Task: Set puzzle
C: #include <stdio.h>
#include <stdlib.h>

char *names[4][3] = {
	{ "red", "green", "purple" },
	{ "oval", "squiggle", "diamond" },
	{ "one", "two", "three" },
	{ "solid", "open", "striped" }
};

int set[81][81];

void init_sets(void)
{
	int i, j, t, a, b;
	for (i = 0; i < 81; i++) {
		for (j = 0; j < 81; j++) {
			for (t = 27; t; t /= 3) {
				a = (i / t) % 3;
				b = (j / t) % 3;
				set[i][j] += t * (a == b ? a : 3 - a - b);
			}
		}
	}
}

void deal(int *out, int n)
{
	int i, j, t, c[81];
	for (i = 0; i < 81; i++) c[i] = i;
	for (i = 0; i < n; i++) {
		j = i + (rand() % (81 - i));
		t = c[i], c[i] = out[i] = c[j], c[j] = t;
	}
}

int get_sets(int *cards, int n, int sets[][3])
{
	int i, j, k, s = 0;
	for (i = 0; i < n; i++) {
		for (j = i + 1; j < n; j++) {
			for (k = j + 1; k < n; k++) {
				if (set[cards[i]][cards[j]] == cards[k])
					sets[s][0] = i,
					sets[s][1] = j,
					sets[s][2] = k,
					s++;
			}
		}
	}

	return s;
}

void show_card(int c)
{
	int i, t;
	for (i = 0, t = 27; t; i++, t /= 3)
		printf("%9s", names[i][(c/t)%3]);
	putchar('\n');
}

void deal_sets(int ncard, int nset)
{
	int c[81];
	int csets[81][3]; 
	int i, j, s;

	do deal(c, ncard); while ((s = get_sets(c, ncard, csets)) != nset);

	printf("dealt %d cards\n", ncard);
	for (i = 0; i < ncard; i++) {
		printf("%2d:", i);
		show_card(c[i]);
	}
	printf("\nsets:\n");

	for (i = 0; i < s; i++) {
		for (j = 0; j < 3; j++) {
			printf("%2d:", csets[i][j]);
			show_card(c[csets[i][j]]);
		}
		putchar('\n');
	}
}

int main(void)
{
	init_sets();
	deal_sets(9, 4);

	while (1) deal_sets(12, 6);

	return 0;
}

C#: using System;
using System.Collections.Generic;
using static System.Linq.Enumerable;

public static class SetPuzzle
{
    static readonly Feature[] numbers  = { (1, "One"), (2, "Two"), (3, "Three") };
    static readonly Feature[] colors   = { (1, "Red"), (2, "Green"), (3, "Purple") };
    static readonly Feature[] shadings = { (1, "Open"), (2, "Striped"), (3, "Solid") };
    static readonly Feature[] symbols  = { (1, "Oval"), (2, "Squiggle"), (3, "Diamond") };

    private readonly struct Feature
    {
        public Feature(int value, string name) => (Value, Name) = (value, name);
        public int Value { get; }
        public string Name { get; }
        public static implicit operator int(Feature f) => f.Value;
        public static implicit operator Feature((int value, string name) t) => new Feature(t.value, t.name);
        public override string ToString() => Name;
    }

    private readonly struct Card : IEquatable<Card>
    {
        public Card(Feature number, Feature color, Feature shading, Feature symbol) =>
            (Number, Color, Shading, Symbol) = (number, color, shading, symbol);

        public Feature Number { get; }
        public Feature Color { get; }
        public Feature Shading { get; }
        public Feature Symbol { get; }

        public override string ToString() => $"{Number} {Color} {Shading} {Symbol}(s)";
        public bool Equals(Card other) => Number == other.Number && Color == other.Color && Shading == other.Shading && Symbol == other.Symbol;
    }

    public static void Main() {
        Card[] deck = (
            from number in numbers
            from color in colors
            from shading in shadings
            from symbol in symbols
            select new Card(number, color, shading, symbol)
        ).ToArray();
        var random = new Random();

        Deal(deck, 9, 4, random);
        Console.WriteLine();
        Console.WriteLine();
        Deal(deck, 12, 6, random);
    }

    static void Deal(Card[] deck, int size, int target, Random random) {
        List<(Card a, Card b, Card c)> sets;
        do {
            Shuffle(deck, random.Next);
            sets = (
                from i in 0.To(size - 2)
                from j in (i + 1).To(size - 1)
                from k in (j + 1).To(size)
                select (deck[i], deck[j], deck[k])
            ).Where(IsSet).ToList();
        } while (sets.Count != target);
        Console.WriteLine("The board:");
        foreach (Card card in deck.Take(size)) Console.WriteLine(card);
        Console.WriteLine();
        Console.WriteLine("Sets:");
        foreach (var s in sets) Console.WriteLine(s);
    }

    static void Shuffle<T>(T[] array, Func<int, int, int> rng) {
        for (int i = 0; i < array.Length; i++) {
            int r = rng(i, array.Length);
            (array[r], array[i]) = (array[i], array[r]);
        }
    }

    static bool IsSet((Card a, Card b, Card c) t) =>
        AreSameOrDifferent(t.a.Number, t.b.Number, t.c.Number) &&
        AreSameOrDifferent(t.a.Color, t.b.Color, t.c.Color) &&
        AreSameOrDifferent(t.a.Shading, t.b.Shading, t.c.Shading) &&
        AreSameOrDifferent(t.a.Symbol, t.b.Symbol, t.c.Symbol);

    static bool AreSameOrDifferent(int a, int b, int c) => (a + b + c) % 3 == 0;
    static IEnumerable<int> To(this int start, int end) => Range(start, end - start - 1);
}