Delete matched pairs in an Array - c

Question

I'm attempting to write a game of Go Fish where points are given for matched pairs of cards. However, I can't seem to find a way to remove the matched pairs from the hand.
I have implemented a little loop to remove single cards used in other aspects of the game, as shown here:
for ( i = position - 1 ; i < user_size - 1 ; i++)
{
user_hand[i] = user_hand[i+1];
user_count[i]--;
user_size--;
}
Where the user_size is the number of cards in the user's hand, and the user_count is how many of each of the 13 card values the user holds. I can't find a way to remove pairs of the same valued cards however.
For example if the user_hand was: 2 2 4 5 6 6 6 6 1 2
I would like to remove all but one of the 2's (one pair) and all four of the 6's (two pairs). Leaving the user_hand as: 4 5 1 2. But for the life of me I can't think of a way to do this. Any help would be greatly appreciated!

Answer 1

In case reordering cards in users hand is not a problem, you could:
int *hand;
int handSize;
...
sort(hand, handSize); // 1. sort
int *newHand = malloc(sizeof(int) * handSize);
int i, newHandSize = 0;
for (i = 1; i < handSize; ++i) {
if (hand[i - 1] == hand[i]) {
hand[i] = -1; // 2. "flag" last value of a pair
continue;
}
newHand[newHandSize] = hand[i - 1]; // 3. copy last examined card to new hand
newHandSize++;
}
if (hand[handSize - 1] != -1) { // 4. copy last element if needed
newHand[newHandSize] = hand[handSize - 1];
newHandSize++;
}
int* handToFree = hand;
hand = newHand; // 5. replace users hand with new hand
free(handToFree); // 6. clean up
but I don't think it can go under O( n*log(n) + n )

Answer 2

Normally you would use 1 array for the cards and one variable to hold the number of cards.
Then you can iterate with a nested loop like this:
for (int i = 0; i < user_size; i++){
for (int j = i+1; j < user_size; j++){
if(user_hand[i] == user_hand[j]){
/* remove card at index i */
for(int z = i; z < user_size - 1; z++) user_hand[z] = user_hand[z + 1];
user_size--;
/* remove card at index j */
for(int z = j; z < user_size - 1; z++) user_hand[z] = user_hand[z + 1];
user_size--;
}
}
}
But in your example, you also got user_count[]. If I understood you right (otherwise please correct me) user_count[] holds how many cards of a specific value the user has on his hand.
So in your example with: 2 2 4 5 6 6 6 6 1 2
user_count[0] = 0
user_count[1] = 1
user_count[2] = 3
user_count[3] = 0
user_count[4] = 1
user_count[5] = 1
user_count[6] = 4
If that's what user_count[] is for, then you could simply do:
for (int i = 0; i < user_size; i++){
for (int j = i+1; j < user_size; j++){
if(user_hand[i] == user_hand[j]){
int cardNumber = user_hand[i];
/* remove card at index i */
for(int z = i; z < user_size - 1; z++) user_hand[z] = user_hand[z + 1];
user_size--;
/* remove card at index j */
for(int z = j; z < user_size - 1; z++) user_hand[z] = user_hand[z + 1];
user_size--;
/* decrement user_count by 2 */
user_count[cardNumber] = user_count[cardNumber] - 2;
}
}
}

Answer 3

void removePairs(int * hand, int size)
{
for(int i = 0; i < size; i++)
{
for(int j = i+1; j < size; j++)
{
if((hand[i] == hand[j]) && hand[i] != -1)
{
hand[i] = -1; // set card "ready to remove"
hand[j] = -1;
break;
}
}
}
for(int k = 0; k < size; k++)
{
if(hand[k] != -1)
{
printf("%d ", hand[k]); // you can store remaining cards here
}
}
}

Answer 4

Start by looping through your user_count array, and keep calling remove_card twice as long as user_count[rank] >= 2:
for(int rank = 1; rank <= 13; rank++)
while(user_count[rank] >= 2) {
remove_card(user_hand, user_count, &user_size, rank);
remove_card(user_hand, user_count, &user_size, rank);
}
For remove_card, just find the first matching card, and call your existing routine:
void remove_card(int *user_hand, int *user_count, int *user_size, int rank) {
for(int pos = 0; pos < *user_size; pos++)
if(user_hand[pos] == rank)
remove_card_at(user_hand, user_count, user_size, pos+1);
}
remove_card_at is the routine you provided in your original post to remove a card at a particular position. Note you'll have to turn user_size into a pointer and dereference it in order to modify the variable in the calling function.
Also, you should really look into using a structure or class to hold the user's hand, depending on if you're using C or C++.