hashMap* mp, char* key)
{
int bucketIndex;
int sum = 0, factor = 31;
for (int i = 0; i < strlen(key); i++) {
// sum = sum + (ascii value of
// char * (primeNumber ^ x))...
// where x = 1, 2, 3....n
sum = ((sum % mp->capacity)
+ (((int)key[i]) * factor) % mp->capacity)
% mp->capacity;
// factor = factor * prime
// number....(prime
// number) ^ x
factor = ((factor % __INT16_MAX__)
* (31 % __INT16_MAX__))
% __INT16_MAX__;
}
bucketIndex = sum;
return bucketIndex;
}
void insert(struct hashMap* mp, char* key, char* value)
{
// Getting bucket index for the given
// key - value pair
int bucketIndex = hashFunction(mp, key);
struct node* newNode = (struct node*)malloc(
// Creating a new node
sizeof(struct node));
// Setting value of node
setNode(newNode, key, value);
// Bucket index is empty....no collision
if (mp->arr[bucketIndex] == NULL) {
mp->arr[bucketIndex] = newNode;
}
// Collision
else {
// Adding newNode at the head of
// linked list which is present
// at bucket index....insertion at
// head in linked list
newNode->next = mp->arr[bucketIndex];
mp->arr[bucketIndex] = newNode;
}
return;
}
void delete (struct hashMap* mp, char* key)
{
// Getting bucket index for the
// given key
int bucketIndex = hashFunction(mp, key);
struct node* prevNode = NULL;
// Points to the head of
// linked list present at
// bucket index
struct node* currNode = mp->arr[bucketIndex];
while (currNode != NULL) {
// Key is matched at delete this
// node from linked list
if (strcmp(key, currNode->key) == 0) {
// Head node
// deletion
if (currNode == mp->arr[bucketIndex]) {
mp->arr[bucketIndex] = currNode->next;
}
// Last node or middle node
else {
prevNode->next = currNode->next;
}
free(currNode);
break;
}
prevNode = currNode;
currNode = currNode->next;
}
return;
}
char* search(struct hashMap* mp, char* key)
{
// Getting the bucket index
// for the given key
int bucketIndex = hashFunction(mp, key);
// Head of the linked list
// present at bucket index
struct node* bucketHead = mp->arr[bucketIndex];
while (bucketHead != NULL) {
// Key is found in the hashMap
if (bucketHead->key == key) {
return bucketHead->value;
}
bucketHead = bucketHead->next;
}
// If no key found in the hashMap
// equal to the given key
char* errorMssg = (char*)malloc(sizeof(char) * 25);
errorMssg = "Oops! No data found.\n";
return errorMssg;
}
// Drivers code
int main()
{
// Initialize the value of mp
struct hashMap* mp
= (struct hashMap*)malloc(sizeof(struct hashMap));
initializeHashMap(mp);
insert(mp, "Yogaholic", "Anjali");
insert(mp, "pluto14", "Vartika");
insert(mp, "elite_Programmer", "Manish");
insert(mp, "GFG", "GeeksforGeeks");
insert(mp, "decentBoy", "Mayank");
printf("%s\n", search(mp, "elite_Programmer"));
printf("%s\n", search(mp, "Yogaholic"));
printf("%s\n", search(mp, "pluto14"));
printf("%s\n", search(mp, "decentBoy"));
printf("%s\n", search(mp, "GFG"));
// Key is not inserted
printf("%s\n", search(mp, "randomKey"));
printf("\nAfter deletion : \n");
// Deletion of key
delete (mp, "decentBoy");
printf("%s\n", search(mp, "decentBoy"));
return 0;
}
C++
#include <iostream>
#include <cstring>
// Linked List node
struct node {
// key is string
char* key;
// value is also string
char* value;
struct node* next;
};
// like constructor
void setNode(struct node* node, char* key, char* value) {
node->key = key;
node->value = value;
node->next = NULL;
return;
}
struct hashMap {
// Current number of elements in hashMap
// and capacity of hashMap
int numOfElements, capacity;
// hold base address array of linked list
struct node** arr;
};
// like constructor
void initializeHashMap(struct hashMap* mp) {
// Default capacity in this case
mp->capacity = 100;
mp->numOfElements = 0;
// array of size = 1
mp->arr = (struct node**)malloc(sizeof(struct node*) * mp->capacity);
return;
}
int hashFunction(struct hashMap* mp, char* key) {
int bucketIndex;
int sum = 0, factor = 31;
for (int i = 0; i < strlen(key); i++) {
// sum = sum + (ascii value of
// char * (primeNumber ^ x))...
// where x = 1, 2, 3....n
sum = ((sum % mp->capacity) + (((int)key[i]) * factor) % mp->capacity) % mp->capacity;
// factor = factor * prime
// number....(prime
// number) ^ x
factor = ((factor % __INT16_MAX__) * (31 % __INT16_MAX__)) % __INT16_MAX__;
}
bucketIndex = sum;
return bucketIndex;
}
void insert(struct hashMap* mp, char* key, char* value) {
// Getting bucket index for the given
// key - value pair
int bucketIndex = hashFunction(mp, key);
struct node* newNode = (struct node*)malloc(
// Creating a new node
sizeof(struct node));
// Setting value of node
setNode(newNode, key, value);
// Bucket index is empty....no collision
if (mp->arr[bucketIndex] == NULL) {
mp->arr[bucketIndex] = newNode;
}
// Collision
else {
// Adding newNode at the head of
// linked list which is present
// at bucket index....insertion at
// head in linked list
newNode->next = mp->arr[bucketIndex];
mp->arr[bucketIndex] = newNode;
}
return;
}
void deleteKey(struct hashMap* mp, char* key) {
// Getting bucket index for the
// given key
int bucketIndex = hashFunction(mp, key);
struct node* prevNode = NULL;
// Points to the head of
// linked list present at
// bucket index
struct node* currNode = mp->arr[bucketIndex];
while (currNode != NULL) {
// Key is matched at delete this
// node from linked list
if (strcmp(key, currNode->key) == 0) {
// Head node
// deletion
if (currNode == mp->arr[bucketIndex]) {
mp->arr[bucketIndex] = currNode->next;
}
// Last node or middle node
else {
prevNode->next = currNode->next;
}
free(currNode);
break;
}
prevNode = currNode;
currNode = currNode->next;
}
return;
}
char* search(struct hashMap* mp, char* key) {
// Getting the bucket index for the given key
int bucketIndex = hashFunction(mp, key);
// Head of the linked list present at bucket index
struct node* bucketHead = mp->arr[bucketIndex];
while (bucketHead != NULL) {
// Key is found in the hashMap
if (strcmp(bucketHead->key, key) == 0) {
return bucketHead->value;
}
bucketHead = bucketHead->next;
}
// If no key found in the hashMap equal to the given key
char* errorMssg = (char*)malloc(sizeof(char) * 25);
strcpy(errorMssg, "Oops! No data found.\n");
return errorMssg;
}
// Drivers code
int main()
{
// Initialize the value of mp
struct hashMap* mp = (struct hashMap*)malloc(sizeof(struct hashMap));
initializeHashMap(mp);
insert(mp, "Yogaholic", "Anjali");
insert(mp, "pluto14", "Vartika");
insert(mp, "elite_Programmer", "Manish");
insert(mp, "GFG", "GeeksforGeeks");
insert(mp, "decentBoy", "Mayank");
printf("%s\n", search(mp, "elite_Programmer"));
printf("%s\n", search(mp, "Yogaholic"));
printf("%s\n", search(mp, "pluto14"));
printf("%s\n", search(mp, "decentBoy"));
printf("%s\n", search(mp, "GFG"));
// Key is not inserted
printf("%s\n", search(mp, "randomKey"));
printf("\nAfter deletion : \n");
// Deletion of key
deleteKey(mp, "decentBoy");
// Searching the deleted key
printf("%s\n", search(mp, "decentBoy"));
return 0;
}
Manish Anjali Vartika Mayank GeeksforGeeks Oops! No data found. After deletion : Oops! No data found.Explanation:
The time complexity of hash table insertion and deletion operations is O(1) on average. There is some mathematical calculation that proves it.
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