Array, avoid pulling Duplicates, simple code, C++ - c++

Question

Its a card game, I draw 2 cards from the deck (the array) with the help of two functions.
Each element of the array represent one card with a symbol Spades, Hearts, Diamonds or Clubs.
These numbers in the array \5 \4 \3 \6, represent Clubs, Dimaond, Heatrs, spades (just if ur curious)
The problem!
When I draw a card two times I sometimes get Duplicates.. The same card twice.
How do I make sure the same card cant be drawn twice?
How do I avoid getting duplicates??
This is how the code Looks like.
Some INFO about the array...
The further in, in the array, the higher value the element has.
I have shortened the array... for easy testing of a solution... later the array will be 52 elements.
array<string, 3> cards = { "Ess \5", "Ess \4", "Ess \3" };
//The function.
pair<string, int> draw_card()
{
int random_index = rand() % 52;
string card = cards[random_index];
return { card, random_index };
}
int main()
{
// Seed, random.
srand((unsigned int)time(NULL));
// Calling the function 2 times.
pair<string, int> you_drew = draw_card();
cout << "You drew: " << you_drew.first << endl;
pair<string, int> comp_drew = draw_card();
cout << "Computer drew: " << comp_drew.first << endl;
// Deciding the winner.
int your_score{ 0 };
int the_computers_score{ 0 };
if (you_drew.second > comp_drew.second) {
cout << "You Won!" << endl;
your_score++;
}
else if (you_drew.second < comp_drew.second) {
cout << "You Lost!" << endl;
the_computers_score++;
}
return 0;
}
Everything is working fine, EXCEPT sometimes I get duplicates... I want to make sure I can Not get that...
Somehow when I draw a card the element in the array should not be able to be drawn.. I want to avoid getting duplicates. Please help me!
Shouldnt something like this work? its not but.. shouldnt it?
pair<string, int> comp_drew = draw_card();
if (you_drew == comp_drew) {
bool run8 = true;
while (run8) {
pair<string, int> comp_drew = draw_card();
if (comp_drew != you_drew) {
cout << "Computer drew: " << comp_drew.first << endl;
run8 = false;
}
}
}
Or maybe another solution..
Perhaps after calling the function one time i can delete the return index from the array?

Answer 1

You can swap the drawn card to the end and only choose an index smaller than 51 the next time.
int array_len = 52; // global variables are not great, but it's easier here
pair<string, int> draw_card()
{
int random_index = rand() % array_len;
--array_len;
string card = cards[random_index];
std::swap(cards[random_index], cards[array_len]);
return { card, random_index };
}

Answer 2

There are many different ways to do this.
std::random_shuffle
#include <iostream>
#include <algorithm>
using namespace std;
class CardMachine {
static unsigned constexpr DECK_SIZE = 4;
int cards[DECK_SIZE] = {1,2,3,4};
int lastUsedCardIndex = -1;
public:
int getCard() {
if (lastUsedCardIndex == DECK_SIZE - 1)
shuffle();
return cards[++lastUsedCardIndex];
}
void shuffle() {
random_shuffle(begin(cards), end(cards));
}
};
int main()
{
CardMachine cardMachine = CardMachine();
cout << "unhsuffled - 1,2,3,4 without duplicates:" << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << "Shuffled. Random order, and still no duplicates:" << endl;
cardMachine.shuffle(); // or call getCard which can shuffle
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
return EXIT_SUCCESS;
}
Mark Card as Taken
#include <iostream>
#include <random>
using namespace std;
class CardMachine {
static unsigned constexpr DECK_SIZE = 4;
inline static int constexpr cards[DECK_SIZE] = {1,2,3,4};
bool cardIsGone[DECK_SIZE] = {false, false, false, false};
public:
int getCard() {
while(true) {
int const RANDOM_INDEX = rand()%DECK_SIZE;
if(!cardIsGone[RANDOM_INDEX]) {
cardIsGone[RANDOM_INDEX] = true;
return cards[RANDOM_INDEX];
}
}
}
void shuffle() {
for(bool &isGone : cardIsGone)
isGone = false;
lastUsedCardIndex = -1;
}
};
int main()
{
CardMachine cardMachine = CardMachine();
cout << "Shuffled - Random order, and still no duplicates:" << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << "Shuffled. Random order, and still no duplicates:" << endl;
cardMachine.shuffle(); // Causes infinite loop if you call getCard DECK_SIZE + 1 times
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
return EXIT_SUCCESS;
}
It's not the most efficient solution, but with small values like 52, it should be instant.
Swap Used Cards to Back and Keep Track of Range
#include <iostream>
#include <random>
using namespace std;
class CardMachine {
static unsigned constexpr DECK_SIZE = 4;
int cards[DECK_SIZE] = {1,2,3,4};
int lastCardIndexExlusive = DECK_SIZE;
public:
int getCard() {
int const RANDOM_INDEX = rand()%lastCardIndexExlusive;
swap(cards[RANDOM_INDEX],cards[lastCardIndexExlusive - 1]);
return cards[--lastCardIndexExlusive];
}
void shuffle() {
lastCardIndexExlusive = DECK_SIZE;
}
};
int main()
{
CardMachine cardMachine = CardMachine();
cout << "Shuffled - Random order, and still no duplicates:" << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << "Shuffled. Random order, and still no duplicates:" << endl;
cardMachine.shuffle(); // Causes exception if getCard DECK_SIZE + 1 times
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
return EXIT_SUCCESS;
}
Remember Last Card
#include <iostream>
#include <random>
using namespace std;
class CardMachine {
static unsigned constexpr DECK_SIZE = 4;
int cards[DECK_SIZE] = {1,2,3,4};
int lastCard = -1;
public:
int getCard() {
int const RANDOM_INDEX = rand()%DECK_SIZE;
if (lastCard == RANDOM_INDEX) {
return getCard();
}
lastCard = cards[RANDOM_INDEX];
return lastCard;
}
void shuffle() {
lastCard = -1;
}
};
int main()
{
CardMachine cardMachine = CardMachine();
cout << "Shuffled - Random order, and still no duplicates, but only able to produce 2:" << endl;
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
cout << "Shuffled. Random order, and still no duplicates:" << endl;
cardMachine.shuffle(); // Causes potential duplicates if getCards 3+ times
cout << cardMachine.getCard() << endl;
cout << cardMachine.getCard() << endl;
return EXIT_SUCCESS;
}
All of these solutions make use of a class to hold state for use in getCard to make sure we get the right behavior.