Hash Table in C and C++

What is Hash Table?

A hash function maps a key to a bucket; collisions require a strategy such as chaining or open addressing.

Important points

State the invariants before implementing operations.
Handle empty, full, and allocation-failure cases.
Measure time complexity and memory ownership together.

See how it works

Type a key and store or look it up. The key is hashed to a bucket number, so the program jumps straight to the right bucket.

C and C++ code examples

C

Run code →

main.c

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

typedef struct Item {
    const char *key;
    int count;
    struct Item *next;
} Item;

unsigned hash(const char *text) {
    unsigned value = 5381;
    while (*text) {
        value = value * 33u ^ (unsigned char)*text++;
    }
    return value % 5;
}

int main(void) {
    const char *words[] = {"c", "cpp", "c"};
    Item items[2] = {{"c", 0, NULL}, {"cpp", 0, NULL}};
    Item *buckets[5] = {0};

    for (int index = 0; index < 2; ++index) {
        unsigned bucket = hash(items[index].key);
        items[index].next = buckets[bucket];
        buckets[bucket] = &items[index];
    }
    for (int index = 0; index < 3; ++index) {
        Item *item = buckets[hash(words[index])];
        for (; item; item = item->next) {
            if (strcmp(item->key, words[index]) == 0) {
                ++item->count;
                break;
            }
        }
    }
    printf("c=%d cpp=%d\n", items[0].count, items[1].count);
}

Result of running the code

c=2 cpp=1

C++

Run code →

main.cpp

#include <iostream>
#include <string>
#include <unordered_map>

int main() {
    std::unordered_map<std::string, int> counts;
    for (const std::string word : {"c", "cpp", "c"}) {
        ++counts[word];
    }
    std::cout << "c=" << counts["c"]
              << " cpp=" << counts["cpp"] << '\n';
}

Result of running the code

c=2 cpp=1

C and C++ comparison

The compact C example uses fixed known keys to focus on buckets and chaining. std::unordered_map owns nodes, grows automatically, and supports arbitrary string keys.

C

Structures and operations are usually separate and allocation is explicit.

C++

Classes and containers can preserve invariants and manage resources automatically.

Practice exercises

Run both versions, then modify them to observe the different language guarantees.

Add empty and capacity-boundary tests.
Implement cleanup and verify no leaks remain.
Compare operation complexity with a standard container.

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