Hash Map

1# Python's built-in dict is a hash map: O(1) average2# insert, lookup, and delete.3inventory = {"apple": 3, "banana": 7}4
5# Insert and update6inventory["cherry"] = 57inventory["apple"] += 28
9# Lookup, with .get for a safe default on missing keys10print("apple: ", inventory["apple"])11print("mango: ", inventory.get("mango", 0))12
13# Membership test and delete14print("banana in stock:", "banana" in inventory)15del inventory["banana"]16print("banana in stock:", "banana" in inventory)17
18# Iterate over key-value pairs19for fruit, count in sorted(inventory.items()):20    print(f"{fruit}: {count}")21
22# Classic hash map use case: counting frequencies23words = "the quick brown fox jumps over the lazy dog the end".split()24freq = {}25for word in words:26    freq[word] = freq.get(word, 0) + 127
28print("Occurrences of the:", freq["the"])29print("Most common word:  ", max(freq, key=freq.get))

1// The built-in Map is the idiomatic hash map in JavaScript:2// any key type, insertion-ordered iteration, O(1) average lookups.3const ages = new Map();4
5ages.set("Alice", 30);6ages.set("Bob", 25);7ages.set("Carol", 35);8ages.set("Bob", 26); // set on an existing key updates the value9
10console.log("Bob:", ages.get("Bob"));11console.log("Has Dave?", ages.has("Dave"));12console.log("Size:", ages.size);13
14ages.delete("Carol");15console.log("Size after delete:", ages.size);16
17for (const [name, age] of ages) {18    console.log(`${name} is ${age}`);19}20
21console.log("Keys:", [...ages.keys()].join(", "));

1import java.util.HashMap;2import java.util.Map;3
4public class Main {5    public static void main(String[] args) {6        Map<String, Integer> ages = new HashMap<>();7        ages.put("Alice", 30);8        ages.put("Bob", 25);9        ages.put("Carol", 35);10
11        System.out.println("Bob -> " + ages.get("Bob"));12        System.out.println("contains Carol: " + ages.containsKey("Carol"));13        System.out.println("Dave or default: " + ages.getOrDefault("Dave", -1));14
15        // getOrDefault makes frequency counting a one-liner16        String[] words = {"apple", "banana", "apple", "cherry", "apple"};17        Map<String, Integer> counts = new HashMap<>();18        for (String w : words) {19            counts.put(w, counts.getOrDefault(w, 0) + 1);20        }21        System.out.println("apple count: " + counts.get("apple"));22
23        ages.remove("Bob");24        System.out.println("size after remove: " + ages.size());25
26        for (Map.Entry<String, Integer> entry : ages.entrySet()) {27            System.out.println(entry.getKey() + " is " + entry.getValue());28        }29    }30}

1#include <algorithm>2#include <iostream>3#include <string>4#include <unordered_map>5#include <vector>6
7int main() {8    std::unordered_map<std::string, int> ages;9
10    // Insert and update11    ages["alice"] = 30;       // operator[] inserts or overwrites12    ages.insert({"bob", 25}); // insert keeps an existing value13    ages.emplace("carol", 41);14    ages["alice"] = 31;15
16    // Lookup17    auto it = ages.find("bob");18    if (it != ages.end()) {19        std::cout << "bob is " << it->second << "\n";20    }21    std::cout << "has dave: " << ages.count("dave") << "\n";22
23    // Remove24    ages.erase("carol");25
26    // Iterate (sort keys first: unordered_map has no defined order)27    std::vector<std::string> keys;28    for (const auto& entry : ages) keys.push_back(entry.first);29    std::sort(keys.begin(), keys.end());30    for (const std::string& name : keys) {31        std::cout << name << " = " << ages[name] << "\n";32    }33    std::cout << "size: " << ages.size() << "\n";34    return 0;35}

1#include <stdbool.h>2#include <stdio.h>3#include <stdlib.h>4#include <string.h>5
6#define BUCKETS 167
8typedef struct Node {9    char key[24];10    int value;11    struct Node* next;12} Node;13
14// A small string -> int hash map with a reusable put/get/remove API15typedef struct {16    Node* buckets[BUCKETS];17    int size;18} HashMap;19
20unsigned int hashKey(const char* key) {21    unsigned int h = 5381;22    for (; *key; key++) h = h * 33 + (unsigned char)*key;23    return h % BUCKETS;24}25
26void mapPut(HashMap* map, const char* key, int value) {27    unsigned int i = hashKey(key);28    for (Node* n = map->buckets[i]; n != NULL; n = n->next) {29        if (strcmp(n->key, key) == 0) {30            n->value = value;31            return;32        }33    }34    Node* n = malloc(sizeof(Node));35    strcpy(n->key, key);36    n->value = value;37    n->next = map->buckets[i];38    map->buckets[i] = n;39    map->size++;40}41
42bool mapGet(const HashMap* map, const char* key, int* out) {43    for (Node* n = map->buckets[hashKey(key)]; n != NULL; n = n->next) {44        if (strcmp(n->key, key) == 0) {45            *out = n->value;46            return true;47        }48    }49    return false;50}51
52void mapRemove(HashMap* map, const char* key) {53    Node** link = &map->buckets[hashKey(key)];54    while (*link != NULL) {55        if (strcmp((*link)->key, key) == 0) {56            Node* old = *link;57            *link = old->next;58            free(old);59            map->size--;60            return;61        }62        link = &(*link)->next;63    }64}65
66int main(void) {67    HashMap map = {0};68    mapPut(&map, "alice", 30);69    mapPut(&map, "bob", 25);70    mapPut(&map, "carol", 41);71    mapPut(&map, "alice", 31);  // updates the existing key72    int age;73    if (mapGet(&map, "bob", &age)) printf("bob is %d\n", age);74    printf("has dave: %s\n", mapGet(&map, "dave", &age) ? "yes" : "no");75    mapRemove(&map, "carol");76    printf("size: %d\n", map.size);77    return 0;78}