# Properly choosing a random discard

Sometimes an effect causes you to discard a card at random. In casual games, it's usually acceptable to have the player hold their hand out (or face down) and the other player picks a card. However, this is not really random. For example I often play mind games to influence their decision like sticking one card out really far while staring at another one.

If my opponent wants to resolve a random discard that way, am I within my right to demand that it be settled with something more random, like a dice roll?

• Try that strategy if you like, but any decent opponent knowledgeable in Game Theory would figure out your strategy in a few instances. You are always better off in such a situation, against a knowledgeable opponent, to provide no information what-so-ever. – Forget I was ever here Aug 31 '13 at 23:16
• @PieterGeerkens: But it's more like this: youtube.com/watch?v=U_eZmEiyTo0 In any case, my actions could influence the selection, for better or worse. Or a number of other things. So, not random. – Wesley Murch Aug 31 '13 at 23:18
• But against a skilled opponent, random is the very best you can do. Google game theory if you don't believe me. – Forget I was ever here Aug 31 '13 at 23:23
• @PieterGeerkens I disagree, "the best I could do" would be to trick my opponent into picking the card I want them to, perhaps using reverse-psychology. Which, of course, is not random, but it's often done. – Wesley Murch Aug 31 '13 at 23:27
• @PieterGeerkens Unfortunately children, novices and fools are about 96% of my opponents ;) – Wesley Murch Aug 31 '13 at 23:33

Yes. Many of us use dice even in casual games, because we can't actually make random decisions, and it's easy.

Random means, well, random, so a nonrandom method is not following the rules. I don't think this is explicitly in the rules anywhere, though both die rolls and coin flips are mentions as methods of randomization. It's left implicit that a nonrandom method does not suffice for a card which asks for a random choice; it's not what the card said to do.

705.3. ... Other methods of randomization may be substituted for flipping a coin as long as there are two possible outcomes of equal likelihood and all players agree to the substitution. For example, the player may roll an even-sided die and call "odds" or "evens," or roll an even-sided die and designate that "odds" means "heads" and "evens" means "tails."

It's also sometimes mentioned in rules clarifications, e.g. for Goblin Test Pilot from the Dragon's Maze FAQ:

To choose a target at random, all possible legal targets (including creatures and players) must have an equal chance of being chosen. There are many ways to do this, including assigning each possible legal target a number and rolling a die.

But again, I don't think it ever says explicitly "nonrandom methods are not random", because that's supposed to be obvious. Any method is okay, as long as all outcomes are equally likely. Determining what methods satisfy that criterion is left up to the players, or if necessary, a judge.

• OK this is what I suspected. So it seems I am well within my right to say that manual selection (albeit face-down cards) is not sufficiently random. You think a judge would back me up on that one? – Wesley Murch Aug 31 '13 at 23:23
• @WesleyMurch Yeah, assuming you have a die there. But of course, if your opponent asks you to choose, you can always just roll a die. And if you're trying to ask them to, you can offer them the die and it seems unlikely they'll refuse. – Cascabel Sep 1 '13 at 0:38

If you don't care that much about actual randomness, but you want to be sure your opponent has no way to know which card is which, you can just make sure you don't know either. That way, all mind tricks or tells won't work anymore.

What I would do is shuffle my hand before presenting it to my opponent. Since I don't know in which order the card are after shuffling, there is no information I can give to the opponent about it, whether willingly or not. As a bonus, since the opponent saw you shuffle, he also knows there's no point in trying to think about which card to pick or read your mind or possible clues or trickeries.

It's also going quite fast since nobody has to think about anything, which means the time you lost shuffling is not a problem any longer; chances are the whole process will be faster than using a dice.

• Whilst this is a helpful thing to do (if you're not the person having to choose), it's not really addressing the question. – doppelgreener Sep 19 '13 at 9:31
• Right; I thought it was worth mentioning but it probably would have been better as a comment than an answer. shrug – Joubarc Sep 20 '13 at 4:06
• In that case I request you make it a comment, since as it stands it's not an answer to this question. – doppelgreener Sep 20 '13 at 12:10
• @JonathanHobbs I think this answers the question just fine. It's so simple, I didn't even consider it. – Wesley Murch Sep 20 '13 at 15:06
• I'll go with the opinion of the one who asked the question then :-) – Joubarc Sep 21 '13 at 18:16

In situations where a die roll is impossible, a series coinflips can be used. this can be achieved by representing each number as binary.

First, decide on which of heads or tails to represent 0/1.

Then, flip the coin, recording the result, this gives you the "ones" column, or a value of 0 / 1.

Then, flip again for the "twos" column, giving you a value of 0 or 2, add this to the previous (giving 0, 1, 2 or 3 respectively)

Then, flip again for the "fours" column, giving you a value of 0 or 4. Add this to the previous (giving 0, 1, 2, 3, 4, 5, 6, 7). This should be enough for 90% of situations. It is zero indexed, so either add one to the final result, or label your cards from 0 to x. As with any other solution that doesn't perfectly fit the cards present, repeat if you hit a number in excess of the choice of cards. If you have less than 4 cards in hand you can skip the last step. If you have more than 8 you can add another flip to cope with up to 16 cards etc etc.

I have no idea of a situation where you would be able to comfortably lay out magic cards but not roll a die, but if you find yourself in this particular magical Christmas land, then this gives you a legitimate way to resolve the decision randomly.

Having said all of this, if you can feasibly use a die, this is the best way to do it.

You can and should demand the decision be made based on die rolls, the cards should still be shuffled for this.

The rules demand that decisions like this be decided in a way that cannot under any circumstances be pre-determined.

That means that a method where it is possible - even if it relies on an opponent's ineptitude - to be pre-determined. That is quite simply called an attempt to cheat, so don't do it.

Another way to obtain a random decision, in the absence of a die, is to:

• Have both players secretly write down a number from 0 to N-1 (where N is the number of cards to choose from);
• Reveal them simultaneously and add them;
• Take the result modulo N; and
• Select a card this many places over from the left-most card of the owning player's hand.

One can see how this works as follows: whatever number one player might choose, each possible number chosen by the other player will map that to a single unique card from the pool, exactly once. And vice-versa.

Update:
Excerpts from my comments to the original question:

• Try that strategy if you like, but any decent opponent knowledgeable in Game Theory would figure out your strategy in a few instances. You are always better off in such a situation, against a knowledgeable opponent, to provide no information what-so-ever.
• against a skilled opponent, random is the very best you can do. Google game theory if you don't believe me.
• Against children, novices and fools you might do better; however a skilled opponent will have determined which card to select in such a circumstance before the game even started. They will deviate from this pre-game selection only if they have identified your tell. Watch The Cincinnati Kid sometime, or the original Rounders with Matt Damon.
• More on Game Theory is readily available on the web, or try Googling Prisoners' Dilemma.
• "in the absence of a die"... What world do you live in? ;) Yes this is basically how I prefer to do it, I usually assign each card a number then keep rolling until that number is hit. – Wesley Murch Aug 31 '13 at 23:29
• A strong player will have decided on a number like 43 in advance (any prime number much greater than the number of cards possible to be selected from will do); and then when presented with the choice will determine the remainder on division by N, and select the card that number of places over from your (or their) leftmost card. If they are really good they will have made a separate determination of which card to select if there are only two to choose from. – Forget I was ever here Aug 31 '13 at 23:47
• @WesleyMurch: If you are playing in the car, for instance, either a die or a suitable place to roll it (where bed-rolling is not possible) might not be available. – Forget I was ever here Aug 31 '13 at 23:50
• Of course you might not have a die, I was just making a joke (hence the winking emoticon). However, I'm not sure I understand your previous comment, "decided on a number like 43", not sure what you mean. I meant that we assign a number (1, 2, 3, etc.) to each face-down card and roll a D6 or something. EDIT: Oh you meant with your "pick a secret number" method. OK. – Wesley Murch Aug 31 '13 at 23:53
• Not sure if I'd be playing anything but a casual game in a car.... – Wesley Murch Aug 31 '13 at 23:58