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Understanding and implementing a Hash Table (in C)

Understanding and implementing a Hash Table (in C)

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data structures – Implementing a HashMap in C – Stack Overflow

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• Summary of article content: Articles about data structures – Implementing a HashMap in C – Stack Overflow When you access the hash table with a key, you process the key with a custom hash function which will return an integer. You then take the modulus of the result … …
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data structures – Implementing a HashMap in C – Stack Overflow

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• Summary of article content: Articles about data structures – Implementing a HashMap in C – Stack Overflow First, we need to define the structure of hashmap. We record the capacity and length of hashmap. The capacity of hashmap means the length of the list. The … …
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Hash Table Program in C

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• Summary of article content: Articles about Hash Table Program in C Hash Table Program in C … Hash Table is a data structure which stores data in an associative manner. In hash table, the data is stored in an array format where … …
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Implementation in C

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Hash Table Program in C

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• Summary of article content: Articles about Hash Table Program in C hashmap c ; 1. #include ; 2. #include ; 3. struct node ; 4. { ; 5. int key;. …
• Most searched keywords: Whether you are looking for Hash Table Program in C hashmap c ; 1. #include ; 2. #include ; 3. struct node ; 4. { ; 5. int key;. Hash Table Program in C, Hash Table is a data structure which stores data in an associative manner. In hash table, the data is stored in an array format where each data value has its ow
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Implementation in C

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performance – Simple Hashmap in C – Code Review Stack Exchange

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• Summary of article content: Articles about performance – Simple Hashmap in C – Code Review Stack Exchange Simple Hashmap in C ; include ; include ; include “hashmap.h” ; vo* key) ; vo* key1, vo* key2) … …
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HashMap trong java – Học lập trình Java online – VietTuts

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• Summary of article content: Articles about HashMap trong java – Học lập trình Java online – VietTuts Key và value là kiểu dữ liệu bất kỳ, và bạn có thể try cập các giá trị của HasMap bằng … …
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Java Collections

Lớp HashMap trong Java

Hierarchy của lớp HashMap trong Java

Khai báo của lớp HashMap trong Java

Constructor của lớp HashMap trong Java

Phương thức của lớp HashMap trong Java

Khởi tạo một HashMap

Duyệt và hiển thị các phần tử có trong HashMap

Truy cập phần tử của HashMap

Cập nhật giá trị của phần tử HashMap

Xóa phần tử HashMap

Tạo HashMap có kiểu generic là đối tượng do người dùng định nghĩa

Sự khác nhau giữa HashSet và HashMap

HashMap trong Java hoạt động như thế nào

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hashmap.c

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• Summary of article content: Articles about hashmap.c #include “cache.h” #include “hashmap.h” #define FNV32_BASE ((unsigned int) … return hash; } unsigned int strihash(const char *str) { unsigned int c, … …
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Tìm hiểu HashMap trong Java qua ví dụ cụ thể

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  Tìm hiểu HashMap trong Java qua ví dụ cụ thể
  Hướng dẫn tìm hiểu về Hashmap thông qua ví dụ, các tạo hashmap, các xóa key, value khỏi Hashmap, cách tìm tất cả các key, value và cách tham chiếu đến giá … …
• Most searched keywords: Whether you are looking for
  Tìm hiểu HashMap trong Java qua ví dụ cụ thể
  Hướng dẫn tìm hiểu về Hashmap thông qua ví dụ, các tạo hashmap, các xóa key, value khỏi Hashmap, cách tìm tất cả các key, value và cách tham chiếu đến giá … Tìm hiểu HashMap trong Java qua ví dụ, hashmap trong java, java hashmapHướng dẫn tìm hiểu về Hashmap thông qua ví dụ, các tạo hashmap, các xóa key, value khỏi Hashmap, cách tìm tất cả các key, value và cách tham chiếu đến giá trị cụ thể thông qua key.
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1 HashMap trong Java là gì

2 Tính năng của HashMap

3 Cách tạo một Hashmap trong Java

3 Ví dụ về Hashmap trong Java

4 Ví dụ cách xóa value khỏi Hashmap thông qua key

5 Một vài câu hỏi về Hashmap giúp bạn hiểu rõ chúng hơn

Chúc mừng bạn đã hiểu sâu hơn về Hashmap trong Java

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Implementing a HashMap in C

Well if you know the basics behind them, it shouldn’t be too hard.

Generally you create an array called “buckets” that contain the key and value, with an optional pointer to create a linked list.

When you access the hash table with a key, you process the key with a custom hash function which will return an integer. You then take the modulus of the result and that is the location of your array index or “bucket”. Then you check the unhashed key with the stored key, and if it matches, then you found the right place.

Otherwise, you’ve had a “collision” and must crawl through the linked list and compare keys until you match. (note some implementations use a binary tree instead of linked list for collisions).

Check out this fast hash table implementation:

https://attractivechaos.wordpress.com/2009/09/29/khash-h/

Hash Table Program in C

Hash Table Program in C

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Hash Table is a data structure which stores data in an associative manner. In hash table, the data is stored in an array format where each data value has its own unique index value. Access of data becomes very fast, if we know the index of the desired data.

Implementation in C

#include #include #include #include #define SIZE 20 struct DataItem { int data; int key; }; struct DataItem* hashArray[SIZE]; struct DataItem* dummyItem; struct DataItem* item; int hashCode(int key) { return key % SIZE; } struct DataItem *search(int key) { //get the hash int hashIndex = hashCode(key); //move in array until an empty while(hashArray[hashIndex] != NULL) { if(hashArray[hashIndex]->key == key) return hashArray[hashIndex]; //go to next cell ++hashIndex; //wrap around the table hashIndex %= SIZE; } return NULL; } void insert(int key,int data) { struct DataItem *item = (struct DataItem*) malloc(sizeof(struct DataItem)); item->data = data; item->key = key; //get the hash int hashIndex = hashCode(key); //move in array until an empty or deleted cell while(hashArray[hashIndex] != NULL && hashArray[hashIndex]->key != -1) { //go to next cell ++hashIndex; //wrap around the table hashIndex %= SIZE; } hashArray[hashIndex] = item; } struct DataItem* delete(struct DataItem* item) { int key = item->key; //get the hash int hashIndex = hashCode(key); //move in array until an empty while(hashArray[hashIndex] != NULL) { if(hashArray[hashIndex]->key == key) { struct DataItem* temp = hashArray[hashIndex]; //assign a dummy item at deleted position hashArray[hashIndex] = dummyItem; return temp; } //go to next cell ++hashIndex; //wrap around the table hashIndex %= SIZE; } return NULL; } void display() { int i = 0; for(i = 0; ikey,hashArray[i]->data); else printf(” ~~ “); } printf(”

“); } int main() { dummyItem = (struct DataItem*) malloc(sizeof(struct DataItem)); dummyItem->data = -1; dummyItem->key = -1; insert(1, 20); insert(2, 70); insert(42, 80); insert(4, 25); insert(12, 44); insert(14, 32); insert(17, 11); insert(13, 78); insert(37, 97); display(); item = search(37); if(item != NULL) { printf(“Element found: %d

“, item->data); } else { printf(“Element not found

“); } delete(item); item = search(37); if(item != NULL) { printf(“Element found: %d

“, item->data); } else { printf(“Element not found

“); } }

If we compile and run the above program, it will produce the following result −

Output

~~ (1,20) (2,70) (42,80) (4,25) ~~ ~~ ~~ ~~ ~~ ~~ ~~ (12,44) (13,78) (14,32) ~~ ~~ (17,11) (37,97) ~~ Element found: 97 Element not found

hash_data_structure.htm

Simple Hashmap in C

\$\begingroup\$

Urgs, the first impression is already bad:

#ifndef __HASHMAP_H__

Every identifier that starts with a double underscore is reserved for the implementation of the C compilation environment (compiler + operating system). Since you are not working on the implementation but are rather an application developer, you must not define these identifiers. Instead, use the commonly accepted pattern of PROJECT_FILE_H , with PROJECT and FILE being placeholders.

#include #include

It is common to sort the headers alphabetically, at least those from the C standard library. Both stdlib.h and stdbool.h come from the standard library.

typedef size_t (*HashFunction)(void*); typedef bool (*CompareFunction)(void*, void*);

These typedefs are almost reasonable. Since these functions are not supposed to modify their arguments, you should replace void * with const void * .

struct Pair { size_t hash_id; void* key; void* value; struct Pair* next; };

The name Pair is wrong for this struct, it should rather be called HashMapEntry . A pair typically has 2 fields, not 4, and these fields are called first and second .

struct HashMap { struct Pair** buckets; size_t num_buckets; HashFunction hfunc; CompareFunction cfunc; };

To quickly see if the map is empty (if you need it), you could add a size_t size field.

struct HashMap* new_hashmap(HashFunction, CompareFunction); struct HashMap* new_hashmap_c(HashFunction, CompareFunction, size_t);

All these function declarations look good. To get rid of the struct keyword, you should typedef struct HashMap HashMap; .

Next, the implementation.

#include #include “hashmap.h”

Looks great. Putting assertions all over the code is good style. That is much better than comments of the kind “I expect that this variable is never NULL”.

#define DEFAULT_BUCKETS 750000

Whoa, that’s a high number. Other HashMap implementations typically use a default bucket count of 16.

struct HashMap* new_hashmap_c(HashFunction hfunc, CompareFunction cfunc, size_t buckets) { struct HashMap* hmap; hmap = malloc(sizeof(*hmap)); assert(hmap);

This assertion is dangerous since it goes away when you compile the code with the preprocessor flag -DNDEBUG . You must either ensure that your code is always compiled with assertions enabled, or if (hmap == NULL) enomem() .

hmap->buckets = malloc(buckets * sizeof(*hmap->buckets));

This multiplication might overflow if the number of buckets is really high. This will probably not happen though. If your HashMap has to cope with untrusted input, you risk a buffer overflow and a security vulnerable, allowing every attacker to run arbitrary code on your computer.

The rest of new_hashmap_c looks fine, except for the assert .

void free_hashmap(struct HashMap* hmap) { free(hmap->buckets); free(hmap); hmap = NULL; }

There is no point in setting hmap = NULL at the end. This will only influence the local variable hmap inside the function free_hashmap . If the calling function has a variable, that variable will not be influenced at all and still point to the freed memory. That’s ok because the caller is not supposed to do anything with that variable anymore.

void insert_hashmap(struct HashMap* hmap, void* key, void* value) { size_t hashed_key = hmap->hfunc(key); struct Pair* prev = NULL; struct Pair* entry = hmap->buckets[hashed_key];

That’s bad design. Your HashFunction is supposed to return a number in the range [0, hmap->num_buckets) , but the field num_buckets should not be accessed by any code outside your own implementation. Therefore the insert_hashmap should ensure for itself that the return value of the hash function is in the correct range, using this simple one-line change:

struct Pair* entry = hmap->buckets[hashed_key % hmap->num_buckets];

The rest of insert_hashmap looks fine, except for the missing null check after malloc , as explained above.

The function get_hashmap looks perfect.

The function remove_hashmap looks perfect.

Next and last: the test code.

size_t hash(void* key) { … hash = 31 * hash + *((char*) (key + i));

I wonder how you got the compiler to accept this broken code. key is a void pointer, and one cannot do arithmetic using void pointers. The usual way to deal with this is to convert the void pointer into a string pointer first:

for (size_t i = 0; i < strlen(key); i++) { Calling strlen in a loop is expensive. This is because in C, the function strlen is very inefficient. It needs to look at every character of a string until it finds the '\0' that terminates the string. If you have a string with 1_000_000 characters, this will take 500_000_000_000 memory accesses, which is really slow. The usual pattern in C is to start with a pointer to the beginning of the string and to advance this pointer until it points to the '\0' , like this: size_t hash(const void* key) { size_t hash = 0; for (const unsigned char *p = key; *p != '\0'; p++) { hash = 31 * hash + *p; } return hash; } bool compare(void* key1, void* key2) { return *((int*) key1) == *((int*)key2); } Ah, so the HashMap maps string keys to int values. These facts should be represented in the function names. Instead of hash and compare , these should rather be called hash_str and compare_int . int main() { The empty parentheses () mean that this function takes an arbitrary number of arguments. This was a historical accident. In modern C (since 1990), you have to write (void) here to state that the function takes no arguments at all. struct HashMap* my_hmap = new_hashmap(hash, compare); int k = 10; int v = 101; int v2 = 102; insert_hashmap(my_hmap, &k, &v); Nope. As I said above, the code says that the keys to the map are strings, yet you pass an int to it. This invokes undefined behavior, which you must avoid. printf("done!"); There is a missing after the "done! ". Without this newline, there is no guarantee that the "done!" is printed at all. That's for historic reasons, but it's a good rule. Every program that outputs text should do so in whole lines. Summary: you got many things right and many things wrong, but that's to be expected when you submit your code for review. For comparison, here is the header and implementation that's very similar to yours. You can practice reading other people's code using that, and if you find any differences, have a look at the GitHub history of these files. They are more than 27 years old and have evolved a lot in all this time.

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