Group by Key

Build buckets keyed by a shared field, preserving the first-seen key order.

Algorithm

Canonical pairs (a,1), (b,2), (a,3), (c,4), (b,5) print {a: [1, 3], b: [2, 5], c: [4]}. The replay uses the same input in every language, so this C DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.c

#include <stdio.h>

int main(void) {
    char keys[] = {'a', 'b', 'a', 'c', 'b'};
    int values[] = {1, 2, 3, 4, 5};
    int groups[3][3] = {{0}};
    int sizes[3] = {0};
    for (int i = 0; i < 5; i++) {
        int bucket = keys[i] - 'a';
        groups[bucket][sizes[bucket]++] = values[i];
    }
    printf("{a: [%d, %d], b: [%d, %d], c: [%d]}\n",
           groups[0][0], groups[0][1], groups[1][0], groups[1][1], groups[2][0]);
}

Complexity

Time: O(n) average
Space: O(k + n) for buckets and values

Implementation notes

Keep output formatting deterministic. Do not rely on unordered hash-map printing when the lesson needs cross-language comparison.
The trace highlights the hash table state after each write.

bucket map Each key owns a list. A new key creates a bucket; a repeated key appends to the existing bucket.