Question

How to select N unique random indices from an existing array in C

Answer 1

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

/*
    Select N unique random indices from an existing array.

    Approach:
    - Build an array of indices: 0, 1, 2, ..., size-1.
    - Shuffle the indices using Fisher–Yates.
    - Take the first N shuffled indices — guaranteed unique.
    - Return those indices to the caller.
*/

// Shuffle an array of indices in-place using Fisher–Yates
void shuffle_indices(size_t *indices, size_t size) {
    for (size_t i = size - 1; i > 0; --i) {
        size_t j = (size_t)(rand() % (i + 1));   // random index in [0, i]
        size_t tmp = indices[i];
        indices[i] = indices[j];
        indices[j] = tmp;
    }
}

// Return N unique random indices
size_t *pick_unique_indices(size_t arr_size, size_t count) {
    if (count > arr_size) {
        fprintf(stderr, "Cannot pick more unique indices than array size.\n");
        exit(EXIT_FAILURE);
    }

    // Allocate index list
    size_t *indices = malloc(arr_size * sizeof(size_t));
    if (!indices) {
        fprintf(stderr, "Memory allocation failed.\n");
        exit(EXIT_FAILURE);
    }

    // Fill with sequential indices
    for (size_t i = 0; i < arr_size; ++i) {
        indices[i] = i;
    }

    // Shuffle them
    shuffle_indices(indices, arr_size);

    // Allocate result array
    size_t *result = malloc(count * sizeof(size_t));
    if (!result) {
        fprintf(stderr, "Memory allocation failed.\n");
        free(indices);
        exit(EXIT_FAILURE);
    }

    // Copy first N shuffled indices
    for (size_t i = 0; i < count; ++i) {
        result[i] = indices[i];
    }

    free(indices);
    return result;
}

int main(void) {
    // Example array
    int data[] = {5, 12, 5, 19, 5, 33, 47, 5, 58, 61, 17, 3, 5, 74, 83, 90, 6};
    size_t data_size = sizeof(data) / sizeof(data[0]);

    // Number of unique indices to pick
    size_t N = 6;

    // Seed RNG once
    srand((unsigned)time(NULL));

    // Get unique random indices
    size_t *indices = pick_unique_indices(data_size, N);

    // Print results
    printf("Random unique indices and their values:\n");
    for (size_t i = 0; i < N; ++i) {
        printf("index %zu -> value %d\n", indices[i], data[indices[i]]);
    }

    free(indices);

    return 0;
}


/*
run:

Random unique indices and their values:
index 2 -> value 5
index 10 -> value 17
index 8 -> value 58
index 3 -> value 19
index 4 -> value 5
index 6 -> value 47

*/

 

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