Tom's Two-up Two-down | Shuffle a Deck of Cards

Shuffle a Deck of Cards

How do you shuffle a deck of playing cards? I created a shuffle function using the Fisher–Yates Shuffle for a game I was working on, and I've tweaked that code a bit for this example. We will swap a random card from the unshuffled portion of a deck with the last unshuffled card in the deck. We then decrement the number of unshuffled cards and repeat. Got that?

Lets take a look at the code. First, I generate a new deck, which is an array of card objects. Then I pass the deck to a shuffle function.

const newDeck = () => {
  const deck = [];
  const suits = ["♤", "♢", "♧", "♡"];
  const ranks = [
    "2",
    "3",
    "4",
    "5",
    "6",
    "7",
    "8",
    "9",
    "10",
    "J",
    "Q",
    "K",
    "A",
  ];

  // Build fresh deck as an array of objects
  for (let i = 0; i < ranks.length; i++) {
    for (let j = 0; j < suits.length; j++) {
      deck.push({
        card: ranks[i] + suits[j],
        rank: ranks[i],
        suit: suits[j],
        color: j % 2 === 0 ? "black" : "red",
      });
    }
  }
  return deck;
};

Now I shuffle the array of card objects. Call shuffle(newDeck()) passing the deck newDeck() returns.

const shuffle = (deck) => {
  // deck is an array of card objects
  let numUnshuffledCards = deck.length;
  let randomUnshuffledCard;
  let lastUnshuffledCard;

  // While unshuffled cards remain
  while (numUnshuffledCards) {
    // Pick random number betweeen 0 inclusive and number of unshuffled cards
    // Then decrement the number of unshuffled cards by 1
    randomUnshuffledCard = Math.floor(Math.random() * numUnshuffledCards--);
    // Assign the last card in the unshuffled portion to a variable
    lastUnshuffledCard = deck[numUnshuffledCards];
    // Move the random card to the last unshuffled card's position
    deck[numUnshuffledCards] = deck[randomUnshuffledCard];
    // Move the last unshuffled card to the random card's previous position
    deck[randomUnshuffledCard] = lastUnshuffledCard;
  }

  return deck;
};

shuffle(newDeck());

I can simplify shuffle() with array destructuring.

const shuffle = (deck) => {
  // deck is an array of card objects
  let numUnshuffledCards = deck.length;
  let randomUnshuffledCard;

  // While unshuffled cards remain
  while (numUnshuffledCards) {
    // Pick random number betweeen 0 inclusive and number of unshuffled cards
    // Then decrement the number of unshuffled cards by 1
    randomUnshuffledCard = Math.floor(Math.random() * numUnshuffledCards--);
    // Swap the random card with the last unshuffled card in the array
    [deck[randomUnshuffledCard], deck[numUnshuffledCards]] = [
      deck[numUnshuffledCards],
      deck[randomUnshuffledCard],
    ];
  }

  return deck;
};

30 Oct 2019
#note
#code

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Shuffle a Deck of Cards